HYDRAULICS. 



from the discharging pipe at h c, and 

 as soon as the efflux ceases, to shut the 

 cock /, and open m to discharge the 

 water from /, and permit the lower 

 chest e to fill, which will be effected 

 whenever water ceases to flow from m. 

 The cock m must then be shut, and I 

 opened, and so on alternately, which 

 may easily be done mechanically, and 

 without superintendanee, by using a 

 part of the impelling water from d, or 

 that which has been discharged from 

 h c, and which may be employed to 

 turn a small water-wheel, or to fill two 

 small cisterns in which floats are made 

 to act. Mr. John W. Boswell de- 

 vised a contrivance for answering this 

 same purpose, which will be found 

 fully detailed in the second volume of 

 Dr. Gregory's excellent Treatise on 

 Mechanics, where this simple machine 

 is described under several forms and 

 modifications. 



It must not be supposed that filling 

 the middle vessel / with water will dis- 

 charge the whole of the water out of e, 

 otherwise disappointment in its effects 

 will ensue ; because, although water is 

 nearly incompressible, air is highly elas- 

 tic, and the air in e*will be compressed 

 into less than its natural bulk, or will 

 be condensed with a force equivalent to 

 the pressure of the perpendicular co- 

 lumn of water h h, which it has to 

 overcome ; and as atmospheric pressure 

 was shown, when speaking of the pumps 

 under the second head or division, 

 to be only equal to the support of a 

 column of water about 33 feet in 

 height, so if we imagine this to be the 

 height of the pipe h h, that column 

 of water would require one of double 

 atmospheric elasticity to support it, 

 and hence the air in e would be con- 

 densed to half its former volume, and, 

 therefore, discharge but half the volume 

 of water, although / should be com- 

 pletely filled. 



Dr. Gregory further'describes a curi- 

 ous phenomenon which takes place in 

 the working of this machine, and which 

 never fails to create surprize in the 

 strangers who visit it, and to whom it 

 is usually shown. That is, when the 

 efflux at A c has stopped, if the cock n 

 be opened, the water and air rush out 

 together with prodigious violence, and 

 the drops of water are changed into hail 

 or lumps of ice, issuing with such force 

 as frequently to pierce a hat, if held 

 against them, like pistol bullets. This 

 rapid congelation is a remarkable in- 



stance of the general fact, that air, by 

 suddenly expanding, generates cold, its 

 capacity for heat being increased. 



The Centrifugal Pump has several 

 different forms, one of the most simple 

 of which is shown at Jig. 15, in 

 which gh represents an upright spindle, 

 so fixed, that rapid rotatory motion 



may be communicated to it by the 

 winch /, and h m represent any num- 

 ber of curved pipes (each of which con- 

 tains one valve opening upwards) so 

 disposed and fixed to the spindle, that 

 their lowest ends may be near to it, and 

 be covered by the water to be raised ; 

 and their upper ends, which are quite 

 open, are extended to a considerable 

 distance from the centre of motion, and 

 finally bent downwards to prevent the 

 dispersion of the water. The several 

 curved pipes must be filled with water, 

 which will be retained in them by their 

 bottom valves, and are then put into 

 rapid motion by turning the winch, 

 when the higher ends m m of the pipe 

 will describe a much larger circle than* 

 the ends below, and'consequently such 

 a centrifugal force, or tendency to* fly off 

 and empty the pipes, will be induce'd at 

 the upper ends as will produce a va- 

 cuum, capable of raising a column of 

 water. 1 1 1 1 h a circular pan or re- 

 servoir to receive the upper ends of all 

 the pipes and the water they deliver, 

 which runs off by spouts at n n. This 

 machine, according to theory, should 

 deliver water with a velocity nearly 

 equal to that with which the upper ends 

 of the pipes move, but in practice it 

 C 2 



