M. Arago's Biographical Memoir of James Watt. 253 



degree cooled, and the new steam with which we may fill it, 

 lose nothing of its elasticity. 



The condenser entirely absorbs the steam of the cylinder, on 

 the one hand, because it contains cold water; and on the 

 other, because the rest of its capacity contains no elastic fluid. 

 But so soon as the first condensation of the steam is effected, 

 these two conditions of its efficiency have disappeared ; the 

 condensing water is heated by absorbing the latent caloric 

 of the steam, and some vapour is formed from this hot water ; 

 the cold water also contained atmospheric air, which, of course, 

 was disengaged in the process of heating. If, then, after 

 every act of condensation, this hot water, and vapour and 

 an-, which the condenser contains, were not removed, it would 

 become inefficient. Watt accomplished this triple evacuation 

 by means of a common air-pump, where a piston was conve- 

 niently attached to the beam, and so was worked. The force 

 required to keep the air-pump in action, diminishes by so 

 much the power of the machine ; but this forms but a small 

 loss compared ^vith what was sustained by the old method of 

 condensing the steam, through the refrigerated sides of the 

 cylinder. 



One word more, and the advantages of another invention of 

 Watt will become manifest to all. When the piston descends 

 m Newcomen's machine, it is the atmosphere which pushes it. 

 That atmosphere is cold, and must cool down the parietes of 

 the metallic cylinder, open at top, along which it successively 

 moves throughout their whole extent. These sides of the 

 cylinder can recover from this refrigeration, during the as- 

 cending course of the piston, only at the expense of a cer- 

 tain quantity of steam. There exists no loss, however, of 

 this sort, in the modified engines of Watt ; the atmospheric 

 action is wholly eliminated, and in the following manner. 

 The cylinder is closed at the top, by a metallic cover, which 

 has an opening nicely fitted, so that the cylindrical piston- 

 rod moves freely in it, without allowing the slightest pas- 

 sago to air or steam. The piston thus divides the cylinder 

 mto two chambers which are distinct from each other and 

 perfectly closed ; when it is about to descend, the steam 

 from the boiler readily roaches the upper chamber through a 



vol.. XXVII. NO, HV.—OC'iOBER 1839. s 



