ll over the country. Mr. Rolls
made a tour of the country in a motor-car in 1895, with the double
object of impressing people with the stupidity of the law with regard
to locomotion, and of illustrating the practical possibilities of the
motor. You may know that Mr. Rolls was the first man to fly across the
Channel, and back again to Dover, without once alighting.

CHAPTER XXI. The Internal-combustion Engine(Cont.)

I suppose many of my readers are quite familiar with the working of a
steam-engine. Probably you have owned models of steam-engines right
from your earliest youth, and there are few boys who do not know how the
railway engine works.

But though you may be quite familiar with the mechanism of this engine,
it does not follow that you know how the petrol engine works, for the
two are highly dissimilar. It is well, therefore, that we include a
short description of the internal-combustion engine such as is applied
to motor-cars, for then we shall be able to understand the principles of
the aeroplane engine.

At present petrol is the chief fuel used for the motor engine. Numerous
experiments have been tried with other fuels, such as benzine, but
petrol yields the best results.

Petrol is distilled from oil which comes from wells bored deep down
in the ground in Pennsylvania, in the south of Russia, in Burma, and
elsewhere. Also it is distilled in Scotland from oil shale, from which
paraffin oil and wax and similar substances are produced. When the oil
is brought to the surface it contains many impurities, and in its native
form is unsuitable for motor engines. The crude oil is composed of a
number of different kinds of oil; some being light and clear, others
heavy and thick.

To purify the oil it is placed in a large metal vessel or "still". Steam
is first passed over the oil in the still, and this changes the lightest
of the oils into vapours. These vapours are sent through a series of
pipes surrounded with cold water, where they are cooled and become
liquid again. Petrol is a mixture of these lighter products of the oil.

If petrol be placed in the air it readily turns into a vapour, and this
vapour is extremely inflammable. For this reason petrol is always kept
in sealed tins, and very large quantities are not allowed to be stored
near large towns. The greatest care has to be exercised in the use of
this "unsafe" spirit. For example, it is most dangerous to smoke when
filling a tank with petrol, or