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

The parts of birds chiefly concerned in flying are the wings and tail; by the former the bird sustains and wafts itself along, and by the latter it is assisted in ascending and descending, keeping its body poised and upright, and obviating vacillations. It is by the size and strength of the pectoral muscles that birds are so well disposed for quick, strong, and continued flying. These muscles, which in men are scarcely a seventieth part of the muscles of the body, in birds exceed and outweigh all the other muscles taken together; so that if it were possible for a man to fly, his wings must be so contrived and adapted that he may make use of his legs, and not his arms, in managing them.

The tail is supposed by Willoughby, Ray, and others, to be principally employed in steering and turning the body in the air, like a rudder; but Borelli has put it beyond all doubt that this is the least use of the tail, which is chiefly intended to assist the bird in its ascent and descent in the air, and to obviate the vacillations of the body and wings; for as to turning to this or that side, that is performed by the wings and inclination of the body, with but very little help from the tail. The flying of a bird is in fact quite a different thing from the rowing of a vessel. Birds do not vibrate their wings towards the tail, as oars are struck towards the stern, but waft them downwards; nor does the tail of the bird cut the air at right angles as the rudder does the water, but is disposed horizontally, and preserves the same situation whichever way the bird turns. In fact, as a vessel is turned on a centre of gravity to the right, by a brisk application of the oars to the left, so a bird, in beating the air with its right wing alone towards the tail, will turn its fore part to the left. Thus pigeons changing their course to the left, would labour it with their right wing, keeping the other almost at rest. Birds of a long neck alter their course by the inclination of their head and neck; which altering the course of gravity, enables the bird to proceed in a new direction.

Artificial Flying, that which is attempted by men, by means of mechanical contrivances.

The art of flying has been attempted by various persons in all ages. The superstitious Leucadians are reported to have had a custom of precipitating a man from a high cliff into the sea, first fixing feathers, variously expanded, round his body, in order to break his fall.

Friar Bacon not only affirms that the art of flying is possible, but assures us that he himself knew how to make a machine in which a man sitting might be able to convey himself through the air like a bird; and he further adds, that there was then a person who 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 2135½ ounces; so that the globe will not only be sustained in the air, but will carry with it a weight of 378½ 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 were 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 the reign of Charles II. were mightily 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.