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 
J 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’ 
