; a bird the breadth of whose wings should be greater than their
length could hardly fly at all. The wings of a flying machine are called
planes, or aerofoils; the length of the wing is called the span of the
plane; the breadth of the wing is called the chord of the plane. The
proportion of the span to the chord, that is, the proportion of the
length of the wing to its breadth, is called the 'aspect ratio' of the
plane; and a plane, or wing, that is long and narrow is said to have a
high aspect ratio. A higher aspect ratio than is found in any bird or
any flying machine would theoretically improve its powers of flight, but
the practicable span of the plane, or length of the wing, is limited by
the need for rigidity and strength. The albatross, nevertheless, the
king of soaring birds, has enormously long and narrow wings; and the
planes of some flying machines have an aspect ratio almost as high as
the slats of a Venetian blind.

The wings of a flying machine, it has been said, are called planes, but
they are not true planes. Like the wings of a bird, they are 'cambered',
that is to say, they curve upward from the leading edge and downward
again to the trailing edge. Some of the most valuable work contributed
by the laboratory to the science of flight has had for its object the
determination of the best form of camber, or curve of the plane. In the
result, that form of camber has been found to be best which attains its
maximum depth a little way only behind the leading edge, and gradually
becomes shallower towards the trailing edge. Such a form of curve
produces a comparatively smooth and untroubled partial vacuum above the
plane, just behind its leading edge, and this vacuum is the factor of
chief importance in the lift of the plane.

The above is a brief and rough statement of some principles of aviation
which have been ascertained by long experiment and the labour of many
minds. It is by experiment that flight has been achieved. The Newton
who shall reduce all the observed phenomena to a few broad and simple
laws is yet to come. A bird is simpler than an aeroplane in that its
wings both support it and drive it forward, whereas all aerial machines,
both those that are heavier than air and those that are lighter than
air, are at present driven forward by the thrust of an airscrew,
revolving at the rate of some twenty to thirty times a second.

There are only two kinds of flying machine, the lighter than air and the
he