58 Experimental Philosophy. [Lecture 5. 



up the valve b in the box H, and rises into the 

 body or barrel of the pump above H. When the 

 piston g, therefore, (which we must observe has 

 no hole or valve in it) is depressed to H, the valve 

 b being closed by this action, the water in the 

 barrel of the pump, finding no other vent, is 

 forced into the pipe M M, and so up through the 

 pipe. If there is no occasion for a continued 

 stream of water, the pipe M is continued to any 

 given height, and then the water would be thrown 

 out like a jet-d'eau at every stroke of the piston. 

 But to make a continued stream a further con- 

 trivance is necessary. 



To this end an air vessel, such as K K, is an- 

 nexed to the pipe M, and into this air-vessel the 

 water is forced by each stroke of the piston. 

 When therefore the water, by this action conti- 

 nued, gets above the lower end of the pipe GHI, 

 which is fixed air-tight, in the top of the vessel, 

 the air in the upper part is proportionably con- 

 densed. The action of the pump being then 

 continued, in proportion as the vessel K K is 

 filled with water, the air above it is compressed, 

 and in return presses on the surface and drives 

 out the water through the pipe at the orifice in its 

 end in a continual stream, and with great force. 



It is upon this principle that the famous and 

 truly useful invention of the fire-engine is found- 

 ed. It consists of two forcing pumps, and a large 

 air vessel which communicates with the pipe. In 

 fig. 27, A B is the body of the engine, in which 



