But it happens that water-power is exposed to frequent interruptions, so to speak, iu its efficiency ; and that in summer, when the largest quantity of water must be raised, the power is the least, whilst in winter the action of the wheels way be entirely suspended in conse quence of floods or of frost. In Englaud, therefore, where coal is cheap, and the use of steam-power is habitual, the latter is almost always resorted to, and it certainly presents many advantages, on the score of certainty of action, over all other sources of motive power. In France, Germany, Switzerland, and the United States, however, many water-wheels for tho supply of towns are to be found ; and even within a very few years London itself was dependent for a large portion of its water supply upon the water-wheels of old London Bridge. The towns of Toulouse, Geneva, Richmond, Va., and Philadelphia, may be mentioned as amongst the most important of these supplied with water by water-power; and in the works lately executed by Mr. liawkaley, at Weymouth, a very ingenious application of the turbine was made. The majority of the English towns are supplied by means of steam-engines; and tho machinery lately erected for the supply of London may be cited as being the most extensive and the most beauti ful of any hitherto erected. It is usually considered that there are advantages in the use of the Cornish pumping-engines when the power to be exercised exceeds 25 horses, and that for lower powers direct acting engines, or fly-wheel engines, are 'preferable; but the remark able results obtained by Messrs. Simpson and Boulton and Watt, show that the preference for large Cornish engines is at least open to question.
In estimating the power to be provided, and consequently iu de ciding the description of engine to be employed it is necessary to take into account the weight of the water to be raised at the maximum rate of consumption ; the total height to which it has to be raised ; and the various causes of retardation of flow in the pumping main, from the friction upon the sides of the pipes, and the bends or changes of horizontal or vertical direction. It will be necessary to revert to the general laws affecting the flow of water in pipes; but here it is mufti cieut to observe that an allowance equivalent to an increase of 12 feet per mile in length of main, to the total weight to be lifted, is sufficient to cover the effect of these retarding causes. As the various classes of engines do not work up to their full power, calculated theoretically, it is necessary to affect the result of the above method of ascertaining the pumping power by the coefficients of the respective engines; which are, for steam-power, on the average, and for the best water wheels, 015.
In small pumping engines it is found that the most useful action is obtained when the power i6 exercised equably and continuously ; and in order to secure this result, the main shaft is made to carry three cranks placed at angles of 120' from each other, upon which the pump rods are keyed. In the larger engines it seems that the most favourable conditions of motion in the pumps are, that they should begin by raising the load rapidly, in order to overcome its via inertia ; and that when the motion of the ascensions' column has been once esta blished, the effort of the motive power should gradually be diminished.
These objects are effected in the Cornish engines by introducing steam to the underside of the cylinders, through largo openings, at high pres sures; the steam is then cut off when the inertia of the water has been overcome, and it is allowed to expand during the remainder of the stroke, exercising in so expanding sufficient power to maintain the °seen. aional movement ; the return stroke is made by the dead weight of the plunger and of the rods. The initial pressure of these engines is usually from 2} to 3 atmospheres ; the expansion begins at from to of the stroke of the piston ; and at the end of the stroke the pressure of steam is not more than between and of an atmosphere; engines so worked being said to produce a duty equivalent to about 90,000,000 lbs. raised 1 foot high by the combustion of 1 ewe of coal. But it is to be observed, that in consequence of the interposition of the air-vessel upon the delivery-pipe of a town pumping-engine, the delivery is, iu fact, constant ; and it is probable that the extraordinary results ob tained of late by the application of the beam and fly-wheel engine, on a large scale, to pumping operations, may bo explained by the fact that as they supply water to the air-vessel in a more continuous manner than the Cornish engines do, they allow the air-vessel to produce its effect with greater regularity. In the large engines erected by .Messrs. Simpson for the Chelsea and Lambeth Water-works, the steam works at high pressure in the first of two cylinders, and at low pressure iu the second ; there is a beam and fly-wheel to each engine; and it is said that the duty they perform sonic-times reaches 120,000,000 lbs. raised one foot high by the combustion of 1 cwt. of coal. Fly-wheel engines, it may be added, possess this great advantage over Cornish engines for the purpose of town distributions, namely, that they are capable of being worked at more variable rates of delivery than are the latter ; and in a town delivery the rate of consumption is subject to such singular irregularities, that this susceptibility of variation in the power may often be a matter of importance. But again, it must be stated that the best modern Cornish engines have been so carefully and skilfully constructed, that they have even allowed the regulating service reservoirs to be dispensed with ; and both the Kent and the East London Water-works pump directly into their supply mains by night and by day, trusting entirely to the skill of the engine-drivers to adapt the motive power to the variable conditions of tho consumption of their districts. These variable conditions are of much greater im portance than is generally considered to be the case ; for it appears from careful examination that about three-fourths of the total daily consumption of a town takes place between the hours of eight in the morning and eight in the evening ; and experienced engineers make their pipes and engines large and powerful enough to discharge one fourth of the total daily consumption in one hour. Evidently, then, it must be desirable to employ the motive power which is able to adapt itself to ouch variable demands with the greatest elasticity.