ATMOSPHERIC ENGINES. 381 



selves, therefore, exclusively with the means by the aid 

 of which Papin proposed to induce his oscillating motion. 



Let us imagine a large vertical cylinder, open at top, 

 and its base resting on a metallic table, pierced with a 

 hole that a cock can either close or open at will. 



Introduce a piston into this cylinder, that is to say, a 

 circular plate, filling it entirely but movable, that shall 

 exactly close it. The atmosphere will rest with all its 

 weight on the upper surface of this species of diaphragm, 

 and will push it from the top to the bottom. The portion 

 of atmosphere that occupies the lower part of the cylin- 

 der will tend, by reaction, to produce an inverse move- 

 ment. This second force will be equal to the first if the 

 tap is open, because gas presses equally in all directions. 

 The piston will thus find itself urged by two opposite 

 forces, which will equalize each other. It will descend, 

 however, though only by reason of its gravity. A coun- 

 terpoise, slightly heavier than the piston, will suffice to 

 raise it, on the contrary, up to the summit of the cylin- 

 der, and keep it there. Suppose the piston to have 

 reached this extreme position. Let us seek the means to 

 make it descend from thence with great force, and carry 

 it up again. 



Let us imagine after having shut the lower cock, we 

 succeed in suddenly annihilating all the air contained in 

 the cylinder, in a word, to render it a vacuum. A 

 vacuum once made, the piston not receiving any pressure 

 but from the external atmosphere which presses it from 

 above, will descend rapidly. On this movement Ibeing 

 achieved, the cock will be opened. The air will thereby 

 return underneath to counterbalance the upper atmos- 

 phere. As at the beginning, the counterpoise will make 

 the piston remount to the cylinder, and all the various 



