ON HYDRAULIC SrACHINES. 333 



is placed within the piston itself, so that the same barrel serves for the ascent 

 of the water, which rises in one continued line, while the piston is raised, 

 and rests on the fixed valve while it is depressed. The velocity of the stroke 

 ought never to be less than 4 inches in a second, nor greater than two or 

 three feet; the stroke should also be as long as possible, in order to avoid 

 unnecessary loss of water during the descent of the valves. The diameter of 

 the pipe, through which the water rises to the barrel, ought not to be less than 

 two thirds of the diameter of the barrel itself. (Plate XXIII. Fig. 309.) 



A bag of leather has also been employed for connecting the piston of a 

 pump witli the barrel, and in this manner nearly avoiding all friction: but it 

 is probable that the want of durability would be a great objection to such a 

 machine. (Plate XXIII. Fig. 310.) 



Where the height, throvigh which the water is to be raised, is considerable, 

 some inconvenience might arise from the length of the barrel through which 

 the piston lod of a sucking pump would have to descend, in order that the 

 piston might remain within the hmits of atmospheric pressure. This may be 

 avoided by placing the moveable valve below the fixed valve,and introducing 

 the piston at the bottom of the barrel. Such a machine is called a lifting pump : 

 in common with other forcing pumj)s, it has the disadvantage of thrusting the 

 piston before the rod, and thus tending to bend the rod, and produce an 

 unequal friction on the piston, while, in the sucking pump, the principal force 

 always tends to straighten the rod. (Plate XXIII. Fig. 311.) 



The rod of a sucking pump may also be made to work in a collar of leather, 

 and the water may be forced througli a valve into an ascending pipe. By 

 applying an air vessel to this, or to any other forcing pump, its motion may 

 be equalised, and its performance improved ; for if the orifice of the air 

 vessel be sufficiently large, the water may be forced into it, during the stroke 

 of the pump, with any velocity that may be required, and with little resistance 

 from friction, while the loss of force, from the frequent accelerations and 

 retardations of the whole body of water, in a long pipe, must always be conr 

 siderable. The condensed air, reacting on the water, expels it more gra- 

 dually, and in a continual stream, so that the air vessel has un effect analo^ 

 gous to that of a fly wheel in mechanics. (Plate XXIII. Fig. 312.) 



