HYDRODYNAMICS. 



pf the head of the ram at F is made flat. an<l lias 

 several valves which allow Uie water to pass freely 

 from the pipe AH, but prevent its return. On each 

 tide of the head of the rain, at the part opposite 

 to these valves is a hollow enlargement, shown 

 by the dotted lines K, forming a circular bason, 

 through Uie centre of which the pipe ABR passes. 

 The pipe is here made flat instead of circular, for 

 forming the seats of the valves, and the bason KK is 

 i-overed uiUi nn air vessel FF. This air vessel 

 oiiMiiiuiiicates all round the pipe B, with the bason 

 KK, and with the vertical pipe M. The machine 

 being thus constructed, let us suppose the pipe ABR 

 full of water, and the valve C to be opened, the 

 water will lift Uie valve D, and escape with a ve- 

 locity due to Uie height of the reservoir. In a short 

 time, the water having acquired an additional ve- 

 locity, raises the valve G, which shuts the passage, 

 and prevents the escape of the water. The conse- 

 quence of this is, that all the included water exerts 

 suddenly a hydrostatical pressure on every part of 

 the pipe, compressing at the same time the air in the 

 annular space i ;', which by its elasticity diminishes 

 Uie violence of the shock. This hydrostatical pres- 

 sure opens the valves at E, and a portion of Uie 

 water flows into the air vessel F, and condenses the 

 air which it contains. The valves at E now close, 

 preventing the return of the water into the pipe, and 

 the water recoils a little in the tube with a slight 

 motion from B to A, in consequence of the reaction 

 or elasticity of the compressed air in 1 i, and also of 

 the metal of the pipe, which must have yielded a 

 little to the force exerted upon it in every direction. 

 The recoil of the water towards A produces a slight 

 aspiration within the head R of the ram, which causes 

 the valve D to descend by its own weight, and pre- 

 vent Uie water X which covers it from descending 

 into the tube. The air, however, passes through the 

 pipe /Ar, opens the valve fc, and a small quantity is 

 sucked into the annular space i i; but the quantity is 

 very small, as the valve k closes as soon as the cur- 

 rent of air becomes rapid. During the recoil towards 

 A, the valve C, being unsupported, falls by its own 

 weight; and when the force of recoil is expended by 

 acting on the water in the reservoir PQ, the water 

 begins again to flow along ABR, and the very same 

 operation which we have described is repeated with- 

 out end, a portion of water being driven into the air 

 vessel F at every ascent of the valve C. The air in 

 this vessel being thus highly compressed, will exert 

 a force due to its elasticity upon the surface of the 

 water in the vessel F, and will force it up through 

 the pipe M to a height which is sufficient to balance 

 the elasticity of the included air. 



The small quantity of air which is drawn into the 

 annular space i i through the air tube / k at each as- 

 piration, causes an accumulation of air in the space 

 ii; and when the aspiration of recoil takes place, a 

 small quantity of air passes from i , and proceeds 

 along the pipe till it arrives beneath the valves at E, 

 and lodging in the small space beneath the valves, 

 it is forced into the air vessel at the next stroke, 

 and thus affords a constant supply of air to the 

 vessel. The valves make in general from fifty to 

 seventy pulsations in a minute. 



When the fall of water, or PQ,is five feet, and the 



C'pe AB six inches in diameter and fourteen feet 

 ng, a machine with its parts proportioned as in the 

 fitrure will raise water to the height of 100 feet. It 

 ill expend about seventy cubic feet per minute in 

 working it, and will raise about two and a third cubic 

 feet per minute to the height of 100 feet. 



The Uiird general division of the subject relates to 

 the mean* by which motion and power may be ob- 

 tained from liquids, and includes the general consid- 



eration of water-wheels and other contrivances for 

 moving machinery. Motion is generally obtained 

 from water, either by exposing obstacles to theae.tion 

 of its current, as in water-wheels, or by arresting it' 

 progress in movable buckets, or receptacles which 

 retain it during a part of its descent. 



Water-wheels have three denominations, depend- 

 ing on their particular construction, on the manner 

 in which they are set or used, and on the manner in 

 which the water is made to act upon them ; but all 

 water-wheels consist, in common, of a hollow cylin- 

 der or drum, revolving on a central axle or spindle, 

 from which the power to be used is communicated, 

 while their exterior surface is covered with vanes, 

 float-boards, or cavities, upon which the water is to 

 act. The undershot wheel is the oldest construction 

 of this kind : it is merely a wheel, furnished with u 

 series of plane surfaces or floats projecting from its 

 circumference, for the purpose of receiving the im- 

 pulse of the water which is delivered under the 

 wheel. As it acts chiefly by the momentum of the 

 water, the positive weight of which is scarcely called 

 into action, it is only proper to be used where there 

 is a great supply of water always in motion. It is 

 the cheapest of all water-wheels, and is more applica- 

 ble to rivers in their natural state than any other 

 form of the wheel; it is also useful in tide-currents, 

 where the water sets in opposite directions at different 

 times, because it receives the impulse equally well 

 on either side of its floats. In the overshot wheel, 

 the circumference is furnished with a series of cavi- 

 ties or buckets, into which the water is delivered 

 from above. The buckets on one side, being erect, 

 will be loaded with water, and the wheel will be 

 thus set in motion ; the mouths of the loaded buckets, 

 being thus turned downwards by the revolution of 

 the wheel, will be emptied, while the empty buckets 

 are successively brought under the stream by the 

 same motion, and filled. The breast- wheel differs 

 from this in receiving the water a little below the 

 level of the axle, and has floats instead of buckets. 

 In these two wheels, the weight and motion of the 

 water are used, as well as its momentum, and a 

 much greater power is, therefore, produced with a 

 less supply of water than is necessary for the under- 

 shot wheel. In order to permit these wheels to work 

 with freedom, and to the greatest advantage, it is 

 necessary that the back or tail water as it is called, 

 or that which is discharged from the bottom of the 

 wheel, should have an uninterrupted passage oft'; for 

 otherwise it accumulates, nnd forms a resistance to 

 the float-boards. One of the simplest methods of 

 removing it consists of forming two drains through 

 the masonry, each side of the water-wheel, so as to 

 permit a motion of the upper water to flow down into 

 the tail, in front of the wheel. The water, thus 

 brought down with great impetuosity, drives the 

 tail-water before it, and forms a hollow place, in 

 which the wheel works freely, even if the state of 

 the water be such that it would otherwise form a 

 tailing of from twelve to eighteen inches. The 

 drains may be closed whenever the water is scarce. 

 Numerous other contrivances are in use, which our 

 limits will not permit us to describe. See Briant, 

 Over-shot and Under- shot, and tVheel. 



In Barker's centrifugal mill, Uie water does not 

 act, as in the contrivances above noticed, by its 

 weight or momentum, but by its centrifugal force 

 and the reaction that is produced by the flowing ot 

 the water on the point immediately behind the ori- 

 fice of discharge. It consists of a revolving vertical 

 tube, which receives the water at the top, and at the 

 bottom of which is a horizontal tube, extending on 

 each side of it, and having apertures opening in op- 

 posite sides, near the ends. The water spouting 



