XII.—FUNDAMENTAL PRINCIPLES OF FLIGHT.

In contemplating the boundless atmosphere, we perceive it to be tenanted by a multitude of creatures of varied form and size, who move and direct themselves with marvellous ease and skill. These beings, so different in their nature, form and construction—from the proud eagle to the "blood-thirsty" mosquito—resemble one another in the possession of three important fundamental principles which constitute the power of flight. These are—weight or gravity, surface or resistance of the atmosphere against it, and force or power of projection.

The medium in which the phenomenon of flight is produced—the air—is an invisible, impalpable, comparatively imponderable fluid, and its density is nearly 800 times less than that of water. Hence a movement through it can be made far more rapidly than through its sister medium. Nevertheless, if agitated, it is capable of exerting great pressure, as the tempestuous storms, overturning fences, unroofing houses, uprooting trees, and carrying even large animals into the air, teach us. Hereon then, that is, the proper manipulation principally in creating artificial currents of air, hinges the secret of flight, because this phenomenon is reproduced in a manner identical, if a surface is moved against it, as we see in the wings of flying creatures.