We are fond of comparing flying machines with birds, with fish,
and with ships: and there are useful analogies with all three. A
drifting balloon is like a becalmed ship or a dead fish. It moves at
the speed of the aerial fluid about it and the occupants perceive no
movement whatever. The earth's surface below appears to move in the
opposite direction to that in which the wind carries the balloon. With
a dirigible balloon or flying machine, the sensation is that of being
exposed to a violent wind, against which (by observation of landmarks)
we find that we progress. It is the same experience as that obtained
when standing in an exposed position on a steamship, and we wonder if a
bird or a fish gradually gets so accustomed to the opposing current as
to be unconscious of it. But in spite of jar of motors and machinery,
there is a freedom of movement, a detachment from earth-associations,
in air flight, that distinguishes it absolutely from the churning of a
powerful vessel through the waves.
[Illustration: VIEW FROM A BALLOON]
[Illustration: ANATOMY OF A BIRD'S WING
(From Walker's _Aerial Navigation_)]
[Illustration: FLIGHT OF A BIRD]
Birds fly in one of three ways. The most familiar bird flight is
by a rapid wing movement which has been called oar-like, but which
is precisely equivalent to the usual movement of the arms of a man
in swimming. The edge of the wing moves forward, cutting the air; on
the return stroke the leading edge is depressed so as to present a
nearly flat surface to the air and thus propel the bird forward. A
slight downward direction of this stroke serves to impel the flight
sufficiently upward to offset the effect of gravity. Any man can learn
to swim, but no man can fly, because neither in his muscular frame nor
by any device which he can attach thereto can he exert a sufficient
pressure to overcome his own weight against as imponderable a fluid
as air. If air were as heavy as water, instead of 700 times lighter,
it would be as easy to fly as to swim. The bird can fly because of
the great surface, powerful construction, and rapid movement of its
wings, in proportion to the weight of its body. But compared with
the rest of the animal kingdom, flying birds are all of small size.
Helmholz considered that the vulture represented the heaviest body that
could possibly be raised and kept aloft by the exercise of muscular
power, and it is understood that vultures have considerable difficulty
in ascending; so much so that unless in a position to take a short
preliminary run they are easily captured.
Every one has noticed a second type of bird flight--soaring. It is this
flight which is exactly imitated in a glider. An aeroplane differs
from a soaring bird only in that it carries with it a producer of
forward impetus--the propeller--so that the soaring flight may last
indefinitely: whereas a soaring bird gradually loses speed and descends.
[Illustration: IN A METEORIC SHOWER]
A third and rare type of bird flight has been called _sailing_. The
bird faces the wind, and with wings outspread and their forward edge
elevated rises while being forced backward under the action of the
breeze. As soon as the wind somewhat subsides, the bird turns and
_soars_ in the desired direction. Flight is thus accomplished without
muscular effort other than that necessary to properly incline the
wings and to make the turns. It is practicable only in squally winds,
and the birds which practice "sailing"--the albatross and frigate
bird--are those which live in the lower and more disturbed regions of
the atmosphere. This form of flight has been approximately imitated in
the man[oe]uvering of aeroplanes.
Comparison of flying machines and ships suggests many points of
difference. Water is a fluid of great density, with a definite upper
surface, on which marine structures naturally rest. A vessel in the air
may be at any elevation in the surrounding rarefied fluid, and great
attention is necessary to keep it at the elevation desired. The air has
no surface. The air ship is like a submarine--the dirigible balloon
of the sea--and perhaps rather more safe. An ordinary ship is only
partially immersed; the resistance of the fluid medium is exerted over
a portion only of its head end: but the submarine or the flying machine
is wholly exposed to this resistance. The submarine is subjected to
ocean currents of a very few miles per hour, at most; the currents to
which the flying machine may be exposed exceed a mile a minute. Put a
submarine in the Whirlpool Rapids at Niagara and you will have possible
air ship conditions.
A marine vessel may _tack_, _i.e._, may sail partially against the
wind that propels it, by skillful utilization of the resistance to
sidewise movement of the ship through the water: but the flying machine
is wholly immersed in a single fluid, and a head wind is nothing else
than a head wind, producing an absolute subtraction from the proper
speed of the vessel.
[Illustration: HOW A BOAT TACKS
The wind always exerts a pressure, perpendicular to the sail,
which tends to drift the boat sidewise (R) and also to propel
it forward (L). Sidewise movement is resisted by the hull.
An air ship cannot tack because there is no such resistance
to drift.]
Aerial navigation is thus a new art, particularly when heavier-than-air
machines are used. We have no heavier-than-water _ships_. The flying
machine must work out its own salvation.