248 M. A.Ymgo's Biographical Memoir of James Watt. 



nal atmospheric pressiu*e, the ascending movement of the 

 piston was effected by means of a counterpoise. Finally, in 

 the English machine, in imitation of Papin's, so soon as the 

 piston reached the limit of its ascending stroke, the steam 

 which had impelled it was refrigerated ; a vacuum was thus 

 produced throughout the whole space it had traversed, and 

 the external atmosphere immediately forced it to descend. 



To produce the necessary cooling, Papin, as we have already 

 stated, did nothing more than remove the brasier which heated 

 the bottom of his small metallic cylinder. Newcomen and 

 Cawley introduced a process greatly preferable in every re- 

 spect. They caused a large quantity of cold water to flow 

 freely in an annular space formed between the external wall 

 of the cylinder of their machine, and a second cylinder, some- 

 what larger, with which they surrounded it. The cold com- 

 municated itself by degi'ees to the whole thickness of the metal, 

 and finally reached the steam itself.* 



Papin's machine, thus perfected in so far as it regarded the 

 method of cooling the steam, or of condensing it, excited, in a 

 high degree, the attention of anine proprietors. It was 

 rapidly introduced into many counties of England, where it 

 was of very considerable service. The want of rapidity in its 

 movements, however — the necessary consequence of the slow- 

 ness with which the A^apour cooled, and so lost its elasticity, 

 was, at the same time, the subject of great complaint. Acci- 

 dent happilj'^ indicated a A'ery simple means of overcoming 

 this inconvenience. 



At the commencement of the eighteenth centiu'y, the art of 

 boring great metallic cylinders, and of closing them hermeti- 

 cally by means of moveable pistons, was as yet in its infancy. 

 Moreover, in the early machines of Newcomen, the piston was 

 covered by a sheet of water, which was intended to fill the 

 spaces included between the circulai* contour of this move- 



* Savery had previously had recourse to a cun-ent of cold water which he 

 threw upon the external surface of a metallic vessel, thereby condensing 

 the steam which the vessel contained. This, in fact, was the origin of his 

 connection with Newcomen and Cawley; but it ought not to be forgotten, 

 that the patent of Savery, his machines, and the work in which they are 

 described, are all many years later then the memoirs of Papin 



