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.