and valves are in the
same relative positions as formerly, and the same series of operations
is repeated again and again. Of course, it is not always the case that
both air _and_ gas valve are opened on the charging stroke; that depends
upon the method employed to govern the speed of the engine. Supposing
it were governed on the hit and miss principle (to be explained
hereafter), the gas valve would be allowed to remain closed during the
charging stroke, and air alone would be drawn into the cylinder, then
compressed, but not being explosive would simply expand again on the
working stroke, giving back nearly all the energy which was absorbed in
compressing it, and finally be exhausted in the same manner as the burnt
gases are.

[Illustration: FIG. 8.--Second out stroke, showing position of valves
during working stroke.]

[Illustration: FIG. 9.--Second inward stroke, showing position of valves
during the exhaust stroke.]

[Illustration: FIG. 10.--First out-stroke, showing position of valves
during the charging stroke.]

Fig. 10 shows diagrammatically the position of crank, piston, and
valves _during_ the charging stroke.

[Illustration: FIG. 11.--Cross Section of Cylinder.]

In figs. 1 and 2 we gave drawings of two gas engines, which are typical
examples of modern practice. Huge strides have been made in recent years
in gas-engine work, as regards both workmanship and efficiency, so that
to-day we have in the gas engine a machine whose mechanical efficiency
compares favourably with that of any other power generator, and whose
thermal efficiency is very much greater.

[Illustration: FIG. 12.--Longitudinal Section of Cylinder.]

Figs. 11 and 12 show respectively a sectional end and side elevation of
the cylinder, from which it will not be difficult for the reader,
however unacquainted he may be with gas-engine work, to see how the
various requirements and peculiarities of the engine should be
considered and provided for.

A most important desideratum in any machine or engine is that it shall
be as simple in construction as ever possible; complicated mechanism
should only be introduced when such addition or complication compensates
adequately for what must necessarily be a higher first cost, and
incidentally the greater wear and tear and attention involved. Figs. 11
and 12 show what has been done to simplify the construction of the gas
engine in recent years. The main feature in this case is the very
get-at-abl