The _piston_ forms, as you will recall, the _bullet_ in the cannon,
which instead of leaving the barrel, was made to travel back and forth
inside of the cylinder under the action of the explosive gas. Owing to
the fact that a solid piece of iron would be very heavy and would get
very warm, the real piston used in a motor is made _hollow_ so that it
is merely a shell. Instead of fastening the rod to the end of it, a
small rod, called the _piston pin_ is in the center of it, and to this
the connecting rod is connected. Fig. 16 shows a section of the piston.
You will notice that the piston pin is kept from sliding sideways by a
bolt that is screwed into it.
[Illustration: Fig. 15--A Piston, Piston Ring, and Piston Pin.]
Owing to the fact that both the cylinder walls and piston get hot, and
that iron expands and contracts according to its temperature, it is
not possible to make a piston alone which would remain air-tight all
the time. Engineers, therefore, found it necessary to put _rings_,
which were cut at some point in their circumference, on the outside
of the piston itself. These _piston rings_, due to the fact that
they are cut, can accommodate themselves to the varying diameters of
the cylinder, and can therefore keep an air-tight fit, even when the
piston is moving back and forth all the time. Most of you, no doubt,
know that the plunger in a pump is made air-tight by one or a set of
leather washers, which, owing to their pliable structure, can expand or
contract so as to always fit air-tightly the pipe in the pump. Piston
rings work in precisely the same manner, and are always kept lubricated
so that they will work smoothly, thus doing away with any friction
which might result.
[Illustration: Fig. 16--A section of a Piston, showing location of
piston pin and end of connecting rods.]