exhausted steam is
too great for economy. Yet we want to utilize as much energy as
possible. How are we to do it?

COMPOUND ENGINES.

In the year 1853, John Elder, founder of the shipping firm of Elder and
Co., Glasgow, introduced the _compound_ engine for use on ships. The
steam, when exhausted from the high-pressure cylinder, passed into
another cylinder of equal stroke but larger diameter, where the
expansion continued. In modern engines the expansion is extended to
three and even four stages, according to the boiler pressure; for it is
a rule that the higher the initial pressure is, the larger is the number
of stages of expansion consistent with economical working.

[Illustration: FIG. 29.--Sketch of the arrangement of a
triple-expansion marine engine. No valve gear or supports, etc., shown.]

In Fig. 29 we have a triple-expansion marine engine. Steam enters the
high-pressure cylinder[4] at, say, 200 lbs. per square inch. It exhausts
at 75 lbs. into the large pipe 2, and passes to the intermediate
cylinder, whence it is exhausted at 25 lbs. or so through pipe 3 to the
low-pressure cylinder. Finally, it is ejected at about 8 lbs. per square
inch to the condenser, and is suddenly converted into water; an act
which produces a vacuum, and diminishes the back-pressure of the exhaust
from cylinder C. In fact, the condenser exerts a _sucking_ power on the
exhaust side of C's piston.

ARRANGEMENT OF EXPANSION ENGINES.

In the illustration the cranks are set at angles of 120 deg., or a third
of a circle, so that one or other is always at or near the position of
maximum turning power. Where only two stages are used the cylinders are
often arranged _tandem_, both pistons having a common piston rod and
crank. In order to get a constant turning movement they must be mounted
separately, and work cranks set at right angles to one another.

COMPOUND LOCOMOTIVES.

In 1876 Mr. A. Mallet introduced _compounding_ in locomotives; and the
practice has been largely adopted. The various types of "compounds" may
be classified as follows:--(1) One low-pressure and one high-pressure
cylinder; (2) one high-pressure and two low-pressure; (3) one
low-pressure and two high-pressure; (4) two high-pressure and two
low-pressure. The last class is very widely used in France, America, and
Russia, and seems to give the best results. Where only two cylinders are
used (and sometimes in the case of three and four), a valve arrangement
perm