The steam-engine is much more intelligible than its name first
suggests. That part by which the machinery is set in motion, may be
compared to a syringe, or squirt, the rod of which is driven up and
down by steam admitted above and below, one end of the rod being
connected with the machinery to be worked. Thus, the piston is made to
turn the wheels of a railway carriage, or the paddles of a steam-boat.
The elastic force of the steam, or vapour, by which the rod is driven
up and down, may be explained by this simple experiment. Provide a test
tube, put into it a little water, hold the thumb over the mouth, and
cause the water to boil by holding it over a spirit-lamp. There will
soon be felt a pressure against the thumb; when, if the tube be dipped
into cold water, the thumb being still held at the end, a kind of
suction will be felt against it. Now, the tube resembles the cylinder
of the steam-engine, in which the piston moves up and down; to imitate
which, wrap a little tow about the end of a piece of stick, grease it
with tallow, and fit it moderately tight into the tube; when the water
is made to boil, the stick will be raised, and when the end is dipped
into cold water, the stick will fall as the piston rises and falls in
the cylinder.