the progressive motion of a bird or other winged animal through the air.

Artificial Flying. Attempts have been made by various persons in all ages to imitate the motions of birds in the air by means of mechanical contrivances, but hitherto with very little success—the balloon being the only known machine that can sustain the body of a man in the air for any considerable length of time. Friar Bacon tells us that he knew how to make a machine by which a man might convey himself through the air like a bird; and he adds, that a person had tried it with success. The secret consisted in a couple of large thin hollow copper globes, exhausted of air; which being much lighter than common air, would sustain a chair on which a person might sit and be buoyed along. Father Francisco Lana, in his Prodromo, proposes the same thing. He computes that a round vessel of plate-brass, fourteen feet in diameter, weighing three ounces the square foot, will only weigh 1848 ounces, whereas a quantity of air of the same bulk will weigh 2155½ ounces, so that the globe will not only be sustained in the air, but will carry with it a weight of 307½ ounces; and by increasing the bulk of the globe without increasing the thickness of the metal, he conceives that a vessel might be made to carry a much greater weight. But the fallacy is obvious. A globe of the dimensions he describes, as shown by Dr Hook, would not sustain the pressure of the air, but be crushed inwards. Besides, in whatever ratio the bulk of the globe is increased, in the same ratio must the thickness of the metal, and consequently the weight, be increased; so that there would be no advantage in such augmentation.

The philosophers of Charles II.'s time were much busied about this art. The celebrated Bishop Wilkins was so confident of success in the attempt, that he says he does not question but in future ages it will be as usual to hear a man call for his wings when he is going a journey as it is now to call for his boots. Among such contrivances may be noticed another, consisting of huge wings, the cavities of which are to be filled with hydrogen gas, and which the flying man is to move with his arms. It is almost needless to observe that this machine is quite incapable of effecting the object in view, as must be every other constructed upon the same principle.

The flying apparatus constructed by Sir George Cayley can scarcely be considered as a successful experiment, since the wings of that ingenious mechanician acted rather on the principle of the parachute, merely floating the experimenter, who started from a moderate elevation, by a very gradual descent towards the earth.