Hilda saw the wistful look in Maud's eyes. "Let us go with Lady Taieri," she said; and so it was arranged.
VIII.
AIR-CRUISERS.
CHAPTER VIII. AIR-CRUISERS.
We trust our readers will not be wearied because it is necessary to give them at some length an explanation concerning the aerial machines to which reference has so often been made as air-cruisers. It need scarcely be said that from time immemorial a great deal of attention has been directed to the question whether aerial travelling could he made subservient to the purposes of man. Balloons, as they were called, made of strong fabrics filled with a gas lighter than air, were to some extent used, but rarely for practical purposes. They were in considerable request for military objects, and it is recorded that Gambetta managed to get out of Paris in a balloon when that city was beleaguered by the German army in 1871. The principle of the balloon was the use of a vessel which, weighing, with all its contents, less than a similar volume of the atmosphere, would consequently rise in the air. But evidently no great progress could be made with such an apparatus. The low specific gravity of the atmosphere forbade the hope of its being possible to carry a heavy weight in great quantity on a machine that depended for its buoyancy on a less specific gravity. Besides, there was danger in using a fabric because of its liability to irreparable destruction by the smallest puncture.
The question then was mooted, Could not an aerial machine be devised to work although of higher specific gravity than the air? Birds, it was argued, kept themselves afloat by the motion of their wings, although their weight was considerably greater than a similar volume of the air through which they travelled. This idea was pursued. The cheap production of aluminium, a strong but light metal, gave an impulse to the experiment; and it was at length proved quite satisfactorily that aerial travelling was practicable in vessels considerably heavier than the air, by the use of quickly revolving fans working in the directions that were found to be suitable to the progress of the vessel. But great power was required to make the fans revolve, and the machinery to yield great power was proportionately heavy. It was especially heavy if applied separately to a portion of the fans, whilst it was dangerous to rely on one set of machinery, since any accident to it would mean cessation of the movement of the whole of the fans and consequently instant destruction. It was considered that, for safety's sake, there should be at least three sets of fans, worked by separate machinery, and that any one set should be able to preserve sufficient buoyancy although the other two were disabled. But whilst it was easy to define the conditions of safety, it was not easy to give them effect. All applications of known engines, whether of steam, water gas, electricity, compressed air, or petroleum, were found to be too bulky; and although three sets of machines were considered necessary, one set only was generally used, and many accidents occurred in consequence. The aerial mode of travelling was much employed by the adventurous, but hundreds of people lost their lives annually.