390 JAMES WATT. 



fluid, and until the cylinder has regained the temperature 

 of 100, its elasticity will be found considerably attenu- 

 ated ; thence will ensue slowness of motion, for the coun- 

 terpoise will not raise the piston until there is sufficient 

 spring contained in the cylinder to counterbalance the 

 action of the atmosphere ; thence there will also arise an 

 increase of expense, for, as I have already said, the price 

 of steam is very high. No doubt will remain on the im- 

 mense importance of this economical consideration, when 

 I shall have stated that the Glasgow model at each oscil- 

 lation expended a volume of steam several times larger 

 than that of the cylinder. The expense of steam, or, 

 what comes to the same thing, the expense of fuel, or, if 

 we like it better, the pecuniary cost of keeping on the 

 working. of the machine, would be several times less if 

 the successive heatings and coolings, the inconveniences 

 of which have just been described, could be avoided. 



This apparently insolvable problem was solved by Watt 

 in the most simple manner. It sufficed for him to add to 

 the former arrangement of the engine a vessel totally dis- 

 tinct from the cylinder, and communicating with it only 

 by a small tube furnished with a tap. This vessel, now 

 known as the condenser, is Watt's principal invention. 

 Notwithstanding my earnest wish to abridge, I feel that 

 I must explain its mode of action. 



If there be a free communication between a cylinder 

 full of steam and a vessel containing neither steam nor 

 air, the steam from the cylinder will partly and very 

 rapidly pass into the empty vessel ; the passage will only 

 cease when the elasticity becomes equal in both. Let us 

 suppose that by an abundant and constant injection of 

 water, the whole capacity and the sides of the vessel be 

 kept constantly cold, then the steam will condense as 



