Pneumatics. 75 



full-blown bladder or a thin glass bubble filled 

 with air is held close to a strong fire, it will burst. 

 That air is a compressible fluid must be 

 evident, when we consider that it is elastic ; and 

 it must be further evident from what was said in 

 the last lecture on the use of the air vessel an- 

 nexed to the forcing pump and common fire 

 engine. There is, however, a beautiful experi- 

 ment expressive of the effects from compressed 

 air, which, with the aid of the plate, I shall 

 endeavour to describe. It is a kind of artificial 

 fountain, which is made to send out a stream or 

 jet of water by means similar to those employed 

 in the fire engine, that is, by a body of compressed 

 air forcing the water contained below it through 

 a small pipe, and out of the jet or orifice of the 

 pipe. In Plate VIII. fig. 31, ABCD, is a 

 copper vessel, which may be made of any con- 

 venient form ; within the vessel is a small pipe 

 or tube N O open at bottom, and with what is 

 called a stop cock *, such as R, at the upper end 

 to keep in the air when it is necessary. To 

 make the fountain play, we first fill it about 

 two- thirds full, with water, then screw in the 

 pipe, which must be made air-tight by oiled 

 leather. The air contained between the surface 



* A stop cock is exactly like the common cocks used in 

 beer barrels, &c. When turned one way there is an 

 orifice through the stopple, which then admits the air, or 

 any fluid ; when turned the other way it is solid, and stops 

 the passage. 



