invented, in which a heavily-loaded piston was forced upward by an
explosion of gas and air drawn in at atmospheric pressure. In its
upward stroke the piston was free to move; but in its downward stroke
it was connected with a ratchet, and the partial vacuum formed after
the explosion beneath the piston, together with its own weight in
falling, operated through a rack, and caused rotation of the flywheel.
This engine (which, in an improved form, uses only about 20 cubic feet
of gas) is still largely employed, some 1,600 having been constructed.
The great objection to it was the noise it produced, and the wear and
tear of the ratchet and rack arrangements. In 1876 the Otto-Crossley
silent engine was introduced. As you are aware, it is a single-acting
engine, in which the gas and air are drawn in by the first outward,
and compressed by the first inward stroke. The compressed mixture is
then ignited; and, being expanded by heat, drives the piston outward
by the second outward stroke. Near the end of this stroke the
exhaust-valve is opened, the products of combustion partly escape, and
are partly driven out by the second inward stroke. I say partly, for a
considerable clearance space, equal to 38 per cent. of the whole
cylinder volume, remains unexhausted at the inner end of the cylinder.
When working to full power, only one stroke out of every four is
effective; but this engine works with only 18 to 22 cubic feet of gas
per horse power. Up to the present time I am informed that about
18,000 of these engines have been manufactured. Several other
compression engines have been introduced, of which the best known is
Mr. Dugald Clerk's, using about 20 feet of Glasgow cannel gas. It
gives one effective stroke for every revolution; the mixture being
compressed in a separate air-pump. But this arrangement leads to
additional friction; and the power measured by the brake is a smaller
percentage of the indicated horse power than in the Otto-Crossley
engine. A number of gas engines--such as Bisschop's (much used for