er is circulated by a
pump[9] (Fig. 43). The heat generated by combustion is so great that the
walls of the cylinder would soon become red-hot unless some of the heat
were quickly carried away. The pistons are of "trunk" form--that is,
long enough to act as guides and absorb the oblique thrust of the piston
rods. Three or more piston rings lying in slots (not shown) prevent the
escape of gas past the piston. It is interesting to notice that the
efficiency of an internal-combustion engine depends so largely on the
good fit of these moving parts, that cylinders, pistons, and rings must
be exceedingly true. A good firm will turn out standard parts which are
well within 1/5000 of an inch of perfect truth. It is also a wonderful
testimony to the quality of the materials used that, if properly looked
after, an engine which has made many millions of revolutions, at the
rate of 1,000 to 2,000 per minute, often shows no appreciable signs of
wear. In one particular test an engine was run _continuously for several
months_, and at the end of the trial was in absolutely perfect
condition.

The cranks revolve in an oil-tight case (generally made of aluminium),
and dip in oil, which they splash up into the cylinder to keep the
piston well lubricated. The plate, P P, through a slot in which the
piston rod works, prevents an excess of oil being flung up. Channels are
provided for leading oil into the bearings. The cranks are 180 deg. apart.
While one piston is being driven out by an explosion, the other is
compressing its charge prior to ignition, so that the one action deadens
the other. Therefore two explosions occur in one revolution of the
cranks, and none during the next revolution. If both cranks were in
line, the pistons would move together, giving one explosion each
revolution.

[Illustration: FIG. 43.--Showing how the water which cools the cylinders
is circulated.]

The valve seats, and the inlet and exhaust pipes, are seen in section.
The inlet valve here works automatically, being pulled in by suction;
but on many engines--on all powerful engines--the inlet, like the
exhaust valve, is lifted by a cam, lest it should stick or work
irregularly. Three dotted circles show A, a cog on the crank shaft; B, a
"lay" cog, which transmits motion to C, on a short shaft rotating the
cam that lifts the exhaust valve. C, having twice as many teeth as A,
revolves at half its rate. This ensures that the valve shall be lifted
only once in