two revolutions of the crank shaft to which it is geared.
The cogs are timed, or arranged, so that the cam begins to lift the
valve when the piston has made about seven-eighths of its explosion
stroke, and closes the valve at the end of the exhaust stroke.

THE CARBURETTER.

A motor car generally uses petrol as its fuel. Petrol is one of the more
volatile products of petroleum, and has a specific gravity of about
680--that is, volume for volume, its weight is to that of water in the
proportion of 680 to 1,000. It is extremely dangerous, as it gives off
an inflammable gas at ordinary temperatures. Benzine, which we use to
clean clothes, is practically the same as petrol, and should be treated
with equal care. The function of a _carburetter_ is to reduce petrol to
a very fine spray and mix it with a due quantity of air. The device
consists of two main parts (Fig. 44)--the _float chamber_ and the _jet
chamber_. In the former is a contrivance for regulating the petrol
supply. A float--a cork, or air-tight metal box--is arranged to move
freely up and down the stem of a needle-valve, which closes the inlet
from the tank. At the bottom of the chamber are two pivoted levers, W W,
which, when the float rests on them, tip up and lift the valve. Petrol
flows in and raises the float. This allows the valve to sink and cut off
the supply. If the valve is a good fit and the float is of the correct
weight, the petrol will never rise higher than the tip of the jet G.

[Illustration: FIG. 44.--Section of a carburetter.]

The suction of the engine makes petrol spirt through the jet (which has
a very small hole in its end) and atomize itself against a
spraying-cone, A. It then passes to the engine inlet pipe through a
number of openings, after mixing with air entering from below. An extra
air inlet, controllable by the driver, is generally added, unless the
carburetter be of a type which automatically maintains constant
proportions of air and vapour. The jet chamber is often surrounded by a
jacket, through which part of the hot exhaust gases circulate. In cold
weather especially this is a valuable aid to vaporization.

[Illustration: FIG. 45.--Sketch of the electrical ignition arrangements
on a motor car.]

IGNITION OF THE CHARGE.

All petrol-cars now use electrical ignition. There are two main
systems--(1) by an accumulator and induction coil; (2) _magneto
ignition_, by means of a small dynamo driven by the engine. A general
a