SCIENTIFIC AMERICAN SUPPLEMENT NO. 595
NEW YORK, MAY 28, 1887.
Scientific American Supplement. Vol. XXIII, No. 595.
Scientific American established 1845
Scientific American Supplement, $5 a year.
Scientific American and Supplement, $7 a year.
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TABLE OF CONTENTS.
I. BOTANY.—The Relation of Tabasheer to Mineral Substances.—The composition of this curious secretion of the bamboo.—Analyses and properties of the material, according to various observers.—Its appearance under the microscope. 1 illustration.
II. CHEMISTRY.—Apparatus for Drying Flour.—An apparatus for determining the moisture in flour. 1 illustration.
III. ELECTRICITY.—Automatic Commutator for Incandescent Lamps.—An apparatus for lighting automatically a new lamp to replace one that has failed. 1 illustration.
Definitions and Designations in Electro-Technics.—Mr. Jamieson's proposed code of electric symbols—literal and graphic. 4 illustrations.
IV. ENGINEERING.—New Dredging Machinery.—The dredger Ajax, recently built in California.—Its dimensions and capacity. 1 illustration.
Reservoir Dams.—By DAVID GRAVELL.—The engineering details of dams.—Typical masonry and earthwork dams of the world. 23 illustrations.
The Flexible Girder Tramway.—A new type of suspended railway—a modification of the wire tramway system. 21 illustrations.
V. HYGIENE.—Climate in its Relation to Health.—By G.V. POORE, M.D.—The third lecture of this series.—Consideration of the floating matter of the air and diseases caused thereby.—Causation of hay fever.
VI. MATHEMATICS.—Radii of Curvature Geometrically Determined.—By Prof. C.W. MACCORD, Sc.D.—No. VII. Path of a point on a connecting rod. 3 illustrations.
VII. MICROSCOPY.—Improved Microscopical Settling Tube.—By F. VANDERPOEL.—New tubes for use in urinary analysis. 4 illustrations.
VIII. MISCELLANEOUS.—Apparatus for Manufacturing Bouquets.—An ingenious machine for facilitating the construction of bouquets. 1 illustration.
Bozerian's Refrigerant Punkas.—A fan worked by the feet, a substitute for the Indian punka. 2 illustrations.
How to Make a Kite without a Tail.—An improved form of kite described and illustrated. 1 illustration.
Punkas.—By J. WALLACE, C.E.—The mechanics of punkas; experiments on their rate of swing.
The Edible Earth of Java.—An account of this curious substance, its taste and appearance.
IX. NAVAL ENGINEERING.—Another Remarkable Torpedo Boat.—Over twenty-eight miles an hour.—Full particulars of the trial of one of the new Italian torpedo boats, built by Yarrow & Co.
Copeman & Pinhey's Life Rafts.—A new life raft for use on steamers, folding into deck settees. 3 illustrations.
X. PHYSICS.—Sunlight Colors—By Capt. W. DE W. ABNEY.—A valuable lecture on the cause of the colors of the sun, and their relative intensities. 3 illustrations.
The Wave Theory of Sound Considered.—By HENRY A. MOTT, Ph.D., LL.D.—Arguments against the generally accepted theory of sound.
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COPEMAN & PINHEY'S LIFE RAFTS.
The experiments with life saving appliances which Mr. Copeman brought before the delegates of the Colonial Conference, on the 13th April, at the Westminster Aquarium, had a particular interest, due to the late and lamentable accident which befell the Newhaven-Dieppe passenger steamer Victoria. In many cases of this nature, loss of life must rather be attributed to panic than to a want of life saving appliances; but, as a general rule, an abundant supply of such apparatus will tend to give passengers confidence, and prevent the outbreak of such discreditable scenes on the part of passengers as took place on the Victoria.
Messrs. Copeman & Pinhey have, for some years past, done good work in this direction, and at the recent meeting of the Institution of Naval Architects, Mr. Copeman showed several models of the latest types of their life saving apparatus, both for use on torpedo boats and passenger steamers. Our illustration (Fig. 1) represents the kind of rafts supplied to her Majesty's troop ships, while Figs. 2 and 3 show deck seats convertible into rafts, which are intended for ordinary passenger steamers. The raft shown in Fig. 1 consists of two pontoons, joined by strong cross beams, and fitted with mast, sail, and oars. When not in use, the pontoons form deck seats, covered by a wooden grating, which in our illustration forms the middle part of the raft. Each pontoon has a compartment for storing provisions, and when rigged as a raft, there is a railing to prevent persons being washed overboard.
The seat life buoy, shown in Fig. 2, serves as an ordinary deck seat, being about 8 ft. long, and it consists of two portions, hinged at the back. When required for use as a life buoy, it is simply thrown forward, the seat being at the same time lifted upward, so that the top rail of the back engages with the two clips, shown at either end of the seat, and the whole structure then forms a rigid raft, as will be seen from Fig. 3. Several other appliances were shown at the Westminster Aquarium on April 13, but the two rafts we have selected for illustration will give a sufficiently correct idea of the general principles upon which the apparatus is based.—Industries.
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ANOTHER REMARKABLE TORPEDO BOAT—OVER TWENTY-EIGHT MILES AN HOUR.
In a recent impression we gave some particulars of the trial trip of a boat built for the Italian government by Messrs. Yarrow & Co., which attained the highest speed known, namely, as nearly as possible, 28 miles an hour. On the 14th April the sister boat made her trial trip in the Lower Hope, beating all previous performances, and attaining a mean speed of 25.101 knots, or over 28 miles an hour. The quickest run made with the tide was at the rate of 27.272 knots, or 31.44 miles per hour, past the shore. This is a wonderful performance.
In the following table we give the precise results:
- - - - - - - Second Boiler. Receiver. Vacuum. Revs. Speed. Means. Means. - - - - - - - per Knots Knots Knots lb. lb. in. min. per hr. per hr. per hr. - - - - - - - 1 130 32 28 373 22.641 24.956 2 130 32 28 372.7 27.272 25.028 24.992 3 130 32 28 372 22.784 25.028 25.028 4 130 32 28 377 27.272 25.248 25.138 5 130 32 28 375 23.225 25.248 25.248 6 130 32 28 377 27.272 - - - - - - - Means 130 32 28 374.5 25.101 - - - - - - -
The boat is 140 ft. long, and fitted with twin screws driven by compound engines, one pair to each propeller. These engines are of the usual type, constructed by Messrs. Yarrow. Each has two cylinders with cranks at 90 deg.. The framing, and, indeed, every portion not of phosphor-bronze or gun metal, is of steel, extraordinary precautions being taken to secure lightness. Thus the connecting rods have holes drilled through them from end to end. The low pressure cylinders are fitted with slide valves. The high pressure valves are of the piston type, all being worked by the ordinary link motion and eccentrics. The engine room is not far from the mid length of the boat, and one boiler is placed ahead and the other astern of it. Each boiler is so arranged that it will supply either engine or both at pleasure. The boat has therefore two funnels, one forward and the other aft, and air is supplied to the furnaces by two fans, one fixed on the forward and the other on the aft bulkhead of the engine room.
The fan engines have cylinders 51/2 in. diameter and 31/2 in. stroke, and make about 1,100 revolutions per minute when at full speed, causing a plenum in the stokeholes of about 6 in. water pressure. Double steam steering gear is fitted, for the forward and aft rudder respectively, and safety from foundering is provided to an unusual degree by the subdivision of the hull into numerous compartments, each of which is fitted with a huge ejector, capable of throwing overboard a great body of water. A body of water equal to the whole displacement of the boat can be discharged in less than seven minutes. There is also a centrifugal pump provided, which can draw from any compartment. The circulating pump is not available, because it has virtually no existence, a very small pump on the same shaft as the centrifugal being used merely to drain the condensers. These last are of copper, cylindrical, and fitted with pipes through which a tremendous current of water is set up by the passage of the boat through the sea. Thus the space and weight due to a circulating pump is saved and complication avoided. The air and feed pumps are combined in one casting let into the engine room floor, quite out of the way, and worked by a crank pin in a small disk on the forward end of the propeller shaft. This is an admirable arrangement, and works to perfection.
The armament of the boat consists of two torpedo tubes in her bows, and a second pair set at a small angle to each—Yarrow's patent—carried aft on a turntable for broadside firing. There are also two quick firing 3 lb. guns on her deck. The conning tower forward is rifle proof, and beneath it and further forward is fixed the steering engine, and a compressing engine, by which air is compressed for starting the torpedoes overboard and for charging their reservoirs. A small dynamo and engine are also provided for working a search light, if necessary. The accommodation provided for the officers and crew is far in advance of anything hitherto found on board a torpedo boat.
The weather on the morning of Thursday, April 14, was anything rather than that which would be selected for a trial, or indeed any, trip on the Thames. At 11 A.M., the hour at which the boat was to leave Messrs. Yarrow's yard, Isle of Dogs, the wind was blowing in heavy squalls from the northeast, accompanied by showers of snow and hail. The Italian government was represented by Count Gandiani and several officers and engineers. In all there were about thirty-three persons on board. The displacement of the vessel was as nearly as might be 97 tons. A start was made down the river at 11:15 A.M., the engines making about 180 revolutions per minute, and the boat running at some 111/2 or 12 knots.
During this time the stokehole hatches were open, but the fans were kept running at slow speed to maintain a moderate draught. The fuel used throughout the trip was briquettes made of the best Welsh anthracite worked up with a little tar. The briquettes were broken up to convenient sizes before being put in the bunkers. This fuel is not of so high evaporative efficiency as Nixon's navigation coal, but it is more suitable for torpedo boat work, because it gives out Very little dust, while the coal in closed stokeholes half smothers the firemen. Watering only partially mitigates the evil. Besides this, the patent fuel does not clinker the tube ends—a matter of vital importance.
During the run down to Gravesend, the small quantity of smoke given out was borne down and away from the tops of the funnels by the fierce head wind, and now and then a heavy spray broke on the bows, wetting everything forward. In the engine room preparations were made for taking indicator diagrams. No attempt was made to drive the boat fast, because high speeds are prohibited by the river authorities on account of the heavy swell set up.
The measured mile on the Lower Hope is on the southern bank of the river, about three miles below Gravesend. Just as the boat passed the town, in the midst of a heavy rain squall, the stokehole hatches in the deck were shut, and the dull humming roar of the fans showed that the fires were being got up. The smoke no longer rose leisurely from the funnels. It came up now with a rush and violence which showed the powerful agency at work below. A rapid vibrating motion beneath the feet was the first evidence that the engines were away full speed. As the boat gathered way she seemed to settle down to her work, and the vibration almost ceased. The measured mile was soon reached, and then in the teeth of the northeaster she tore through the water. The tide and wind were both against her. Had the tide and wind been opposed, there would have been a heavy sea on. As it was, there was quite enough; the water, breaking on her port bow, came on board in sheets, sparkling in the sun, which, the rain squall having passed, shone out for the moment. As the wind was blowing at least thirty miles an hour, and the boat was going at some twenty-six miles an hour against it, the result was a moderate hurricane on board. It was next to impossible to stand up against the fury of the blast without holding on. The mile was traversed in less than 21/2 minutes, however; but the boat had to continue her course down the river for nearly another mile to avoid some barges which lay in the way, and prevented her from turning. Then the helm was put over, and she came round. There was no slacking of the engines, and astern of her the water leaped from her rudder in a great upheaved, foaming mass, some 7 ft. or 8 ft. high. Brought round, she once more lay her course. This time the wind was on her starboard quarter, or still more nearly aft. The boat went literally as fast as the wind, and on deck it was nearly calm. The light smoke from the funnels, no longer beaten down by wind, leaped up high into the air. Looking over the side, it was difficult to imagine that the boat was passing through water at all. The enormous velocity gave the surface of the river the appearance of a sheet of steel for 1 ft. or more outside the boat. Standing right aft, the sight was yet more remarkable. Although two 6 ft. screws were revolving at nearly 400 revolutions per minute almost under foot, not a bubble of air came up to break the surface. There was no wave in her wake; about 70 ft. behind her rose a gentle swelling hill.
Her wake was a broad smooth brown path, cut right through the rough surface of the river. On each side of this path rose and broke the angry little seas lashed up by the scourging wind. Along the very center of the brown track ran a thin ridge of sparkling foam, some 2 ft. high and some 20 ft. long, caused by the rudder being dragged through the water. There was scarcely any vibration. The noise was not excessive. A rapid whirr due to the engines, and a rythmical clatter due to the relief valve on one of the port engine cylinders not being screwed down hard enough, and therefore lifting a little in its seat at each stroke, made the most of it. The most prominent noise perhaps was the hum of the fans. Standing forward, the deck seems to slope away downward aft, as indeed it does, for it is to be noted that at these high speeds the forefoot of the boat is always thrown up clean out of the water—and the whole aspect of the boat: the funnels vomiting thin brown smoke, and occasionally, when a fire door is opened, a lurid pillar of flame for a moment; the whirr in the engine room; the dull thunder of the fans, produce an impression on the mind not easily expressed, and due in some measure no doubt to the exhilaration caused by the rapid motion through the air.
The best way to convey what we mean is to say that the whole craft seems to be alive, and a perfect demon of energy and strength. Many persons hold that a torpedo boat is likely to be more useful in terrifying an enemy than in doing him real harm, and we can safely say that the captain of an ironclad who saw half a dozen of these vessels bearing down on him, and did not wish himself well out of a scrape, has more nerve than most men.
The second mile was run in far less time than that in which what we have written concerning it can be read, and then the boat turned again, and once more the head wind with all its discomforts was encountered. Events repeated themselves, and so at last the sixth trip was completed, and the boat proceeded at a leisurely pace back again to Poplar. Mr. Crohn, representing Messrs. Yarrow on board, and all concerned, might well feel satisfied. We had traveled at a greater speed than had ever before been reached by anything that floats, and there was no hitch or impediment or trouble of any kind.
The Italian government may be congratulated on possessing the two fastest and most powerful torpedo boats in the world. We believe, however, that Messrs. Yarrow are quite confident that, with twin screw triple expansion engines, they can attain a speed of 26 knots an hour, and we have no reason to doubt this.—The Engineer.
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[Footnote: Paper, with slight abbreviation, read by Mr. David Gravell, Assoc. M. Inst. C.E., before the Society of Civil and Mechanical Engineers. The paper brings together in a convenient form the sections and salient facts concerning many dams. It was illustrated by numerous diagrams, from which our engravings have been prepared.—The Engineer.]
By DAVID GRAVELL.
The construction of dams, in some form or other, may probably rank among the very earliest of engineering works. Works of this character are not infrequently referred to in the accounts of the earliest historians; but it is to be feared that they are not always perfectly trustworthy. The subscribers to the Mudie of the period had to be considered, and their taste for the marvelous was probably not much inferior to that of our own day. When, therefore, Herodotus describes the reservoir of Moeris as formed for the control of the river floods of Nile-nourished Egypt, and of another constructed by Nebuchadnezzar at Sippara, of 140 miles in circumference, we must make allowances. But there is no question as to the existence in the East at the present day, and especially in India and Ceylon, of the remains of what may correctly be termed stupendous works; and the date of the construction of which, as regards India, is in many cases prehistoric. In Spain also the Moors, whose occupation of the peninsula terminated in the thirteenth century, have left reservoir dams of great magnitude, situated mostly in the south-eastern provinces of Murcia and Alicante, and many of which are still serviceable.
In India and Ceylon the greater number of the ancient dams or bunds are now in ruins, and this can occasion but little surprise, considering the meteorological condition of these countries. In Ceylon, for instance, the whole rainfall of the year occurs within a period of six to eight weeks, and often amounts to as much as 12 in. in the twenty-four hours, and has been known, comparatively recently, to reach nearly 19 in., the latter an amount only 2 in. or 3 in. less than the average rainfall of Lincolnshire for the whole year. In London it is only 25 in. and in the wettest district in Great Britain, viz., Cumberland, averages not more than 70 in. per annum.
The rainfall in Bombay is from 80 in. to 100 in. per annum, and throughout India may be taken as from 50 in. to 130 in., varying, as is the general rule, in direct ratio with the altitude, and limited to a few weeks in the year. Notwithstanding this, there still exist in the Madras Presidency a not inconsiderable number of ancient bunds which serve their intended purpose at the present day as well as ever. Slight mistakes did occasionally occur, as they ever will till no more dams are wanted, as is proved by the remains of some works in Ceylon, where the failure was evidently due to error, possibly due to the instruments being out of adjustment, as their base is at a higher level than the bed of the stream at the point where water from the latter was to be diverted to afford the supply.
Among the most remarkable of these ancient works is the Horra-Bera tank, the bund of which is between three and four miles in length and from 50 to 70 ft. in height, and although now in ruins would formerly impound a reservoir lake of from eight to ten miles long and three to four miles broad. There is also the Kala-Weva tank, with a bund of twelve miles in length, which would, if perfect, create a lake of forty miles in circumference. Both of these ruined works are situated in Ceylon. The third embankment of a similar character is that of the Cummum tank, situated in the Madras Presidency, and which, though ranking among the earliest works of Hindoo history, is still in such a condition as to fulfill its original intention. The area of the reservoir is about fifteen square miles, the dam about 102 ft. high, with a breadth at the crest of 76 ft., and of the section shown in the diagram.
The by-wash is cut in the solid rock altogether clear of the dam; the outlet culverts, however, are carried under the bank. We will now consider generally the methods employed in determining the site, dimensions, and methods of construction of reservoir dams adapted to the varying circumstances and requirements of modern times, with a few references to some of the more important works constructed or in progress, which it will be endeavored to make as concise and burdened with as few enumerations of dimensions as possible.
The amount of the supply of water required, and the purposes to which it is to be applied, whether for household, manufacturing, or irrigation uses, are among the first considerations affecting the choice of the site of the reservoir, and is governed by the amount of rainfall available, after deducting for evaporation and absorption, and the nature of the surface soil and vegetation. The next important point is to determine the position of the dam, having regard to the suitability of the ground for affording a good foundation and the impoundment of the requisite body of water with the least outlay on embankment works.
It has been suggested that the floods of the valley of the Thames might be controlled by a system of storage reservoirs, and notice was especially drawn to this in consequence of the heavy floods of the winter of 1875. From evidence given before the Royal Commission on Water Supply, previous to that date it was stated that a rainfall of 1 in. over the Thames basin above Kingston would give, omitting evaporation and absorption, a volume of 53,375,000,000 gallons. To prevent floods, a rainfall of at least 3 in. would have to be provided against, which would mean the construction of reservoirs of a storage capacity of say 160,000,000,000 gallons. Mr. Bailey Denton, in his evidence before that commission, estimated that reservoirs to store less than one tenth that quantity would cost L1,360,000, and therefore a 3 in. storage as above would require an outlay of, say, L15,000,000 sterling; and it will be seen that 3 in. is by no means too great a rainfall to allow for, as in July of 1875, according to Mr. Symons, at Cirencester, 3.11 in. fell within twenty-four hours. Supposing serious attention were to be given to such a scheme, there would, without doubt, be very great difficulty in finding suitable situations, from an engineering and land owner's point of view, for the requisite dams and reservoir areas.
In Great Britain and many European countries rain gauges have been established at a greater or less number of stations for many years past, and data thereby afforded for estimating approximately the rainfall of any given district or catchment basin. The term "watershed" is one which it appears to me is frequently misapplied; as I understand it, watershed is equivalent to what in America is termed the "divide," and means the boundary of the catchment area or basin of any given stream, although I believe it is frequently made use of as meaning the catchment area itself. When saying that the rain gauges already established in most of the older civilized countries afford data for an approximate estimate only, it is meant that an increase in the number of points at which observations are made is necessary, previous to the design of a reservoir dam on the catchment area above, the waters of which are proposed to be impounded, and should be continuous for a series of five or six years, and these must be compared with the observations made with the old established rain gauges of the adjacent district, say for a period of twenty years previously, and modified accordingly. This is absolutely necessary before an accurate estimate of the average and maximum and minimum rainfall can be arrived at, as the rainfall of each square mile of gathering ground may vary the amount being affected by the altitude and the aspect as regards the rainy quarter.
But this information will be of but little service to the engineer without an investigation of the loss due to evaporation and absorption, varying with the season of the year and the more or less degree of saturation of the soil; the amount of absorption depending upon the character of the ground, dip of strata, etc., the hydrographic area being, as a rule, by no means equal to the topographic area of a given basin. From this cursory view of the preliminary investigations necessary can be realized what difficulties must attend the design of dams for reservoirs in newly settled or uncivilized countries, where there are no data of this nature to go on, and where if maps exist they are probably of the roughest description and uncontoured; so that before any project can be even discussed seriously special surveys have to be made, the results of which may only go to prove the unsuitability of the site under consideration as regards area, etc. The loss due to evaporation, according to Mr. Hawksley, in this country amounts to a mean of about 15 in.; this and the absorption must vary with the geological conditions, and therefore to arrive at a satisfactory conclusion regarding the amount of rainfall actually available for storage, careful gaugings have to be made of the stream affected, and these should extend over a lengthened period, and be compounded with the rainfall. A certain loss of water, in times of excessive floods, must, in designing a dam, be ever expected, and under favorable conditions may be estimated at 10 per cent. of the total amount impounded.
As regards the choice of position for the dam of a reservoir, supposing that it is intended to impound the water by throwing an obstruction across a valley, it may be premised that to impound the largest quantity of water with the minimum outlay, the most favorable conditions are present where a more or less broad valley flanked by steep hills suddenly narrows at its lower end, forming a gorge which can be obstructed by a comparatively short dam. The accompanying condition is that the nature of the soil, i.e., the character, strata, and lie of the rock, clay, etc., as the case may be, is favorable to assuring a good foundation. In Great Britain, as a rule, dams for reservoirs have been constructed of earthwork with a puddle core, deemed by the majority of English engineers as more suitable for this purpose than masonry.
Earthwork, in some instances combined with masonry, was also a form usual in the ancient works of the East, already referred to; but it would appear from the experience of recent years that masonry dams are likely to become as common as those of earthwork, especially in districts favorable to the construction of the former, where the natural ground is of a rocky character, and good stone easily obtained.
As to the stability of structures of masonry for this purpose, as compared with earthwork, experience would seem to leave the question an open one. Either method is liable to failure, and there certainly are as many cases on record of the destruction of masonry dams as there are of those constructed of earthwork, as instanced in Algeria within the past few years. As regards masonry dams, the question of success does not seem so much to depend upon their design, as far as the mere determination of the suitable profile or cross section is concerned, as that has been very exhaustively investigated, and fairly agreed upon, from a mathematical point of view, but to be principally due to the correctness of the estimate of the floods to be dealt with, and a sufficient provision of by-wash allowed for the most extreme cases; and, lastly, perhaps the most important of all, the securing a thoroughly good foundation, and a careful execution of the work throughout.
These remarks equally apply to earthwork dams, as regards sufficient provision of by-wash, careful execution of work, and security of foundation, but their area of cross section, supposing them to be water-tight, on account of the flatness of their slopes and consequent breadth of base, is, of course, far in excess of that merely required for stability; but in these latter, the method adopted for the water supply discharge is of the very greatest importance, and will be again referred to.
Before commencing the excavation for the foundations of a dam, it is most essential that the character of the soil or rock should be examined carefully, by sinking a succession of small shafts, not mere borings, along the site, so that the depth to which the trench will have to be carried, and the amount of ground water likely to be encountered, can be reliably ascertained, as this portion of the work cannot be otherwise estimated, and as it may bear a very large proportion of the total expense of construction, and in certain cases may demonstrate that the site is altogether unsuitable for the proposed purpose.
The depth to which puddle trenches have been carried, for the purpose of penetrating water-bearing strata, and reaching impenetrable ground, in some cases, has been as much as 160 ft. below the natural surface of the ground, and the expense of timbering, pumping, and excavation in such an instance can be easily imagined. This may be realized by referring to Fig. 4, giving a cross-section of the Yarrow dam, in which the bottom of the trench is there only 85 ft. below the ground surface. In the Dale Dyke dam, Fig. 2, the bottom of the trench was about 50 ft. below the ground surface.
There is one other point which should be mentioned in connection with the form of the base of the puddle trench—that instead of cutting the bottom of the trench at the sides of the valley in steps, it should be merely sloped, so that the puddle, in setting, tends to slide down each inclined plane toward the bottom of valley, thereby becoming further compressed; whereas, should the natural ground be cut in steps, the puddle in setting tends to bulge at the side of each riser, as it may be termed, and so cause fissures. It will be noticed that the slopes of these earthwork dams vary from 7 to 1 to 2 to 1.
The depths to which some puddle trenches are carried has been objected to by some engineers, and among them Sir Robert Rawlinson, as excessive and unnecessary, and, in the opinion of the latter, the same end might be obtained by going down to a depth say of 30 ft. only, and putting in a thick bed of concrete, and also carrying up the concrete at the back of the puddle trench, with a well for collecting water, and a pipe leading the same off through the back of the dam to the down stream side. An arrangement of this kind is shown in the Yarrow dam, Fig. 4.
The thickness of the puddle wall varies considerably in the different examples given in the diagrams before you, a fair average being the Row bank of the Paisley Water Works, Fig. 6; and although in instances of dams made early in the century, such as the Glencorse dam—Fig. 5—of the Edinburgh Water Works, the puddle was of very considerable thickness, and it would appear rightly so. This practice does not seem to have been followed in many cases, as, for instance, again referring to the Dale Dyke dam, Fig. 2, where the thickness of the top was only 4 ft., with a batter of 1 in 16 downward, giving a thickness of 16 ft. at the base. For a dam 95 ft. in height this is very light, compared with that of the Vehui dam at Bombay, of which the engineer was Mr. Conybeare—Fig. 7—where the puddle wall is 10 ft. wide at the top, with a batter downward of 1 in 8, the Bann reservoir—Fig. 8—of Mr. Bateman's design, where the puddle is 8 ft. broad at the top, and other instances. The same dimension was adopted for the puddle wall of the Harelaw reservoir, at Paisley, by Mr. Alexander Leslie, an engineer of considerable experience in dam construction.
There appears to be a question as to what the composition of puddle should be, some advocating a considerable admixture of gravel with clay. There is no doubt that clay intended for puddle should be exposed to the weather for as long previous to use as possible, and subject to the action of the air at any rate, of sunshine if there be any, or of frost. When deposited in the trench, it should be spread in layers of not more than 6 in. in thickness, cut transversely in both directions, thoroughly watered, and worked by stamping.
The position of the puddle wall is, as a rule, in the center of the bank and vertical; but laying a thickness of puddle upon the inner or up stream slope, say 3 ft. thick, protected by a layer of gravel and pitching, has been advocated as preventing any portion of the dam from becoming saturated. There are, however, evident objections to this method, as the puddle being comparatively unprotected would be more liable to damage by vermin, such as water rats, etc.; and in case of the earthwork dam at the back settling, as would certainly be the case, unless its construction extended over a very lengthened period, the puddle would be almost certain to become fissured and leaky; in addition, the comparative amounts of puddle used in this manner, as compared with the vertical wall, would be so much increased. With the puddle wall in the position usually adopted, unequal settlement of the bank on either side is less liable to affect the puddle, being vertical.
It would be interesting to refer to the embankment of the Bann, or Lough Island Reavy reservoir, Fig. 8, designed by Mr. Bateman, now nearly fifty years ago, where a layer of peat was adopted both on the slope, 15 in. thick, and in front or on the up stream side of the puddle wall, 3 ft. thick. The object was, that should the puddle become fissured and leaky, the draught so created would carry with it particles of peat, which would choke up the cracks and so reduce the leakage that the alluvial matter would gradually settle over it and close it up. On the same diagram will be noticed curved lines, which are intended to delineate the way in which the earthwork of the embankment was made up. The layers were 3 ft. in thickness, laid in the curved layers as indicated.
It is a moot question whether, in making an earthwork embankment, dependence, as far as stanchness is concerned, should be placed upon the puddle wall alone or upon the embankments on either side, and especially upon the up-stream side in addition. Supposing the former idea prevails, then it can be of little moment as to how or of what material the bank on either side is made up—whether of earth or stone—placed in thin layers or tipped in banks of 3 ft. or 4 ft. high; but the opinion of the majority of engineers seems to be in favor of making the banks act not merely as buttresses to the puddle wall, and throwing the whole onus, as it may be termed, of stanchness upon that, but also sharing the responsibility and lessening the chances of rupture thereby. But to insure this, the material must be of the very best description for the purpose. Stones, if allowed at all—and in the author's opinion they should not be—should be small, few, and far between. Let those that are sifted out be thrown into the tail of the down stream slope. They will do no harm there, but the layers of earth must not approach 3 ft. in thickness nor 1 ft.—the maximum should be six in., and this applies also to the puddle. Let the soil be brought on by say one-horse carts, spread in six inch layers, and well watered. The traffic of the carts will consolidate it, and in places where carts cannot traverse it should be punned. In the Parvy reservoir dam a roller was employed for this purpose. It comprised a small lorry body holding about a yard and a half of stone, with two axles, on each of which was keyed a row of five or six wheels.
At the Oued Meurad dam, in Algeria, 95 ft. high, constructed about 23 years ago, the earthwork layers were deposited normal to the outer slope, and as the bank was carried up the water was admitted and allowed to rise to near the temporary crest, and as soon as the bank had settled, the earthwork continued another grade, and the same process repeated.
It was the practice until comparatively recently to make the discharge outlet by laying pipes in a trench under the dam, generally at the lowest point in the valley, or constructing a culvert in the same position and carrying the pipes through this, and in the earlier works the valves or sluices regulating the outflow were placed at the tail of the down stream bank, the pipes under the bank being consequently at all times subject to the pressure of the full head of the water in the reservoir. An instance of the first mentioned method is afforded by the Dale Dyke reservoir, Fig. 2, where two lines of pipes of 18 in. diameter were laid in a trench excavated in the rock and resting upon a bed of puddle 12 in. in thickness, and surrounded by puddle; the pipes were of cast iron, of the spigot and faucet type, probably yarned and leaded at the joints as usual, and the sluice valves were situated at the outer end of the pipes. As the failure of this embankment was, as we all know, productive of such terrible consequences, it may be of interest to enter a little more fully into the details of its construction. It was situated at Bradfield, six or seven miles from Sheffield, and at several hundred feet higher level. Its construction was commenced in 1858, the puddle trench was probably taken down to a depth of 40 ft. to 50 ft., a considerable amount of water being encountered. This trench was 15 ft. to 20 ft. broad at the top, and of course had to be crossed by the before mentioned line of pipes; and although the trench was filled with puddle, and the gullet cut in the rock already mentioned for carrying the pipes under the site of the dam was "padded" with a layer of 12 in. of puddle, we can imagine that the effect of the weight of the puddle wall and bank upon this line of pipes would be very different at the point where they crossed the puddle trench to what it would be where they were laid in the rock gullet and partially protected from pressure by the sides of the latter. At the trench crossing there would be a bed of puddle 50 ft. in thickness beneath the pipe, in the gullet a bed of 1 ft. in thickness. So much as regards the laying of the pipes.
The embankment had scarcely been completed when, on March 11, 1864, a storm of rain came on and nearly filled it up to the by-wash, when the bank began slowly to subside. The engineer was on the crest at the very time, and remained until the water was running over his boots; he then rushed down the other slope and was snatched out of the way as the bank burst, and the whole body of water, about 250,000,000 gallons, rushed out through the trench, carrying with it in the course of about twenty minutes 92,000 cubic yards, or say one fourth of the total mass of earthwork, causing the death of 250 human beings, not to mention cattle, and destruction of factories, dwellings, and bridges, denuding the rock of its surface soil, and, as it were, obliterating all the landmarks in its course. The greatest depth of the bank from ground level to crest was 95 ft., the top width 12 ft., and the slopes, both on the up stream and down stream sides, 21/2 to 1, and the area of the reservoir 78 acres.
Mr.—now Sir Robert—Rawlinson, together with Mr. Beadmore, were called in to make a report, to lay before Parliament, upon this disaster; and having made a careful examination of the ruins, and taken evidence, they were of opinion that the mode of laying the pipes, and in such an unprotected way, was faulty, and that subsidence of the pipes probably occurred at the crossing of the puddle trench. A fissure in the puddle was created, affording a creep for the water, which, once set up, would rapidly increase the breach by scour; and this event was favored by the manner in which the bank had been constructed and the unsuitability of the material used, which, in the words of one engineer, had more the appearance of a quarry tip than of a bank intended to store water. This opinion of the cause of failure was, however, not adopted universally by engineers, the line of pipes when examined being found to be, although disjointed, fairly in line; and there having occurred a land slip in the immediate neighborhood, it was suggested that the rupture might be caused by a slip also having taken place here, especially as the substratum was of flagstone rock tilted at a considerable angle. The formation was millstone grit. This catastrophe induced an examination to be made of other storage reservoir dams in the same district, and a report on the subject was presented to Parliament by Sir Robert Rawlinson.
The dam of Stubden reservoir, of the Bradford water supply, also on the millstone grit, was constructed about 1859, and caused considerable anxiety for a length of time, as leakage occurred in the culvert carrying the pipes, under the embankment at a point a short distance on the down stream side of the puddle trench. This was repaired to some extent by lining with cast iron plates; and an entirely independent outlet was made by driving a curved tunnel into the hill side clear of the ends of the dam and lining it with cast iron plates. In this tunnel was then laid the main of 2 ft. diameter, and as the original culvert again became leaky, the water had to be lowered, the old masonry pulled out, and the space filled in with puddle.
The Leeming compensation reservoir of the same water supply, with a dam of 50 ft. in height, and culvert outlet, had to be treated somewhat in the same manner, as, although the reservoir had never been filled with water, in 1875, when it was examined previous to filling, it was found that the culvert was cracked in all directions; and it was deemed best to fill it up with Portland cement concrete, and drive a tunnel outlet through the hill side, as described in the case of the Stubden reservoir. The Leeshaw dam, which was being constructed at that time upon the same lines, viz., with culvert outlet under the dam, was, at the advice of Sir Robert Rawlinson, altered to a side tunnel outlet clear of the dam.
Some years previous to the failure of the Dale Dyke reservoir there occurred, in 1852, a failure of a similar character—though, as far as the author is aware, unattended by such disastrous results—at the Bilberry reservoir at Holmfirth, near Huddersfield, which had never been filled previous to the day of its failure, and arose from the dam having sunk, and being allowed to remain at a level actually below that of the by-wash; so that when the storm occurred, the dam was topped and destroyed. An after examination proved that the bank was badly constructed and the foundation imperfect.
Besides the above instances, there have been numerous failures within recent times of earthwork dams in Spain, the United States, Algeria, and elsewhere, such as that which occurred at Estrecho de Rientes, near Lorca, in Murcia, where a dam 150 ft. high, the construction of which for irrigation purposes was commenced in 1755 and completed in 1789, was filled for the first time in February, 1802, and two months later gave way, destroying part of the town of Lorca and devastating a large tract of the most fertile country, and causing the death of 600 people. The immediate cause of failure in this case the author has been unable to ascertain. In Algeria the Sig and Tlelat dams were destroyed in 1865; and in the United States of America, at Williamsburg, Hampshire Co., Massachusetts, in 1874, an earthwork dam gave way, by which 159 lives were lost and much damage done to property. In another case, viz., that of the Worcester dam, in the United States of America—impounding a volume of 663,330,000 gallons, and 41 ft. high, 50 ft. broad at the crest, and formed with a center wall of masonry, with earthwork on each side—which gave way in 1875, four years after its completion; here, as in almost all other instances of failure, the leakage commenced at a point where the pipes traverse the dam. In this case they were carried in a masonry culvert, and the leak started at about 20 ft. on the up stream side of the central wall. The opinion of Mr. McAlpine as to the cause of failure, which agrees with that of the most eminent of our own water engineers, was to the effect that "earthen dams rarely fail from any fault in the artificial earthwork, and seldom from any defect in the natural soil. The latter may leak, but not so as to endanger the dam. In nine tenths of the cases, the dam is breached along the line of the water outlet passages."
The method of forming the discharge outlet by the construction of a masonry culvert in the open has no doubt many advantages over that of tunnel driving through the hill side clear of the dam, permitting as it does of an easy inspection and control of the work as it proceeds; but a slight leakage in the instance of a side tunnel probably means nothing more than the waste of so much water, whereas in the case of the culvert traversing the site of the bank, the same amount or less imperils the stability of the bank, and in ninety-nine cases out of a hundred would, if not attended to, sooner or later be the cause of its destruction. I think the majority will therefore agree that the method of discharge outlets under the site of embankments should not be tolerated where it is possible to make an outlet in the flank of the hill, to one side, and altogether clear of the dam.
At Fig. 9 is a diagram of the Roundwood dam of the Vartry Water Works, supplying Dublin, which is a fair specimen of the class of earthwork dam with the outlet pipes carried in a culvert under the embankment, and which, perhaps, is one of the most favorable specimens of this method of construction, as the inlet valves are on the up stream of the dam, and consequently when necessary the water can be cut off from the length of pipes traversing the dam. A short description will be given. This dam is 66 ft. high at the deepest point and 28 ft. wide at the crest, having to carry a public road. The slope on the inner face is 3 to 1, and on the outer 21/2 to 1. The by-wash is 6 ft. below the crest, which is about the average difference. The storage capacity of the reservoir is 2,400,000,000 gallons, or sufficient for 200 days' supply to the city. The puddle wall is 6 ft. wide at the top and 18 ft. at ground level, the bottom of the puddle trench about 40 ft. below the surface of the ground. The culvert was formed by cutting a gullet 14 ft. wide with nearly vertical sides through the rock, and covering it with a semicircular arch 4 ft. in thickness. Through this tunnel are laid a 33 in. and 48 in. main; the former for the water supply, and the latter for scouring or for emptying the reservoir on an emergency. There is a plugging of brickwork in cement under the center of the dam in the line of the puddle wall, and then stop walls built at the end of the plugging, projecting 25 ft. beyond the sides of the culvert and 8 ft. above, the space between them being filled up with cement concrete tied into the rock, and on this the puddle wall rests. This bank, like almost all others pierced by outlet pipes or culverts, was not destined to be perfect. In 1867, four years after the completion, spurts of water showed themselves in the culvert in front of the puddle wall, which began to settle, and the water had to be drawn off to admit of repairs. Diagram No. 10 shows a structure of a different character to any of these already described. This character of work is adopted on the North Poudre Irrigation Canal, in N.E. Colorado. Timber is there plentiful, and a dam of this character can be rapidly constructed, although probably not very durable, owing to liability to decay of timber. That represented is about 25 ft. high.
The author has now concluded the consideration of earthwork dams, and proposes making a few remarks upon those of masonry or concrete, with reference to some of the most important, as shown on the diagrams. Their stability, unlike those of earthwork, may be considerably increased where the contour and nature of the ground is favorable by being curved in plan, convex toward the water, and with a suitable radius. They are especially suitable for blocking narrow rocky valleys, and as such situations must, from the character of the ground, be liable to sudden and high floods, great care is necessary to make sufficient provision for overflow.
When of masonry, the stones should be bonded, not merely as they would be in an ordinary vertical wall, where the direction of the stress is perpendicular, but each course should be knit in with that above and below it in a somewhat similar manner to what is termed "random" work. And lastly, if hydraulic mortar be used, a sufficient time should elapse after construction before being subjected to strain, or in other words, before water is allowed to rise in the reservoir. For this latter reason, and also the liability to damage by sudden floods during the progress of the works, dams of Portland cement concrete, on account of their quick consolidation, possess advantages over those of hydraulic masonry apart from the necessity in the latter instance of constant supervision to prevent "scamping" by leaving chinks and spaces vacant, especially where large masses of stone or Cyclopean rubble are used.
Again, should the dam be drowned by flood during its erection, no harm would accrue were it composed of Portland cement concrete, whereas should it be of hydraulic mortar masonry, the wall would probably be destroyed or, at all events, considerably injured by the mortar being washed out of the joints. Portland cement, however, is only suitable for situations where the foundation is absolutely firm, as, should there be the slightest settlement, fissures would certainly be produced.
As regards foundations, the dam of the Puentes reservoir in Spain is somewhat remarkable—see Fig. 12. Its height is 164 ft., and the profile or cross section is of precisely the same character as that of the Alicante dam, the latter being 135 ft. in height, 65 ft. wide at the crest, and 65 ft. at the base, and erected about 300 years ago. At the Puentes dam the flanks of the valley were reliable, but, as must be frequently the case in such situations, the bed of the valley was composed to a great depth of gravel, debris, and shaky strata. The difficulty was overcome by throwing an arch, or arches, across the valley, the abutments being formed by the solid rock on each side, and building the dam upon this arching and filling in below the latter down to a sufficient depth with walling.
Bearing in mind the sudden and great floods to which dams constructed in such situations must be subjected, and, if the valley be very narrow, the probability that sufficient space at the side for a by-wash will be difficult to obtain, it would seem reasonable that in the calculation for their section allowance should be made for the possible condition of the whole length of the dam being converted into a weir, over which the waters may flow without risk of injury to the dam, to a depth of, say, at least twice that ever probable.
The topping of dams by floods is not uncommon, and if the extra strain thus induced has not been allowed for, their destruction is nearly certain, as instanced in more than one case in Algeria, where, although the average rainfall is only 15 in. yearly, a depth of 61/4 in., or more than one-third of the annual total, has been known to fall in twenty-four hours.
The Habra dam—see Fig. No. 13—completed in 1871, was destroyed by a sudden flood of this kind in December, 1881. This reservoir, with a storage capacity of 6,600,000,000 gallons, was intended for the irrigation of a cultivated bordering on the Mediterranean and the storage of floods. The height of the dam was 116.7 ft. and was provided with a by-wash of 394 ft. in length, and outlets for irrigation formed by four cast iron pipes of 311/2 in. diameter through the dam. It was composed of rubble set in hydraulic mortar, the latter composed of two parts of sand to one of hydraulic lime.
For getting rid of the large deposits of sand to which all reservoirs in that country are liable, two scouring outlets were provided of the same description as those in the old Moorish dams. The profile was calculated from Delocre's formula, and was correct in this respect, supposing the by-wash to have been sufficient. But as it was otherwise, and the flood swept over the crest to the depth of about 3 ft., the enormous extra strain thus induced overthrew the dam and caused the destruction of several villages and the death of 209 persons. It must be mentioned that when the reservoir was filling, the water percolated through the masonry, giving the face wall the appearance of a huge filter, which at the time was attributed to the porous nature of the sandstone used in construction, but which more probably was due to the washing of the green mortar out of the joints.
At the Hamiz dam, also in Algeria, the water was admitted in 1884, but it showed immediately signs of weakness, so that the water had to be run out and an immense retaining wall erected to strengthen the main dam. Algeria seems to have been singularly unfortunate as regards the success of works of this description. Water was admitted to the Cheurfas reservoir in January, 1885, and it at once began to make its way through permeable ground at one end of the dam. The flushing sluice in the deepest part of the dam had become jammed, so that the pressure could not be relieved, and in February 30 ft. length of the dam was carried away, causing a flood in the river below. At some distance down stream was the Sig reservoir. The flood rushing down, topped this dam by 18 ft. and overthrew it also.
Allusion has been made to provision for scouring out sand and deposit, especially in the dams of Algeria and of Spain. The amount of sand, etc., brought down by the floods is something enormous, and the question of the best means of getting rid of it has occupied much attention. In the old Moorish reservoirs the flushing gallery, piercing the lower part of the dam, was closed by iron doors on the down stream face and blocked with timber at the upper end. When required to be flushed out, laborers passed through the gallery and broke down the timber barrier, the silt forming a wall sufficiently thick to resist the pressure of the water for the time being, and allow of the retreat of the Forlorn Hope—if the latter had luck—before giving way.
One method adopted in Algeria, which has the advantage of permitting the sediment to be utilized together with the irrigation, this sediment being very fertilizing, is to pump air down through hose extending to the bottom of the reservoir, the pumps being actuated by steam power or turbine, and the sediment thus stirred up and run off with the water through the irrigation pipes. As an example of one of the early types of masonry dams in France, reference may be made to Fig. 13, on which is shown an elevation and cross section of the Lampy dam, forming a large reservoir for feeding the Languedoc canal.
I will now refer to some of the most notable masonry dams in existence, commencing with France, where perhaps the finest is that known as the Furens, in connection with the St. Etienne Water Works, constructed between the years 1859-66, and designed by the engineers Graiff and Grandchamps. It is curved in plan, struck with a radius of 828 ft. from a center on the down stream side, and founded upon compact granite, the footings being carried down to a depth of 3 ft. 3 in. below the surface of the rock. It is of rubble masonry, in hydraulic mortar, carried up in courses of 5 ft. in depth.
The height is 170 ft. on the up stream side and 184 ft. high on the lower side, with a breadth of 9 ft. 8 in. at the crest and 110 ft. at the base, and the cross section is so designed that the pressure is nearly constant in all parts, and nowhere exceeds 93 lb. to the square inch—13,392 lb. to the square foot. The contents is equal to 52,000 cubic yards of masonry, and the cost of erection was L36,080. The capacity of the reservoir is equal to 352,000,000 gallons.
The reservoir discharges into two tunnels (see Fig. 11), driven one above the other through a hill into an adjacent valley. The lower tunnel contains three cast iron pipes, with a masonry stopping of 36 ft. long. Two of these pipes are 16 in. diameter, with regulating valves, and discharge into a well, from whence the water can be directed for the town supply or into the river. The third pipe, of 81/2 in. diameter, is always open, and serves to remove any deposit in the reservoir, and to furnish a constant supply for the use of manufacturers.
The author drew attention to the difference in the section of the Furens dam, Fig. 11, as compared with that of Alicante, and of Puentes, which is similar to the latter. These two last illustrate the ancient Moorish type, and the former that of the present day. The Gileppe dam at Verviers, in Belgium, Fig. 14, although quite recently erected, viz., between the years 1869 and 1875, differs very much from the Furens type, in so far as it is of very much larger sectional area in proportion to its height, but this is accounted for by the desire of the engineer, M. Bodson, to overcome the opposition to its construction, and meet the objections and combat the fears of those whose interests—and those serious ones, no doubt—would be affected in the event of its rupture, the body of water stored being 2,701,687,000 gallons, or about eight times as much as the capacity of the Furens reservoir.
In addition to this, there was another reason, which was quite sufficient in itself to account for the extra substantiality of the dam. This reservoir is for supplying water to the cloth factories of Verviers, on the Belgian-German frontier. It is curved in plan to a radius of 1,640 ft., with a length of 771 ft., and the additional strength of the structure due to so flat a curve is probably slight.
It is built of rubble masonry, with ashlar facework, laid in hydraulic mortar. The total amount of masonry is 325,000 cubic yards. There are two weirs, at a level of 6 ft. below the crest, each 82 ft. wide. The total height, including the foundations, which are carried down from 3 ft. to 5 ft. into the rock, is 154 ft., and the breadth of the crest, which carries a road, is 49 ft. 3 in., and at the base 216 ft. The outlet pipes are carried through tunnels, which are driven on the curve into the hill side a considerable distance clear of each end of the dam.
Another very important structure is the Villar dam, Fig. 15, in connection with the water supply of Madrid, and situated on the river Lozoya. The storage capacity of this reservoir is very considerable, viz., 4,400,000,000, or nearly thirteen times as great as that of Furens. The height of the dam is 162 ft., with a breadth of 14 ft. 9 in. at the crest. It is built on the curve to a radius of 440 ft., and the length of the dam measured along the crest is 546 ft., of which 197 ft. is by-wash, thus describing nearly one-fifth of a circle, and consequently well designed to resist pressure. The dam is built of rubble masonry in hydraulic mortar, and cost L80,556.
The Stony Creek lower reservoir dam of the Geelong water supply, Fig. 16, colony of Victoria, is interesting as being constructed of concrete, in the proportion of 1 to 81/2. Its erection occupied eighteen months, and cost about L18,000. It is curved in plan to a radius of 300 ft., and the greatest depth or head of water is 52 ft. 4 in. The width at the crest is only 2 ft. 8 in., although surmounted by a heavy coping of bluestone 3 ft. 3 in. broad and 1 ft. 9 in. deep. There being no facility for making a by-wash at the side, the center of the dam is dished to form a weir 30 ft. long. There are both outlet and scour pipes, and valves of 2 ft. diameter, and the capacity of the reservoir is 143,145,834 gallons.
The Paramatta dam, in New South Wales, built of masonry in hydraulic mortar, is another instance of a dam built on the curve, and which has resisted a flood of water 4 ft. in depth over the crest; and in the case of a dam of about 40 ft. high across the river Wyre, in connection with the Lancaster Water Works, made of cement concrete in proportion of 4 to 1, there has, according to Mr. Mansergh, frequently been a depth of 5 ft. of flow over it. This dam is built to a radius of 80 ft. only, and as it measures 100 ft. along the crest, must include about the fifth of a circle.
There now remain only two other examples of masonry dams, the first being that in connection with the Liverpool water supply, and known as the Vyrnwy dam, Fig. 17, this being thrown across a stream of that name in North Wales. It is now under construction, and when completed will impound an area of 1,115 acres.
The dam will be 1,255 ft. long, and formed of Cyclopean rubble set in cement mortar, and the interstices or spaces between the large masses of stone, which are rough hewn and not squared, are filled with cement concrete. The proportion of the cement mortar is 21/2 to 1. These masses of stone weigh from two to eight tons each, and it is expected that the wall will be of a most solid description, as great care is being taken to fill up all spaces. The face next to the water is cemented. The area of the cross section shown on the diagram, which is at one of the deepest points, is 8,972 square feet, and the height from foundation to flood level is 129 ft., the breadth at the base being 117 ft. 9 in.
The existing dam of the New York water supply, Fig. 18, known as the Croton reservoir, is shown on the diagram. Its capacity is 364,000,000 gallons and the area 279 acres. The height is 78 ft. and width at crest 8 ft. 6 in., and is built of masonry in hydraulic mortar. The face walls are of stone laid in courses of 14 in. to 26 in., and are vertical on the up stream side, and with a batter of 1 in 21/2 on the down. The hearting is of concrete for a depth of 45 ft. from the top, and the remaining depth is in Cyclopean rubble.
At Fig. 19 is shown the section of the Quaker Bridge dam, which when completed will be the largest structure of the kind in existence. It is situated on the Croton River, which is a tributary of the Hudson, about four miles below the present Croton dam. The length will be 1,300 ft. and the height 170 ft. above the river bed, or 277 ft. above the foundation. The water by-wash is 7 ft. below the crest, and the dam is 26 ft. broad at the crest and 216 ft. at the base. The capacity of the reservoir will be 32,000,000,000 gallons, or nearly a hundred times as great as that of Furens. The geological formation at the site is sienitic gneiss. The cost of the dam is estimated at L500,000.
The accompanying table gives the pressures to which various dams are subjected, and it may be noted with regard to the weight of water, generally assumed as 62.4 lb. per cubic foot, that it will, in some districts, in time of flood, carry so much matter in suspension as to be increased to as much as 75 lb. weight, or an addition of 20 per cent., which, it may be easily imagined, will affect the conditions of stability very seriously.
TABLE OF MAXIMUM PRESSURES.
Lb. per sq. in. Gileppe (Verviers). 88 Furens (St. Etienne). 93 Puentes. 112 De Ban. 113 St. Chamond. 114 Alicante. 154 Hamiz (Algeria)—failed. 157 Habra (Algeria)—failed. 185
A diagram comparing the section derived from Molesworth's formula and those of Furens, Gileppe, Vyrnwy, and Quaker Bridge, is given at Fig. 20, the limit of pressure assumed for the masonry being 93 lb. per square inch, which is that of the Furens, the Gileppe being 88.
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NEW DREDGING MACHINERY.
We illustrate the new dredger Ajax, recently built for Mr. Geo. F. Smith, of Stockton, Cal.
The dredger has now been working for two weeks at Wakefield, and, we are informed, is giving entire satisfaction; having been repeatedly timed to be discharging clay at the rate of 220 cubic yards per hour.
The Ajax is almost a duplicate of the last dredger designed by Mr. Ferris for levee building on Roberts Island, with such modifications and improvements as have suggested themselves in the two years it has been working.
The hull, oval in plan, is 36 ft. 10 in. by 60 ft. over all; it has four solid fore and aft bulkheads, and a well hole 5 x 12 ft. at one end for the bucket ladder.
The main engine is 10 x 24, operating, by bevel gearing and a 31/2 in. vertical shaft, a 4 sided upper tumbler with 21 in. sides. This engine works also a gypsy shaft for swinging, and the conveyer that carries the mud ashore. A steam hoist with 6 x 11 engines raises and lowers the bucket ladder. The buckets, at 4 foot centers, have a struck capacity of 5 cubic feet, and are speeded to deliver from 18 to 20 a minute, according to the character of the material being handled. They are of boiler iron, with a 5 in. steel nosing. The links are of wrought iron, with cast bushings. The lower tumbler is hexagonal, on a 4 in. shaft.
The conveyer, projecting 72 ft. from the center of the boat, consists of a 5 ply rubber belt 36 in. wide; running over iron drums at each end and intermediate iron friction rollers at 3 foot centers. Ratchet and pinion on each side of conveyer ladder give means for taking up the slack of the belt and adjusting the drums to maintain them parallel.
This conveyer is the important feature of the dredge. It is entirely satisfactory in its working and delivers its material, as nearly as may be, in a dry state upon the levee. It was feared the rubber belt would be shortlived, but a 4 ply belt ran continuously for over two years on the Roberts Island dredge before it needed replacing.
The boiler is of the marine type, 52 in. by 10 ft. 6 in., with 3 in. tubes and 14 in. flues; and burns about 1,400 lb. of steam coal in a day of 12 hours. There are three pumps aboard—a hand force pump for washing boiler, a plunger pump for boiler feed, and an Evans steam pump to throw a jet of water into the delivery hopper when digging in any very tenacious material. All three are connected with the boiler.
Water tanks below deck serve to trim the boat and furnish a supply for the boiler. The dredger cuts by swinging on a center spud 16 in. in diameter, and moves forward from 8 to 10 ft. at each fleet.
The Roberts Island dredger, of which the Ajax is an improved copy, handles steadily 700 yards per day of 12 hours, in the stiffest and most tenacious clay in which it has been worked; and ranges from that average to 1,500 yards per day in soft, peaty mud.
The Ajax was built by Farrington, Hyatt & Co., of the Stockton Iron Works.
This type of dredger can be built for about $12,500, and we are informed can be relied on for a monthly average of 26,000 yards in any material met with in the overflowed lands near Stockton, delivered 50 ft. ashore, at a height of 10 or 12 ft. above the ground line.—Min. and Sci. Press.
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THE FLEXIBLE GIRDER TRAMWAY.
This is an ingenious proposition for utilizing a modification of the wire tramway system for overcoming obstacles (while retaining the ordinary wire tramway or any light railway on other parts of the line), made by Mr. Charles Ball, of London.
The flexible girder tramway is an improved system of constructing a modification of the well known and extensively used rope or wire tramway, and it is claimed that it will revolutionize the transport of the products of industrial operations from the place of production to the works or manufactory, railway station, shipping ports, or place of consumption; and that in the result the introduction of the flexible girder tramway will in many cases enable profits to be earned in businesses which have hitherto been unremunerative. It is declared to be at once simple, cheap, durable, and efficient. The improvement consists in the employment, in addition to the usual tram wire (a hempen rope, a wire rope, or a metallic or other rod), along which the load is transported, of a second or suspension wire or rope to which the tram wire is connected by tension rods or their equivalent at intervals between the rigid supports or piers, the object being to diminish or distribute the sagging or deflection of the tram wire, and thus lessen the steepness of the gradients over which the load has to be transported. The combined tram wire, tension rods, suspension wire, and accessories are, for convenience, designated a "flexible girder."
Another improvement consists in using, when a double line is employed, stretchers or crossheads to keep the flexible girders nearly parallel to each other, so that when necessary the load to be transported may be suspended from or borne by both tram wires jointly or simultaneously, thus permitting a load of greater weight than that for which each single tram wire is intended to be carried over the system. One indisputable claim for confidence in the flexible girder principle is said to be that, although the peculiar combination of parts constitutes a striking and valuable novelty, it contains nothing that has not been proved by the experience of years—nay, generations—to be useful, economic, and reliable. The usual practice followed in erecting suspension bridges is applicable in mounting the line, and the carriers, supports, and carriages may be of any of the usual forms. For the rapid removal of limited loads wire tramways are in universal favor, and are recognized not only as very economic and quickly constructed, but also as being in many cases the only means of transport available except by the adoption of elaborate and costly engineering works.
It has, it seems, been suggested by some who have examined the construction of the flexible girder tramway for mineral and produce traffic that it would be an additional advantage if arrangements were made for the carriage of small loads—half a dozen or so—of passengers, the primary intention being to carry the workpeople backward and forward between comparatively inaccessible mines, works, or plantations and a neighboring village or town. Compared with every other system where the line over which the load travels is elevated, the flexible girder tramway is claimed to possess many advantages—the center of gravity is kept well down, the liability of the wheels leaving the line is reduced to the minimum, the gradients are the easiest that can be obtained, there is an entire absence of jolting and extremely little vibration, and the motion is altogether smooth and regular; yet it is very questionable whether, when human life is at stake, any but an ordinary ground line should be relied upon. A living freight is far more liable than a dead freight to move during the journey; and as the safety of all overhead lines depends upon what is scientifically designated "unstable equilibrium," the flexible girder tramway is not recommendable for passenger lines, although it can, of course, be fitted for passenger traffic, a suitable vehicle and ten or a dozen good stout workmen coming well within a two-ton load, which can be readily carried.
Rope traction or animal traction—practically speaking—is alone available for wire tramways (that is to say, if the trains are each to be propelled by its own locomotive—whether steam, springs, or electricity—the cost of construction and maintenance becomes so serious that overhead lines, however well designed, are no longer economic); and experience gained with rope traction in numerous collieries in the North of England and Lancashire districts—where it is highly appreciated—has shown that, all circumstances considered, the endless rope is preferable. The chief objection urged against wire tramways as hitherto constructed has been that the "sag" of the rope has sometimes caused annoyance to those using the property passed over, and has always added much to the cost of traction, owing to the increased power required for moving the load; this has also resulted in vastly increased wear and tear and the rapid deterioration and destruction of the wire rope. The flexible girder system so reduces the "sag" that the maximum economy and durability are obtained, and the gradients over which the load has to travel can be made as easy and regular as those upon an ordinary railway. This advantage will be the more readily appreciated when it is considered that with a given load on a gradient of 1 in 30 the resistance due to gravity alone is 200 per cent. greater than on a gradient of 1 in 150, and that the retardation and wear and tear due to friction, greater curves, and imperfections increase still more rapidly with increase of gradient, soon rendering the old sagging wire line practically worthless.
To construct an entire line of flexible girders would be not only unnecessary, but so costly as to neutralize any advantage which it may possess, yet for surmounting occasional obstacles the claim made for it—that it will sometimes permit of a line otherwise impracticable being cheaply made—seems justified. One can readily imagine a light narrow gauge line costing L1,000 per mile being laid, for example, between a mine and the shipping place, and that a swamp, river, or valley would cost more to bridge over than the whole line besides. If at this obstacle the trucks or carriages could be lifted bodily, passed along the flexible girder, and again placed on the line the other side of the obstacle, the advantage to be derived is obvious; and as the flexible girder is really little more than a suspension bridge minus the platform, and having but two suspension wires, the cost and the difficulties should both be very small.—Industrial Review.
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BOZERIAN'S REFRIGERANT PUNKAS.
Punkas (also called pankasor tankas) are apparatus that serve for fanning rooms throughout the entire extent of English India. These devices consist of a light wooden frame covered with canvas, from the bottom of which depends a fringe. These frames are suspended from the ceiling in such a way as to occupy nearly the entire width and length of the room. To the base of the frame is attached a cord which passes over a wheel, and which is pulled by a Hindoo domestic. After the frame has been lifted, a weight fastened to the lower part causes it to fall back again. The result of the continuous motion of this colossal fan is a coolness that is highly appreciated in a country where the temperature is at times incredibly high, and where, without the factitious breeze created by the punka, living would not be endurable. This breeze prevents perspiration, or evaporates the same as soon as it is formed. Sometimes it sinks to a light zephyr; then, if you are reading or writing, you may continue your work, but in a distracted way, with a moist brow, and with a feeling of annoyance that soon makes you leave book or pen.
Looking around you, you find the punka immovable. The bahi still holds the cord that pulls it, but it is because he has tied it to his hand. He has gently slid to the floor in a squatting posture. He is asleep and you are burning. A vigorous exclamation brings him to his feet all standing, and he begins to pull the punka with all his might, and you have a feeling of ease and coolness. It is like the passage from an attack of fever to a state of comfort in an intermittent disease. So the punka is seen everywhere—in the temple and court room and other public places, as well as in private dwellings. It is one of the first things to astonish the European upon his arrival in India, and it is not long before he has to bless the happy invention.
Although, in a country where the temperature generally reaches, and even often exceeds, 40 deg. C., it is absolutely necessary to obtain by every means possible a factitious coolness without which the Indies would not be habitable for Europeans; and although there is no hesitancy in putting up these punkas everywhere to be maneuvered by bahis, the elevation of the temperature is not such in France that we are obliged to have recourse to such processes. But, without being forced thereto by nature, it is none the less true that we are often the more incommoded by heat in that we are not accustomed to it, and that in southern France, at certain hours of the day, such heat becomes absolutely unbearable. We can, it is true, obtain a little air by moving a fan, but, aside from the fact that this exercise soon becomes tiresome, it prevents the use of the hand that is fanning.
The new apparatus which have just been devised by Mr. G. Bozerian permit of one's fanning himself all day long if he wants to, without any fatigue, and while he is eating, reading, writing, etc.
In one of these apparatus, designed to be used in the open air (Fig. 1), we find a table, a tent, and a fan combined; but as each part is independent, we can have the table and fan without tent, or the fan and pedals alone without table or tent. Under the tent there is arranged a frame which pivots freely in apertures formed in the uprights that support both the tent and table. This frame is connected, through two levers, with the pedals upon which one's feet rest. The motion of the pedals is an alternating one like those of sewing machines; but while in the case of the latter a pressure has to be exerted that soon becomes very tiresome, the motion in Mr. Bozerian's apparatus is so easy that it is only necessary to raise the toes of each foot in succession in order to produce a swing of the fan through the weight alone of the foot that is pressing. The frame, which when at rest hangs perpendicularly, describes about a quarter of a circle when the extremity of the foot is raised about an inch. In consequence of the absence of passive resistances, motion occurs without any stress, and almost mechanically, giving air not only to him who is actuating the fan, but also to his vis-a-vis.
Fig. 2 represents an apartment apparatus designed to be placed in front of a table or desk, in order that one can fan himself while eating or writing. Being mounted upon casters, it can be readily moved about from one place to another. At the extremity of a wooden support, whose height may be varied at will, there is arranged a flexible fan whose handle is fixed near a pulley. A small piece of lead forms the counterpoise of the fan, which is thus completely balanced. Over the pulley runs a cord, each end of which is attached to a pedal. It will be seen that the alternate motion of these pedals must cause a rotation of the pulley in one direction or the other, and that consequently the fan will rise or fall more or less rapidly, and give a quantity of air that varies according to the rapidity with which the toes are moved.—La Nature.
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[Footnote: Extract from a lecture recently delivered at Bombay.]
By J. WALLACE, C.E.
The function of a punka is to cause a current of air to pass the human body so that the animal heat may escape more rapidly. This has nothing to do with ventilation; for if the punka were used in a closed room, it would still produce a cooling effect on the skin.
Let us for a moment examine into what takes place in this operation, for a clear idea of the cause of our sensations of heat is absolutely necessary to enable us to go directly to the simplest and best form of remedy. The heat we feel, and which sometimes renders us uncomfortable, is produced within us by the slow combustion of the food we eat.
This heat continues to escape from the whole surface of the body during the whole lifetime, and if anything occurs to arrest it to any great extent, the result is fatal.
In cold weather, and especially when there is much wind, the animal heat escapes very rapidly from the body, and extra clothing is used, not for any heat it imparts, but simply because it interrupts the escape of the heat, and thus maintains the temperature of the skin—that part of us which is most sensible of change of temperature. It is a wonderful fact that the heat of the interior of the body varies very little in a healthy man between India and Greenland.
The skin may bear a good many degrees of change of temperature with impunity, but the blood will only suffer a very small variation from the normal temperature of 98-4/10 deg. Fahrenheit without serious consequences.
Well, to keep the skin at an agreeable temperature in India we generally wear a minimum of clothing, and when there is no breeze, we try to produce one with the punka.
The escape of animal heat from the body forms a subject which is much more complicated, and much more important, than the one we have met to consider, but it is impossible within the limits of our time to refer to it, except in the measure that is strictly necessary to elucidate the principles that should control the construction of the punka.
It has often been said that every engineer on his arrival in India sets about improving this useful apparatus; but if we may judge from the endless variety of forms which may be seen in shops and offices, in public and in private buildings, no general principle of construction has been recognized, and the punka, as we see it, seems to depend, for its form, more upon the taste of the workman who makes it than on anything else.
We shall begin by directing our attention to the suspended punka, which is usually hung from the ceiling, and put in movement by a cord. The object of this class of punka is to produce a downward current of air by swinging to and fro, and the best punka is the one which throws downward the greatest quantity of air with the smallest applied force.
The swinging punka is one of the simplest forms of mechanism; it can be fitted up with the most primitive materials, and however badly made, it will always have some effect. This fact has its good and its bad aspects; it brings a certain comfort within the reach of all, but it removes a great part of that necessity which, as we all know, is the mother of invention.
There are some very important natural laws which are illustrated in the punka. The first is that which governs the movement of the pendulum. The number of swings it makes per minute depends on the length of the suspending cords; a pendulum three feet long will swing 621/2 times per minute, and a pendulum six feet long will swing 441/4 times per minute. Whether the swings are long ones or short ones, the number per minute is still the same. You cannot, therefore, alter the natural rate of movement of a punka unless you pull it at both sides.
The next law is that which determines that the angles of incidence and of reflection are equal. This in simple language means that it is useless to expect a good downward current of air from a slow moving and heavy punka, with long suspending cords which keep it nearly always in a vertical position to its plane of movement. Striking the air squarely as it does in its forward and backward movement, it throws almost as much air upward as downward, and of course all the air that is propelled in any other than a downward direction represents just so much power wasted.
One more law, and then we may proceed to demonstration.
As the air weighs 0.072 lb. per cubic foot at 82 deg. Fahrenheit, and as a considerable quantity of air is put in motion, the power required to drive a punka depends upon the quantity of air it puts in motion in a given time.
The useful effect is a separate matter; it depends on the amount of air thrown in a downward direction.
To summarize; all punkas of the same size or surface, and going at the same speed, require the same amount of pulling. The best one is that which will throw down more air than any other of the same size.
To obtain the greatest result from the power expended in driving it, the punka should be placed as near as possible to the person to be cooled, as the loss of effect, due to distance, increases not in direct ratio, but in proportion to the square of the distance between punka and person. If at two feet of distance he receives one eighth of the total effect, he will at four feet of distance obtain only one thirty-second part.
In practice, the punka should just clear his head when standing, and the weighting of the curtain should be of some yielding material, so as not to damage any person who might stand in its course.
We shall now proceed to examine several forms of punka, all made to the same size, and, for purposes of comparison, we shall drive them all at the same speed. And in order that their effects may be visible to you, I have prepared an indicator which resembles more than anything else the keyboard of a piano. It consists of a series of balanced levers with blades or keys attached, forming a keyboard four feet long. The levers, each three feet long, are delicately hung on fine brass centers, and each lever is counterbalanced by a weight hung in a vessel of water, which acts as a hydraulic brake, and checks any spasmodic movement in the apparatus.
On the end of each blade is fixed a disk of white Bristol board four inches in diameter, forming a row which faces the audience.
This apparatus is so sensitive that a slight change in the humidity of the atmosphere is sufficient to throw it out of balance.
The power required to drive a punka is nearly all due to the resistance of the air; that part due to the force of gravity, and the friction of the suspending joints, is scarcely worth counting. We may readily observe the effect of the resistance of the air by swinging two pendulums of equal length and having each a large cardboard disk attached. One of the disks shall present its edge to the line of movement, and the other its face.
Exp. 1.—They are now swinging, and being both of the same gravity length, they should swing together and for an equal length of time. This they would do in a vacuum, but you have already observed that one of them is lagging, and will evidently soon come to a standstill. It is the one facing the air.
If punkas were pulled from both sides, they might be made very much lighter than they are at present, but for the sake of simplicity a single pull is preferred. They must, therefore, be made of such a weight that they will swing nearly as far on the opposite side as they are pulled on the near side; any greater weight is useless and only serves to wear out the suspending cords, which, by the way, are nearly always too numerous and too thick for their purpose.