252 M. Arago's Biographical Memoir of James Watt. 



effect of the atmospheric pressure. When, afterwards, steam 

 at the temperature of 212° rushes into the same cylinder, if 

 the parietes be cold, this steam in heating them is partially 

 liquefied, and until the moment that their temperature like- 

 wise rises to 212°, the elasticity of the steam is decidedly 

 diminished ; hence slowness of movement must be the result ; 

 for the counterpoise does not raise the piston till there exist 

 within the cylinder a force which is capable of counter- 

 balancing the action of the atmosphere ; and hence also in- 

 creased expense ; steam being very dear, as I have already ex- 

 plained. No doubt will be entertained of the great im- 

 portance of this economical consideration, when I state that 

 the Glasgow model at each stroke, used a volume of steam 

 many times greater than that of the cylinder. The expense 

 of the steam, or, which comes to mk same thing, the expense 

 of the combustible, or, in other words, the pecuniary cost in- 

 dispensable for maintaining the action of the machine, would 

 be many times less if we could succeed in doing away with 

 the successive heatings and coolings, the inconveniences of 

 which have just been described. 



Watt in a very simple way resolved this apparently unsolv- 

 able problem. All he did was to add to the machine, as 

 previously constructed, a vcsisel perfectly distinct from the 

 cylinder, and communicating with it only by means of a 

 narrow tube, supplied with a stopcock. This vessel, known 

 by the name oitlie condenser, is the chief of Watt's inventions ; 

 and, in spite of all my desire to be short, I cannot but explain 

 its mode of action. 



If there exist a free communication between a cylinder 

 filled with steam, and a vessel void of steam and air, the steam 

 of the cylinder will pass in part and very rapidly into the 

 vessel, and the current will not stop till the elasticity be the 

 same throughout. Suppose, then, that with the help of an 

 abundant and continual injection of water, the vessel be kept 

 constantly cold throughout its whole capacity, the steam 

 will be condensed there so soon as it enters ; — the whole of 

 the steam with which the cylinder was originally filled will thus 

 come to be successively annihilated ; — the cylinder will thus 

 be freed of its steam without its sides being in the slightest 



