Engineering Wonders of the World
Volume III

DIRIGIBLE BALLOONS. IN the minds of a good many persons there undoubtedly exists a confusion as re- gards the terms “ airships ” and “ flying machines.” That this should be so is some- _ . , what curious, as a little thought Terminology. ° must make it evident that a “ ship ” implies something that floats by virtue of its own buoyancy in the medium through which it moves; and the term airship, therefore, must apply only to the dirigible balloon. On the other hand, every living thing that flies is heavier than air, and supports itself only by the action of moving parts on the air. Hence the words “ flying machine ” obviously refer to contrivances which lift as well as propel themselves by the development of power. The airship has its counterpart in the submarine boat ; the flying machine may be compared to the hydroplane, which is sup- ported when moving at high speed by the resistance to water of more or less oblique horizontal surfaces, and not by buoyancy. If the atmosphere surrounding our globe were untroubled by currents, the dirigible balloon would have “ arrived ” many years ago. To make a cigar-shaped envelope, attach thereto a car, and provide motive power of some kind would not have presented very serious difficulties ; and the improvement of motors would have greatly increased the, at first, unavoidably low speeds. Unfortunately, from the point of view of the “ dirigible,” the air ocean has a constant motion, at times al- most imperceptible, at others terrifying in its velocity. Even the more gentle of the inter- mediate strengths of current have to be reckoned with. The resistance of the air to a large body moving through it demands that the shape of a dirigible should be considered carefully. A sphere has greater volume than a body of any other shape proportionately to its surface. But to drive a sphere through the atmosphere requires half the power needed to propel a circular plane of equal diameter flatways on ; and therefore a not suited for a hand, the more adopted, though an envelope that is heavy relatively to the volume of gas imprisoned. Its efficiency is, however, augmented by a general increase in dimensions—the proportions being constant— as tho doubling of surface area of the envelope far more than doubles the cubical contents. To consider for a moment the shape. Experiment has shown that a hemispherical Shape of Airships. spherical form is evidently “ dirigible.” On the other or less cigar-shaped form offering less resistance, has