body, like the tiles of a house; and are arranged from the fore-part backwards, by which the animals are enabled the more conveniently to cut their way through the air. For the purpose of giving warmth to the body, a short and extremely soft down fills up all the vacant spaces between the shafts of the feathers. Their elevation from the earth is also aided by their bones being hollow, and very light compara tively with those of terrestrial animals. That they may the more easily make their way through the air, the head is small and the bill somewhat wedgeshaped. The neck is long, and easily moveable in all directions; and the body is slender, sharp on the under side, and flat or round on the back.

They urge themselves forward in the air by means of wings. These are so constructed, that in striking downwards they expand very greatly; and, except that they are somewhat hollow on the under side, they become, in this act, almost two planes. The muscles that move the wings downwards are exceedingly large; and have been estimated, in some instances, to constitute not less than the sixth part of the weight of the whole body. When a bird is on the ground, and intends to fly, he takes a leap, stretches his wings from the body, and strikes them downwards with great force. By this stroke they are put into an oblique direction, partly upwards and partly horizontally forwards. That part of the force tending upwards is destroyed by the weight of the bird; and the horizontal force serves to carry him forwards. The stroke being completed, he moves up his wings; which being contracted, and having

their edges turned upwards, meet with very little resistance from the air. When they are sufficiently elevated, he takes a second stroke downwards, and the impulse of the air again moves him forward. These successive strokes act only as so many leaps taken in air. When the bird wants to turn to the right or left, he strikes strongly with the opposite wing, which impels him to the proper side. The tail acts like the rudder of a ship; except that it moves him upwards or downwards instead of sideways. If the bird wants to rise, he raises his tail; and if to fall, he depresses it: whilst he is in an horizontal position, it keeps him steady.

A bird, by spreading his wings, can continue to move horizontally in the air for some time, without striking; because he has acquired a sufficient velocity, and his wings, being parallel to the horizon, meet with but small resistance; and when he begins to fall, he can easily steer himself upwards by his tail, till the motion he had acquired is nearly spent, when he must renew it by two or three more strokes of his wings. On alighting, he expands his wings and tail full against the air, that they may meet with all the resistance possible.

The centre of gravity in birds is somewhat behind the wings; and, to counterbalance it, most of them may be observed to thrust out their head and neck in flying. This is very apparent in the flight of Ducks, Geese, and several other kinds of water-fowl, whose centre of gravity is farther backwards than in the land birds. In the Heron, on the contrary, whose long head and neck, although folded up in flight,

overbalance the rest of the body, the long legs are extended, in order to give the proper counterpoise, and to supply what is wanting from the shortness of the tail.

Somewhat more than a century ago, many attempts were made to enable man to raise himself into the air by means of artificial wings. This idea, however, was in the highest degree absurd. The pectoral muscles in man are vastly too weak for the purpose, being not a sixtieth part of the muscles of the body; while those of a bird are equal, if not greater than all the others put together. In addition to this circumstance, the centre of gravity in man is so situated, that, allowing him to have sufficient power in his pectoral muscles, he would still never be able to make his way through the air, for his body would always assume an upright position.

The feathers of birds would perpetually imbibe the moisture of the atmosphere; and during rain absorb so much wet, as would almost, if not wholly, impede their flight; had not the wise economy of nature obviated this by a most effectual expedient.They are each furnished on their rump with two glands, in which a quantity of unctuous matter is constantly secreting. This is occasionally pressed out by means of the bill, and used for the lubrication of the feathers. The birds that share, as it were, the habitations of man, and live principally under cover, do not require so great a supply of this fluid; and therefore are not provided with so large a stock as those that rove abroad, and reside in the open clement. It is on this account that poultry, when wet,

make the ruffled and uncomfortable appearance that we observe.

As these animals are continually passing among hedges and thickets, they are provided, for the defence of their eyes from external injuries, as well as from too much light when flying in opposition to the rays of the sun, with a nictitating or winking membrane, which can at pleasure be drawn over the whole eye like a curtain. This covering is neither opake nor wholly pellucid, but is somewhat transparent; and it is by means of this that the Eagle is said to gaze at the sun. In Birds we find that the sight is much more piercing, extensive, and exact, than in the other orders of animals. The eye is much larger in proportion to the bulk of the head, than in any of these. This is a superiority conferred upon thein not without a corresponding utility; it seems even indispensable to their safety and subsistence. Were this organ in birds dull, or in the least degree opake, from the rapidity of their motion they would be in danger of striking against various objects in their flight. In this case their celerity, instead of being an advantage, would become an evil, and their flight must be restrained by the danger resulting from it. Indeed, we may consider the velocity with which an animal moves, as a sure indication of the perfection of its vision. Among the quadrupeds, the Sloth has its sight greatly limited; whilst the Hawk, as it hovers in the air, can espy a Lark sitting on a clod, perhaps at twenty times the distance at which a man or a dog could perceive it.

Birds respire by means of air-vessels, that are ex

tended through the whole body, and adhere to the under-surface of the bones. These, by their motion, force the air through the true lungs, which are very small, somewhat of the shape of the human lungs, and are seated in the uppermost part of the chest, and closely braced down to the back and ribs. The lungs, which are never expanded by air, are destined for the sole purpose of oxidating the blood. Mr. John Hunter made a variety of experiments to discover the use of this general diffusion of air through the bodies of birds and from these he found, that it prevents their respiration from being stopped or interrupted by the rapidity of their motion through a resisting medium. The resistance of the air increases in proportion to the celerity of the motion; and were it possible for a man to move with a swiftness equal to that of a Swallow, the resistance of the air, as he is not provided with reservoirs similar to those of birds, would soon suffocate him.

The abode of these tribes is very various; for they inhabit every corner of the world, from the hottest to the coldest regions. Some species are confined to particular countries; others are widely dispersed ; and many change their abode at certain seasons of the year, and migrate to climates better suited to their temperament or mode of life, for a certain period, than those which they leave. Many of the birds of our own island, directed by a peculiar and unerring instinct, retire, before the commencement of the cold season, to the southern parts of Africa, and again return in the spring. The causes usually assigned for migration are, either a defect of food, or the want