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 years' observation showed 48 inches, and allowing 50 per cent. as collectable, 24 inches was assumed as available; with 3 million gallons per acre of the shed as storage room, 18 million gallons daily was estimated as the capacity of the works. (This was in addition to compensation water to certain mills.) Three dry years in succession reduced the delivery to six million gallons daily, or 9 inches instead of 24 inches over the entire shed. This case is but one of many bearing the same evidence. Before a true average of rainfall can be determined upon in a given locality, of course the proper proportion of dry years should be embraced in the series, otherwise the estimate will be in excess -aside from the fact, that unless under exceptional circumstances, the minimum years of rainfall, and not the average become the true criterion for certainty of supply. And in addition to this, the year of least rain is not necessarily the year in which the season of least flow from the streams occur, and this latter is the measure of reliable supply. Nor is the year of least rainfall necessarily the year of drought, although frequently so; but the year of drought which tests the capacity of a city supply is the year wherein the rainfall is so unequally distributed, that for several months in succession, it may be, the evaporation and absorption and needs of vegetable life, take up all the rain which falls, and thus little or nothing is contributed to the city supply. On the subject of evaporation from reservoir surfaces, it is manifestly an error to assume it as a percentage of the rainfall; for beyond the fact that during rains but little evaporation may be tak ing place, and at such times there may be a lowering of the temperature, which with difficulties, and some of the results attained are thus far so anomalous, that the effort is no longer made to attain absolute accuracy in this respect; and it is assumed, in accordance with some ɔbservations, that the yearly rainfall on a given water surface, will supplement the loss by evaporation; hence, in considering the area of a gathering ground, if the water surfaces are subtracted in estimating the area of the shed, and disregarding the element of evaporation from these surfaces, it will be as near the truth as can be reached by any known method of observing evaporation. And so of the absorption of the soil, the demands of vegetation, and evaporation from ground surfaces, which are usually embraced under one head, can, neither collectively nor singly, bear any relation whatever to the amount of rainfall, and can only be approximately determined by long-continued observation and measurement on areas of similar characteristics, and the result deducted from the yearly rain, and independently of the extent of the latter. Neither of these items, any more than that of the evaporation from water surface, can be properly estimated as a percentage of the rainfall. In the article referred to, reference is constantly made to the average annual rainfall as a basis for computation, and the low years precipitation is derived from it by the simple process of multiplying by 0.8; but unless the observations had extended through a greater number of years than is usual with such records in this country, and had certainly embraced a year or more of the lowest rainfall, or the season of lowest flow, it would scarcely prove a safe guide. The flow from the Merimack, the Passaic, the Delaware, the Schuylkill 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 did not reach the springs. It is estimated that to avail of the 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's 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. T T―t=P and=k; 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- 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 contemplated, similar to the undertakings at 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 efa 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, have 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 supposing that cheap motive power was in itself sufficient to create industries in k P T= -1; t= Р k T t The localities where their essential elements If the ratio be greater than k, the rope were wanting. The author accordingly considers there is not much to be gained will slip on the driving pulley. 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 k a somewhat transmitting power to a distance. He lower value than its real one; which first considered transmission of power by wire ropes, which is merely an extension of the simple case of transmission by ordinary hemp ropes, and the same principles apply to both. Let A and B be the axes of two parallel shafts carrying two pulleys whose planes coincide. The driving power P acts on A, and the resistance Qon B. For simplicity, let it be assumed that those two forces act tangentially at the circumference of the pulleys. The motion is communicated from A to B by means of the rope passing round practically amounts to increasing the tensions T and t a little beyond what is requisite in theory. The tension common to the whole rope when at rest is somewhere intermediate between the tensions T and t of its two spans while running; and by adjusting the rope while at rest to this intermediate tension, its two spans assume of their own accord the required tensions T and t as soon as it begins to run. The section w to be given to the rope, so that it may possess the requisite strength, is regulated by In practice the rope elongates under the P be taken as at least 3 P. It is evident, therefore, that rope transmission renders the shaft friction much greater than does the effect of this friction is much reduced transmission by toothed wheels. But by the large diameter of the pulleys in comparison with that of their shafts, in the shaft bearings, has to be multiplied consequence of which the pressure on by a number not exceeding at most 0.003, in order to obtain the resulting tension on the rope. The author dealt briefly with belting and went on to consider at length the nature of the strains to which ropes are subject. It is difficult to lay down any general rule as to the duration of the ropes, for this depends upon the conditions under which they work. In practice, it must not be assumed that a rope in constant use will last more than a year. In fact, Professor Amsler-Laffon recently wrote to the author on the subject of the ropes at Schaffhausen:— A rope lasts about one year, some a little more, some a little less. But it 66 the I 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 kefa 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 the long tunnels through the Alps. In 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, Hence the average pressure may the advantage of its employment is un mum. T k and T kP k-l' 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 |