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 manœuvering 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.
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.