should stop
while the machine was in mid-air, a terrible disaster would happen. All
petrol engines may be described as fickle in their behaviour, and
so complicated is their structure that the best of them are given to
stopping without any warning. Aeroplane engines are far superior
in horse-power to those fitted to motorcars, and consequently their
structure is more intricate. But if an airman's engine suddenly stopped
there would be no reason whatever why he should tumble down head first
and break his neck. Strange to say, too, the higher he was flying the
safer he would be.

All machines have what is called a GLIDING ANGLE. When the designer
plans his machine he considers the distribution of the weight or the
engine, pilot and passengers, of the petrol, aeronautical instruments,
and planes, so that the aeroplane is built in such a manner that when
the engine stops, and the nose of the machine is turned downwards, the
aeroplane of its own accord takes up its gliding angle and glides to
earth.

Gliding angles vary in different machines. If the angle is one in
twelve, this would mean that if the glide wave commenced at a height of
1 mile, and continued in a straight line, the pilot would come to
earth 12 miles distant. We are all familiar with the gradients shown on
railways. There we see displayed on short sign-posts such notices as
"1 in 50", with the opposite arms of the post pointing upwards and
downwards. This, of course, means that the slope of the railway at that
particular place is 1 foot in a distance of 50 feet.

One in twelve may be described as the natural gradient which the machine
automatically makes when engine power is cut off. It will be evident why
it is safer for a pilot to fly, say, at four or five thousand feet high
than just over the tree-tops or the chimney-pots of towns. Suppose, for
example, the machine has a gliding angle of one in twelve, and that when
at an altitude of about a mile the engine should stop. We will assume
that at the time of the stoppage the pilot is over a forest where it is
quite impossible to land. Directly the engine stopped he would change
the angle of the elevating plane, so that the aeroplane would naturally
fall into its gliding angle. The craft would at once settle itself into
a forward and slightly downward glide; and the airman, from his point of
vantage, would be able to see the extent of the forest. We will assume
that the aeroplane is gliding in a northerly di