12 



HYDRAULICS. 



the action of this machine, as it is pre- 

 sumed the figure will render it suffi- 



ciently obvious ; while the lower end of 

 the working-barrel h h is immersed in 

 water, and the piston i moves upwards 

 and downwards, the barrel will be filled 

 through the piston-valve at each down- 

 stroke, and at each up- stroke its con- 

 tents will be expelled through the stop- 

 valve k into the ascending pipe II; and 

 whatever the diameter of this pipe may 

 be, still its resistance will constantly be 

 equal to the weight of a column of water 

 of the size of the working-barrel, and of 

 a height equal to the perpendicular alti- 

 tude of the water in the ascending pipe ; 

 for this pipe maybe placed horizontally 

 or obliquely so as materially to alter its 

 length, but it is the perpendicular height 

 between the surface of the water to be 

 raised and its point of discharge, which 

 must alone be taken into account in es- 

 timating the load upon a pump, since 

 increase of length, without height in 

 the pipe, produces no other resistance 

 than that of friction, which is easily 

 overcome by increasing the capacity of 

 the pipe. 



It may appear that the above pump is 

 applicable to every purpose and to every 

 situation, such as raising water from 

 mines and the deepest places ; but this 



is not the case, owing to the almost 

 imperceptibly small elasticity of water, 

 and the effects of the vis inertia?, which 

 belongs to fluids in common with solid 

 matter. In working the pump shown in 

 the last figure, if we presume the pipe / / 

 to be full of water, that water has not 

 sufficient elasticity to permit the ban-el 

 h h to discharge its contents through the 

 valve k without putting all the water 

 contained in 1 1 into motion, while, when 

 the piston descends, that motion will be 

 at an end. The water in II will there- 

 fore be in an alternate state of rest and 

 motion ; and if the column is long, and 

 its quantity great, the vis inertice will 

 be very considerable, that is to say, it 

 will require a considerable -exertion of 

 force to get it from a state of rest into 

 motion ; and when it has once begun to 

 move, it will have no immediate ten- 

 dency to return again to rest, but might 

 be continued in its motion with less 

 force than that which was originally 

 employed to move it. The descent of 

 the piston, however, allows sufficient 

 time for all the motion that was com- 

 municated to be completely lost ; and 

 hence in working this pump we not only 

 have the weight of the column to over- 

 come, but the natural inertia to combat 

 with at every stroke. This may in great 

 measure be removed by keeping two, or 

 what is still better, three pumps con- 

 stantly at work by what is called a triple 

 or three-throw crank ; and accordingly 

 this expedient is generally resorted to 

 in all small engines for throwing water 

 to a great height : for by this means the 

 water is never permitted to stand still in 

 the pipes, but a constant flow or stream is 

 maintained. The triple crank is an axle 

 of iron, bent into the form shown ak jig. 

 9, so as to form three elbows v u w y 

 to each of which the piston-rod of a 



fff-9. 



