420 Hydrodynamics. 



no explosion is to be feared. Still, however, it is necessary that 

 the boiler should be made stronger than this limit of resistance 

 supposes. For when the steam rushes into the cold cylinder 

 and is condensed there, this precipitation is so rapid that the 

 new steam formed at the same time in the boiler is not always 

 sufficiently instantaneous to supply its place. For a moment a 

 vacuum is left in the boiler, and the pressure of the external 

 air being no longer counterbalanced, the boiler may burst inward 

 if its sides are not sufficiently thick. This accident sometimes 

 happens, but it may always be prevented by means of a second 

 safely valve, w r hich opens inward whenever the external pressure 

 becomes too great. 



From this explanation it would seem that when the engine 

 was once in operation nothing would remain in the piston or body 

 of the pump but pure steam or a vacuum. But it must be re- 

 marked that the injected water has also some air combined with 

 it which escapes into the body of the pump ; since it is found 

 there in a highly rarified state, being heated to a considerable 

 degree by the great quantity of heat disengaged during the con- 

 densation. Happily this air, being in small quantity, and con- 

 tained in a small space, is easily expelled through the valve S by 

 the first effort of the steam introduced into the cylinder. 



524. The apparatus which we have described is not precisely 

 the one first invented. It appears that the original attempt was 

 simply to employ the force of steam as a moving power. But the 

 more ingenious discovery of the method of condensing the steam 

 by cooling was not made until 1696; and the English attribute 

 it to Capt. Savary, who published an account of it in a treatise 

 entitled The Miner's Friend. The mode of applying this princi- 

 ple was still very imperfect. In 1 705, Newcomen, another Eng- 

 lishman, gave it the form which we have described, in which, 

 under the name of the atmospherical engine it was a long time 

 not unprofitably employed. 



Nevertheless, with the progress we have made in mechanics 

 and the natural sciences, it is easy to perceive that this engine 

 had many theoretical defects. It was a great imperfection that 

 it required an intelligent person to watch it for the purpose turn- 

 ing the stop-cocks to introduce water and steam every time the 



