418 Hydrodyn amics. 



tremity D to another piston P, moving like the first in the body of 

 the pump TT likewise cylindrical ; but suppose the bottom of 

 the body of the pump, instead of being immersed in water, to 

 communicate with an air pump by means of which the air in it 

 can be exhausted. It is manifest that after the exhaustion the 

 pressure of the atmosphere upon the upper surface of the piston 

 P will tend to make it descend ; and will, in fact, make it descend, 

 ii the whole eifect of this pressure exceed the weight of the pis- 

 ton P 7 , together with that of the column of water to be raised. 

 Now the piston P, having descended to the bottom of the body 

 of the pump, let the air be admitted below ; then the pressure on 

 the two surfaces will be equal ; and the excess of the weight of 

 the piston P' beginning to act, P will rise in the bore of the pump ; 

 after which, if we make a new exhaustion under P, we shall 

 cause P to descend and P' to rise, and we can repeat these mo- 

 tions at. pleasure. 



But it will be seen that the air pump could hardly be em- 

 ployed upon so large a scale. To supply its place, we introduce 

 steam into the body of the pump TT. The simplest method of 

 doing this, and which, (although it has not hitherto been most 

 generally employed,) is nevertheless attended with some pecu- 

 252. liar advantages, is the following. Under the body of the pump 

 JT, let there be placed a boiler FF, filled in part with boiling 

 water, the steam of which being equal or a very little superior in 

 elasticity to the pressure of the atmosphere, may be introduced 

 at pleasure into the cylinder TT by turning a stop-cock J?, which 

 opens a communication between the pump and the boiler. Let 

 there be likewise at the bottom of the cylinder a small lateral 

 passage FS, shut by a valve S opening outwards. Now the pis- 

 ton P being forced to the bottom or nearly to the bottom of the 

 cylinder TJ 1 , and the space below being filled with air, turn the 

 stop-cock R which communicates with the boiler. The steam 

 will rush into the cylinder, and by its impulse, together with its 

 elastic force, will in part expel the air remaining in the cylinder 

 by forcing it to open the valve S. In this operation a great quan- 

 tity of steam is suddenly condensed by the cold surface of the 

 cylinder TTand that of the piston P, and being reduced to water, 

 is made to pass out through a descending tube EGS', recurved at 

 the lower end and terminated by a valve 5', opening outwards. 



