MODE OF REFRIGERATING. 385 



counterpoise. In the English machine, in short, in imi- 

 tation of Papin's, as soon as the piston has reached its 

 maximum of ascent, the steam that had tended to push it 

 there is refrigerated ; thus a vacuum is created through- 

 out the space that the piston had traversed, and the ex- 

 ternal atmosphere forces it to descend again. 



To obtain the requisite refrigeration, we already know 

 that Papin contented himself with removing the chafing- 

 dish that heated the base of the little metallic cylinder. 

 Newcomen and Cawley had recourse to an arrangement 

 far preferable in every respect : they poured an ample 

 quantity of cold water into the circular space contained 

 between the outer surface of the cylinder of their ma- 

 chine and the internal surface of a second rather larger 

 cylinder, which served as an envelop to the first. The 

 cold was gradually communicated through all the thick- 

 ness of the metal, and lastly reached the steam itself.* 



Papin's machine, thus perfected as to the manner of 

 cooling or condensing the steam, excited the proprietors 

 of mines to the highest pitch. It spread rapidly through 

 several counties of England, and rendered great service. 

 The slowness of its movements, the necessary conse- 

 quence of the tardiness with which the steam cooled and 

 lost its elasticity, was still a subject of great regret. 

 Chance fortunately indicated a very simple way of obvi- 

 ating this impediment. 



At the beginning of the eighteenth century the art of 

 casting metallic cylinders, and of hermetically sealing 



* Savery had already adopted throwing cold water on the exterior 

 surface of a metal vase, to condense the steam within it. This was 

 the origin of his partnership with Newcomen and Cawley ; yet we 

 must not forget that Savery' s patent, his machines, and the work in 

 which he describes them, are posterior by several years to Papin's 

 memoirs. 



SEC. SER. 17 



