So far we have confined ourselves to the parts of a cannon, but now
that we are going to take up the study of the motor in its details let
us call them by their regular names. The _barrel_ of the cannon we will
call a _cylinder_. In an actual motor a cylinder is made out of cast
iron, carefully bored out inside, so that the hole is perfectly round,
and the sides of the wall as smooth as possible.
[Illustration: Fig. 9.]
You will realize that this is necessary as we want to reduce, as much
as possible, any rubbing or friction, as it is called, between the
piston and cylinder walls. Next we must provide some means of cooling
these walls, as you know that the continuous firing would soon make
them very hot. This is done by surrounding the cylinder with what is
known as a _water jacket_ through which water can be circulated,
thereby carrying off the heat, and keeping the iron from getting red
hot. We must also cut two holes in the side of the cylinder to make
places for the valves and a place for the spark plug.
[Illustration: Fig. 10--End view of Horizontal One-cylinder Motor,
showing piston, valves and valve mechanism.]
A cylinder is generally mounted on its side in a one cylinder engine,
and is set up on end when it is desired to use more than one.
Therefore, in a one cylinder motor you will notice that the piston
moves back and forth, whereas in a two-cylinder, four-cylinder or
six-cylinder type, the pistons move up and down. As far as the action
of the parts is concerned they work in exactly the same way, only that
the valve mechanism has to be changed somewhat.
The cylinder is bolted to a framework called the crank case which
furnishes a solid foundation upon which it can rest.
[Illustration]