ON HYDRAULIC MACHINES. 255 



pump with 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, through which the water is to be raised, is consider- 

 able, some inconvenience might arise from the length of the barrel through 

 which the piston rod of a sucking pump would have to descend, in order 

 that the piston might remain within the limits of atmospheric pressure. 

 This may be avoided by placing the inoveable 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 pumps, 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 through 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 considerable. The condensed air, reacting on 

 the water, expels it more gradually, and in a continual stream, so that the 

 air vessel has an effect analogous to that of a fly wheel in mechanics. 

 (Plate XXIII. Fig. 312.) 



If, instead of forcing the water to a certain height through a pipe, we 

 cause it to form a detached jet, we convert the forcing pump into a fire 

 engine : and in general two barrels, acting alternately, are connected, for 

 this purpose, with the same air vessel ; so that the discharge is thus 

 rendered very nearly uniform. The form of the adjutage, or orifice of the 

 pipe, is by no means indifferent to the effect of the machine, since the 

 height of the jet may be much increased by making it moderately con- 

 tracted, and a little conical rather than cylindrical. When the air vessel 

 is half filled with water, the height of such a jet will be about 30 feet, when 

 two thirds filled, about 60, the height being always nearly proportional to 

 the degree of condensation of the air, or to the excess of its density above 

 that of the surrounding atmosphere. Sometimes a double forcing pump, or 

 fire engine, is formed by the alternate rotatory motion of a flat piston 

 within a cylindrical barrel ; the axis of its motion coinciding with that of 

 the barrel, and the barrel being divided by a partition into two cavities, 

 which are filled and emptied in the same way as the separate barrels of the 

 common fire engine. The mechanical advantage of this machine is nearly 

 the same as that of the more usual constructions, but it appears to be some- 

 what more simple than a common engine of equal force. The partition 

 may be extended throughout the diameter of the cylinder, the opposite pairs 

 of cavities being made to communicate with each other, and thus both sides 

 of the piston may be employed at once. (Plate XXIII. Fig. 313.) 



