ondensed,--that enters it, until
it has from its own heat imparted so much to the cylinder as to raise
it to its own temperature, when it will no longer condense, and not till
then does it begin to exert its elastic power to produce motion. This
was the great object gained by James Watt, when, after various
experiments, he gave up the idea altogether of condensing steam in its
own or working cylinder, and then made use of a separate vessel, now
called the condenser.

The weight of steam is about 1800 times less than water. I may here
perhaps mention also that water will boil at 100 degrees Fahr. in vacuo,
whereas in atmosphere it takes 212 degrees to boil. There is also a
thing perhaps worth knowing to all who wish to get the most stock out of
bones, &c., that if they are boiled in a closed vessel, that is to say,
under a pressure of steam, a very large increase in quantity of the
stock will be produced, because the heat is increased. A cubic inch of
water, evaporated under _ordinary_ atmospheric pressure, will be
converted into a cubic foot of steam; and a cubic inch of water,
evaporated as above, gives a mechanical force equal to raising about a
ton a foot high.

The next great improvement of Watt, in addition to the condenser, is the
air-pump, the use and absolute necessity for which you will understand
when I explain its action. Watt first used it for his atmospheric
engine. The piston of this engine was kept tight by a flow of oil and
water on the top, which tended to make the whole a troublesome and
bad-working machine. The cold atmosphere, as the piston went down, of
course followed it and cooled the cylinder. On the piston again rising,
some steam would of course be condensed and cause waste. If the
engine-room could be kept at the heat of boiling water, this would not
have been the case, but the engineman who could live in this heat would
also require to be invented, and so this had to be given up. Watt's next
and most important step was the one which brings us to talk of the
steam-engine as it now is in the present day. This important step was
the idea, of making the steam draw down the piston, as well as help to
drive it up; in the first engines it was raised by the beam, and steam
used only to cause a vacuum, so as to let the air drive it down. All
before this had been merely steps in advance, like those of children,
who must walk before they can run; so was it with the steam-engine. It
was uphill work for