The lungs of birds have several openings, communicating with corresponding air-bags or cells, which fill the whole cavity of the body from the neck downwards, and into which the air passes and re-passes in the process of breathing. This is not all: the very bones of birds are hollowed out with the design of receiving air from the lungs, from which air-pipes are conveyed to the most solid parts of the body, and even into the quills and plumelets of the feathers, which are hollow or spongy for its reception. As all these hollow parts, as well as the cells, are only open on the side communicating with the lungs, the bird requires only to take in a full breath to fill and distend its whole body with air, which, in consequence of the considerable heat of its body, is rendered much lighter than the air of the atmosphere. By forcing this air out of the body again, the weight becomes so much increased, that birds of large size can dart down from great heights in the air with astonishing velocity. The structure of insects is not a little analogous.—_Ins. Trans._ AIR-GUN, _s._ A species of gun charged with air instead of powder. [Illustration] The _air-gun_, although long known, has never been much used, as its slight report is the only advantage it can possess over fire-arms, while its danger and inefficiency are admitted. Fatal accidents have occurred in charging it, and every attempt to remedy the danger has been unsuccessful. In the old principle, the globe or ball which forms the receiver has frequently blown up while the air was being pumped in, and loss of life or severe injury resulted. Latterly, the attention of several experimentalists has been directed to its improvement. One of them, Mr. Laing, of the Haymarket, imagined he had overcome the great objection; and by a very ingenious alteration, in which the globe was done away with, and the air-receiver transferred to the stock of the gun, he hoped thus to have removed the danger. But in this he was sadly disappointed. While trying the improved air-gun, the receiver burst, injured him severely, and induced him to abandon any farther experiments with this dangerous instrument. Indeed, the manifest inferiority of the air-gun to the rifle, would, independently of its danger, prevent it from coming into general use. As an instrument, it is extremely troublesome to shoot with, and liable to constant derangement. Its power is very weak, and in accuracy of aim the rifle is far superior. Even the advantage of slight report in the discharge is now comparatively done away with, in consequence of the recent introduction of the small-bore rifles for rook and rabbit shooting; which, from the trifling quantity of powder used in charging, are scarcely louder than the air-gun.