You will notice that the pilot's seat is fixed on the lower
plane, and almost in the centre of it, while close by the engine
is mounted. Alongside the engine is a radiator which cools the
water that has passed round the cylinder of the engine in order
to prevent them from becoming overheated.

Above the lower plane is a similar plane arranged parallel to it,
and the two are connected by light upright posts of hickory wood
known as STRUTS. Such an aeroplane as this, which is equipped
with two main planes, known as a BIPLANE. Other types of
air-craft are the MONOPLANE, possessing one main plane, and the
TRIPLANE, consisting of three planes. No practical machine has
been built with more than three main planes; indeed, the triplane
is now almost obsolete.

The Wrights fitted their machine with two long-bladed wooden
screws, or propellers, which by means of chains and
sprocket-wheels, very like those of a bicycle, were driven by the
engine, whose speed was about 1200 revolutions a minute. The
first motor engine used by these clever pioneers had four
cylinders, and developed about 20 horsepower. Nowadays engines
are produced which develop more than five times that power.

In later machines one propeller is generally thought to be
sufficient; in fact many constructors believe that there is
danger in a two-propeller machine, for if one propeller got
broken, the other propeller, working at full speed, would
probably overturn the machine before the pilot could cut off his
engine.