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of years. Even when a compensation would tend to retard evaporation, there is aimed at by large storage facilities, it is nothing in a season of rain influencing is found to be impracticable to retain all to any extent such evaporation. Exposthe average counted on; as in the event ure to winds, hygrometric conditions of of large floods occurring when the reser- the atmosphere, the vicinity of forests voirs are full, the water must be lost, or open plains, the presence or absence and the available supply, obtainable from of elevated surroundings—all may effect reservoirs, is found to fall short of that it more or less, but independently of deduced from computations based upon rainfall, and no connection necessarily average rainfalls. * A single statement exists between the two. The proper defrom the official report of the engineer termination of the amount of evaporaof the Liverpool works will illustrate tion from water surface is so replete this. The average yearly fall for thirty with difficulties, and some of the results years' observation showed 48 inches, and attained are thus far so anomalous, that allowing 50 per cent. as collectable, 24 the effort is no longer made to attain inches was assumed as available; with absolute accuracy in this respect; and it 38 million gallons per acre of the shed as is assumed, in accordance with some obstorage room, 18 million gallons daily was servations, that the yearly rainfall on a estimated as the capacity of the works. given water surface, will supplement (This was in addition to compensation the loss by evaporation; hence, in conwater to certain mills.) Three dry years sidering the area of a gathering ground, in succession reduced 'the delivery to six if the water surfaces are subtracted in million gallons daily, or 9 inches in- estimating the area of the shed, and disstead of 24 inches over the entire shed. regarding the element of evaporation This case is but one of many bearing the from these surfaces, it will be as near same evidence.
the truth as can be reached by any Before a true average of rainfall can
known method of observing evaporation. be determined upon in a given locality, And so of the absorption of the soil, of course the proper proportion of dry the demands of vegetation, and evaporyears should be embraced in the series, ation from ground surfaces, which are otherwise the estimate will be in excess usually embraced under one head, can, --aside from the fact, that unless under neither collectively nor singly, bear any exceptional circumstances, the minimum relation whatever to the amount of rainyears of rainfall, and not the average fall, and can only be approximately debecome the true criterion for certainty of termined by long.continued observation supply. And in addition to this, the year and measurement on areas of similar of least rain is not necessarily the year characteristics, and the result deducted in which the season of least flow from from the yearly rain, and independently the streams occur, and this latter is the of the extent of the latter. Neither of measure of reliable supply. Nor is the these items, any more than that of the year of least rainfail necessarily the year evaporation from water surface, can be of drought, although frequently so; but properly estimated as a percentage of the year of drought which tests the ca
the rainfall. pacity of a city supply is the year In the article referred to, reference is wherein the rainfall is so unequally dis- constantly made to the average annual tributed, that for several months in suc- rainfall as a basis for computation, and cession, it may be, the evaporation and the low years precipitation is derived absorption and needs of vegetable life, from it by the simple process of multitake
up all the rain which falls, and thus plying by 0.8; but unless the observalittle or nothing is contributed to the tions had extended through a greater city supply.
number of years than is usual with such On the subject of evaporation from records in this country, and had certain. reservoir surfaces, it is manifestly an ly embraced a year or more of the lowest error to assume it as a percentage of the rainfall, or the season of lowest flow, it rainfall; for beyond the fact that during would scarcely prove a safe guide. rains but little evaporation may be tak. The flow from the Merimack, the ing place, and at such times there may Passaic, the Delaware, the Schuylkill be a lowering of the temperature, which and the Croton, as well as others of our
rivers, may be computed on an average observation are taken to represent the at one million of gallons daily through- average. There was collected that year in out the year for every square mile of the Croton, 45%, or 21 inches, as available drainage area. But of what use is the rain, and the daily average flow for the knowledge of this fact, when it is equally year was a million gallons per square mile. certain, that save by an amount of stor- Of the rain for that year, 15.8% was stored age room which is wholly impracticable, or used in the City of New York, 30.7% the freshet flows, which supplementing was lost for lack of storage capacity, and the low summer flow brings up the total 53% was'evaporated from the surface of to the average named, cannot be re- the shed (omitting the water surface) or tained.
absorbed, or used for vegetable life and It is estimated that to avail of the did not reach the springs. average yearly flow of the Croton basin The rainfall the last year (1880) on of 338 square miles, would require a the Croton shed was 38.5 inches; and the storage of 41000 million gallons. This following represents the ratio of the would accomplish it upon paper, but for several items of water supply for the obvious reasons, the reservoirs could not several years named, on this basin and be counted on at all times to fulfill their Ridgwood, L. I. In Blodgett's Climatpurpose, were it practicable to build them; ology it appears that the Croton basin is and hence a less discharge from the basin credited with an average annual rain of becomes the true measure of its capacity. 44 inches, and the Long Island basin
From 1864 to the present year, the with 42 inches. In the last 50 years the average yearly rainfall on the Croton observations of the rain gauges have basin has been 46.1 inches. In the year shown an average of 42.69 inches yearly 1877 the rainfall was so nearly the same rain, for the water-shed which supplies (46.03) that the results of that year'si the City of Brooklyn.
It appears by the above, as might In a dry season the Croton basin has be anticipated, that the loss by evapora- yielded a minimum of 100,000 gallons tion and absorption, instead of being per square mile in twenty-four hours, proportional in any degree directly to and during the same season the Brooklyn the amount of rainfall, is really inversely basin has yielded a minimum of 300,000 proportional thereto; and indeed an ac- gallons per square mile for the same quaintance with the detailed operations time, the rainfall for the year being in of almost any well-conducted city water favor of the Croton basin. This dissupply, will show how little the average crepancy in the amounts flowing from a rainfall can be trusted as a safe guide, given area, appearing within so short a and the necessity which exists of refer. distance as the width of the East River, ring, as of controlling importance, to will indicate to what extent local circumthose local characteristics, topographical stances may influence the water supply and geological, as well as meteorological, of a city, independently of the extent of which will be found to modify most rainfall. Much has been written abroad materially the results arrived at by any upon this subject of water supply, but general formula.
scarcely applicable to this latitude.
ON THE VARIOUS MODES OF TRANSMISSION OF POWER
TO A DISTANCE.
By M. A. ACHARD.
A Paper read before the Institution of Mechanical Engineers.
The author in this paper furnished a'the two pulleys; of this the part which summary of the practical results obtained is passing towards the driving pulley is in the transmission of power to a dis- I called the driving span, and the part tance. While the interest attaching to which is passing from the driving pulley this subject is unquestionable, the author is called the trailing span. Let T be is, nevertheless, very doubtful whether a the tension of the driving span, and t successful result can be attained in one that of the trailing span. Neglecting particular application-namely, the es- friction, &c., we should have Q=P; and tablishment of large undertakings for the values of the tensions in the two distributing hydraulic power to a num- spans are given by the equations ber of factories, either existing or con
T templated, similar to the undertakings at
T-t=P and =k; Schaffhausen, Fribourg, and Bellegarde. At the first of these places, in spite of denoting by k the smallest practicable favorable circumstances, rapid extension value of era for the two pulleys, where of working, and good management, the e is the base of Napierian logarithms, f profit has been very small on the capital the coefficient of friction between the outlay. The manufactories at the two pulley and the rope, and a the ratio beother places, being much less favorably tween the arc encircled by the rope and situated, bave failed after a short and the radius of the pulley. Accordingly profitless existence. Their failure has the values of T and t are given by the shown very clearly that their founders following equations:labored under a strange delusion in sup
k P Р posing that cheap motive power was in
rope were wanting. The author accordingly considers there is not much to be gained will slip on the driving pulley. The from this method of transmitting power values of T and t, as above calculated, to a distance, and that it can only suc- when k has its exact value, are only just ceed financially under exceptionally fa- sufficient to prevent slipping, which vorable conditions. Having promised so would occur on any accidental diminumuch, he next proceeded to examine the tion of friction. For safety, therefore, various methods used, or proposed, for it is necessary to assign to ka somewhat transmitting power to a distance. He lower value than its real one; which first considered transmission of power by practically amounts to increasing the wire ropes, which is merely an extension tensions T and t a little beyond what is of the simple case of transmission by or- requisite in theory. The tension com. dinary hemp ropes, and the same princi- mon to the whole rope when at rest is ples apply to both. Let A and B be the 'somewhere intermediate between the tenaxes of two parallel shafts carrying two sions T and t of its two spans while runpulleys whose planes coincide. T'he driv- ning; and by adjusting the rope while ing power P acts on A, and the resistance at rest to this intermediate tension, its Q on B. For simplícity, let it be as- two spans assume of their own accord sumed that those two forces act tangen- the required tensions T and t as soon as tially at the circumference of the pulleys. it begins to run. The section w to be The motion is communicated from A to given to the rope, so that it may possess B by means of the rope passing round the requisite strength, is regulated by
the driving tension T, and must be such be taken as at least 3 P. It is evident, T
therefore, that rope transmission renders that the quotient=shall not exceed the the shaft friction much greater than does working strain which the material of the the effect of this friction is much reduced
transmission by toothed wheels. But rope is suited to bear in practice. It is by the large diameter of the pulleys in evident that, in transmitting a given comparison with that of their shafts, in amount of power, the driving tension, and consequently the section of the rope, the shaft bearings, has to be multiplied
consequence of which the pressure on may be diminished by increasing the by a number not exceeding at most 0.003, speed; for if N denotes the power trans. in order to obtain the resulting tension mitted, and v the speed of the rope, on the rope. The author dealt briefly then
with belting and went on to consider at KP k N Pv=N, and T=
length the nature of the strains to which k-12-10 ropes are subject. It is difficult to lay In practice the rope elongates under the down any general rule as to the duration continuous pull, and requires shortening of the ropes, for this depends upon the from time to time to keep the tension conditions under which they work. In up to the proper amount. The author practice, it must not be assumed that a next took into account the useless resist- rope in constant use will last more than ances neglected for the sake of simplicity. a year. In fact, Professor Amsler-LafThe useful resistance Q is now necessari- fon recently wrote to the author on the ly less than the driving power P, and the subject of the ropes at Schaffhausen:
lasts about one year, some a Q ratio represents the efficiency of the little more, some a little less. But it
must be understood that we do not wait transmission. The useless resistances are till our ropes break, but replace them as two in number. The first is the rigid - soon as we can no longer depend on their idity or stiffness, due to the imperfect strength. They might therefore last flexibility of the rope. This effect, how- rather longer, if we chose to run the ever, is insignificant in the case of rope risk of interruption in our work.” Their transmission, on account of the large short life is certainly a defect in this size of the pulleys employed. The other mode of transmitting power. Accorduseless resistance is the friction of the ing to M. Ziegler, who has considerable two shafts A and B in their bearings, experience on this subject, horizontal oswhich is measured by the resultant F of cillations are very injurious to the duraall the external forces acting on each tion of the ropes, and they appear to last shaft. It appears from the principles longer on pulleys with wide grooves enunciated above that the employment than with narrow grooves. The author of rope transmission renders this fric- next considered transmission by comtion considerable. In fact, under aver- pressed air. Hitherto the method of age conditions of adhesion, the value to transmission by compressed air has only be allowed for k=ea is not more than been used, so far as the author is aware, 2; and since in the limit
for boring the headings of mines, and T KP
the long tunnels through the Alps. In =k and T= k-1
these cases, as is well known, the work
to be done consists in a rapid boring of we have as the least possible values holes for the purpose of blasting the T=2 P, and t=P. These tensions are rock with powder or dynamite. As this parallel to each other, and as the useful kind of work requires a high pressure of resistance Q may also aet in the same di- air, and almost entirely precludes the rection, the total pressure F on the shaft employment of expansion, the utilization may=T+t+P=4 P, as a minimum, of the motive force is necessarily defectwhere the conditions are the most unfavor- ive; but in consequence of the peculiar able; while under the most favorable con- convenience which compressed air offers ditions the pressure on the bearings will for the work, and particularly the be given by F=T+t-P=2 P, as a mini- improved ventilation · which it affords, mum. Hence the average pressure may the advantage of its employment is un
doubted, and leaves in the background must be very highly compressed, and the question of efficiency. M. Achard that in going from the reservoir into the dealt at great length with the somewhat cylinder it passes through a reducing complex mathematics of the subject, but valve, or expander, which keeps the he did not supply much if any new data. pressure of admission at a definite figure, Referring to the motors fed with the so that the locomotive can continue compressed air, the author held that this working so long as the supply of air consubject is still in its infancy from a prac- tained in the reservoir has not come tical point of view. In proportion as the down to this limiting pressure. The air air becomes hot by compression, so it does not pass the expander until after it cools by expansion, if the vessel contain- has gone through the boiler already ing it is impermeable to heat. Under mentioned. Therefore, if the temperathese conditions, it gives out, in expand. ture which it assumes in the boiler is 100 ing, a power appreciably less than if it deg. Cent.—212 deg. Fah.—and if the retained its original temperature, be- limiting pressure is five atmospheres, the sides which the fall of temperature may gas which enters the engine will be a impede the working of the machine, by mixture of air and water vapor at 100 freezing the vapor of water contained in deg. Cent.; and of its total pressure the the air. If it is desired to utilize to the vapor of water will contribute one atutmost the force stored up in the com- mosphere, and the air four atmospheres. pressed air, it is necessary to endeavor Thus this contrivance, by a small exto supply heat to the air during expan- penditure of fuel, enables the air to act sion, so as to keep its temperature con- expansively without injurious cooling, stant. It would be possible to attain and even reduces the consumption of this object by the same means which compressed air, to an extent which comprevent heating from compression, pensates for part of the loss of power namely, by the circulation and injection arising from the preliminary expansion of water. It would, perhaps, be neces- which the air experiences, before its adsary to employ a little larger quantity of mission into the engine. This scheme water for injection, as the water, instead was then mathematically investigated by of acting by virtue both of its heat of the author. Next M. Achard dealt with vaporization and of its specific heat, can the transmission of power by water in this case act only by virtue of the lat- pressure.
Of machines worked by water ter. These methods might be employed pressure, the author referred only to two, without difficulty for air machines of which appear to him in every respect the some size. It would be more difficult to most practical and advantageous. One apply them to small household machines, is the well-known piston machine of M. in which simplicity is an essential ele- Albert Schmid, engineer, Zurich. The ment; and we must rest satisfied with cylinder is oscillating, and the distribuimperfect methods, such as proximity to tion is effected, without an eccentric, by a stove, or the immersion of a cylinder the relative motion of two curved surin a tank of water. Consequently, loss faces fitted one against the other, and of power by cooling and by incomplete having the axis of oscillation for a comexpansion cannot be avoided. The only mon axis. The convex surface, which is way to diminish the relative amount of movable and forms part of the cylinder, this loss is to employ compressed air at serves as a port face, and has two ports a pressure not exceeding three or four in it communicating with the two ends atmospheres. The only real practical of the cylinder. The concave surface, advance made in this matter is M. Me- which is fixed and plays the part of a karski's compressed air engine for tram- slide valve, contains three openings, the ways. In this engine the air is made to two outer ones serving to admit the pass through a small boiler, containing pressure water, and the middle one to water at a temperature of about 120 discharge the water after it has exerted deg. Cent.—248 deg. Fah.—before enter- its pressure. The piston has no packing; ing the cylinder of the engine. It must its surface of contact has two circumbe observed that in order to reduce the ferential grooves, which produce a sort size of the reservoirs, which are carried of water packing acting by adhesion. on the locomotive, the air inside them A small air chamber is connected with