hing,
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 distinct 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.

Still another invention by Watt deserves a word, the advantages of
which will become evident to everybody. When the piston descends in
Newcomen's engine, it is by the weight of the atmosphere. The atmosphere
is cold; hence it must cool the sides of the metal cylinder, which is
open at the top, in proportion as it expands itself over the entire
surface. This cooling is not compensated during the whole ascension of
the piston, without the expense of a certain quantity of steam. But
there is no loss of this sort in the engines modified by Watt. The
atmospheric action is totally eliminated by the following means:

The top of the cylinder is closed by a metal cover, only pierced in the
centre by a hole furnished with greased tow stuffed in hard, but through
which the rod of the piston has free motion, though without allowing
free passage either to air or steam. The piston thus divides the
capacity of the cylinder into two distinct and well-closed areas. When
it has to descend, the steam from the caldron reaches freely the upper
area through a tube conveniently placed, and pushes it from top to
bottom as the atmosphere did in Newcomen's engine. There is no obstacle
to this motion, because, while it is going on, only the base of the
cylinder is in communication with the condenser, wherein all the steam
from that lower area resumes its fluid state. As soon as the piston has
quite reached the bottom, the mere turning of a tap suffices to bring
the two areas of the cylinder, situated above and below the piston, into
communication with each other, so that both shall be filled with steam
at the same degree of elasticity; and the piston being thus equally
acted upon, upward and downward, ascends again to the top of the
cylinder, as in Newcomen's atmospheric engine, merely by the action of a
slight counterpoise.

Pursuing his researches on the mean