to use the spirit near a naked light.
Many motor-cars have been set on fire through the petrol leaking out of
the tank in which it is carried.

The tank which contains the petrol is placed under one of the seats of
the motor-car, or at the rear; if in use on a motor-cycle it is arranged
along the top bar of the frame, just in front of the driver. This tank
is connected to the "carburettor", a little vessel having a small nozzle
projecting upwards in its centre. The petrol trickles from the tank into
the carburettor, and is kept at a constant level by means of a float
which acts in a very similar way to the ballcock of a water cistern.

The carburettor is connected to the cylinder of the engine by another
pipe, and there is valve which is opened by the engine itself and is
closed by a spring. By an ingenious contrivance the valve is opened when
the piston moves out of the cylinder, and a vacuum is created behind it
and in the carburettor. This carries a fine spray of petrol to be sucked
up through the nozzle. Air is also sucked into the carburettor, and the
mixture of air and petrol spray produces an inflammable vapour which is
drawn straight into the cylinder of the engine.

As soon as the piston moves back, the inlet valve is automatically
closed and the vapour is compressed into the top of the cylinder. This
is exploded by an electric spark, which is passed between two points
inside the cylinder, and the force of the explosion drives the piston
outwards again. On its return the "exhaust" or burnt gases are driven
out through another valve, known as the "exhaust" valve.

Whether the engine has two, four, or six cylinders, the car is propelled
in a similar way for all the pistons assist in turning one shaft, called
the engine shaft, which runs along the centre of the car to the back
axle.

The rapid explosions in the cylinder produce great heat, and the
cylinders are kept cool by circulating water round them. When the water
has become very hot it passes through a number of pipes, called the
"radiator", placed in front of the car; the cold air rushing between the
coils cools the water, so that it can be used over and over again.

No water is needed for the engine of a motor cycle. You will notice that
the cylinders are enclosed by wide rings of metal, and these rings are
quite sufficient to radiate the heat as quickly as it is generated.

CHAPTER XXII. The Aeroplane Engine

We have seen that a very important