The Mechanism which enables Birds to fly is singular and instructive. Their bodies are covered with feathers which are much lighter than the hair with which quadrupeds are usually covered; the feathers are so placed as to overlap each other, like the slates or tiles on the roof of a house; they are also arranged from the fore-part backwards, so as to enable the birds to cleave their way through the air more conveniently. Their skeleton is well adapted for aerial existence; the bones are hollow and extremely light in comparison with those of terrestrial animals, the bone tissue containing a much larger proportion of phosphate of lime. Moreover, the long bones contain air instead of marrow, and are filled with air by means of special apertures connected with air-cells supplied from the lungs. This greatly facilitates their rising from the earth; their heads are comparatively small, their bills are wedge-shaped, and their bodies are slender, sharp below and round above. With all these conditions, however, birds could not fly without wings. These are the instruments which confer the power of rapid locomotion. They are constructed in such a manner that they are capable of great expansion when struck in a downward direction; if, in this action, we except the slight hollow which occurs on the under side they almost become two planes. That the downward action may be accomplished to the necessary extent, the muscles which move the wings have been made exceedingly large; so large, indeed, that, in some instances, they have been estimated at not less than a sixth of the weight of the whole body. Therefore, when a bird is on the ground and intends to fly, it takes a leap, and immediately stretching its wings, strikes them out with great force. By this act these are brought into an oblique direction, being turned partly upwards and partly horizontally forwards. That part of the force which has the upward tendency is neutralized by the weight of the bird, whilst the horizontal force serves to carry it forward. The stroke being completed it moves upon its wings which, being contracted and having their edges turned upwards, obviate, in a great measure, the resistance of the air. When it is sufficiently elevated it makes a second stroke downwards, and the impulse of the air again moves it forward. These successive strokes may be regarded as so many leaps taken in the air. When the bird desires to direct its course to the right or left it strikes strongly with the opposite wing, which impels it to the proper side. The tail plays a prominent part in the movements of the bird, acting like the rudder of a ship, except that it moves upwards and downwards instead of sideways; if the bird wishes to rise, it raises its tail; it depresses it when it desires to descend; if it wishes to preserve a horizontal position it keeps its tail steady. For example, a pigeon or a crow will preserve a horizontal flight for some time without any apparent motion of the wings; this is accomplished by the bird having already acquired sufficient velocity and by its meeting with but small resistance from the atmosphere, owing to its wings being parallel to
