THE GROWTH OF THE STEAM-ENGINE. 



2 5 



fected by water boiling under atmospheric pressure, can be subjected 

 to the action of water boiling under high pressure, and thus thoroughly 

 "digested," or cooked. 



The danger of bursting these vessels caused him, in 1681, to in- 



\6\ 



Denys Papin. 



vent and apply the lever safety-valve, 1 now an indispensable appur- 

 tenance to every steam-boiler. 



17. In 1690 he constructed a working model of an engine, consist- 

 ing of a steam-cylinder with a piston which was raised by steam-press- 

 ure, and which descended again when the condensation of the steam 

 produced a vacuum beneath it. 



This apparatus the inventor proposed to use as a motor for working 

 pumps and for driving paddle-wheels ; but he never built a successful 

 working machine on this plan, so far as we can ascertain ; and he did 

 not then propose a separate boiler, but made the same vessel serve 

 at once as a boiler, steam-cylinder, and condenser, evaporating water 

 in the cylinder itself; 2 and, after raising the piston, removing the 

 cylinder from the fire, or the fire from under the cylinder, to effect 

 condensation by the gradual loss of heat by radiation. 



18. The most important advance in actual construction was made 

 by Thomas Savery. 



The constant and embarrassing expense, and the engineering diffi- 

 culties presented by the necessity of keeping the British mines, and 



1 Other forms of safety-valve had been previously used. 



2 " Recueil des diverses Pieces touchant quelques nouvelles Machines et autres Sujets 

 philosophiques," M. D. Papiu, Cassel, 1695. 



