FLIGHT. Flight stridtly speaking, is-prog ress. through the air in any desired direction by any agent or object heavier than the air, opposed to the• floating of an object lighter than tile air, such as a balloon. Animals accomplish flight mainly by means of wings, which may be special organs as in 'the insects, or modified fore legs, as, in the caw of bats and birds. The wings of. insects are always thin membranes supported by a stiffer framework. In bats they consist of extensible membranes stretched over the immensely elongated fingers and joined also to• the sides of the body and the hind legs. The wings of birds are composed of long stiffened feathers attached to the rudimentary fingers and to the bones of the forearm; the individual feathers overlapping one another so as to form a continuous more or less attheci surface The tail feathers constitate another sail area which plays an important part•n flight, while in many bats a continuation of the flight-membrane be tween the hind legs has much the same func tion.
Besides these special organs the structure of aerial creatures is modified in many other re spects with a view to facilitating flight. Thus the hollow bones of birds are lighter than the solid bones of terrestrial animals and relieve birds of much unnecessary weight; while the general shape of the bird's body and the smooth rounded contour of its feather covering are calculated to give the least possible resistance to the air during flight Birds are also pro vided with large distensible air sacs under the skin, the function of which has been a matter of much dispute, but •which, as suggested by von Leudenfeld, may.aid in shifting the centre gravity of the body, a matter of the utmost importance to the bird,in holding to its course or in turning in flight The centre of gravity, we may, note, is also kept well below the plane of the wings by the distribution of the heavy parts of the body on the pectoral side: So that as the bird flies the heavy breast muscles, which con stitute the bulk of its weight, are well down while the wings are attached to the back of the thorax. Thus the body becomes essentially a
weight hung directly'below an outstretched sail area.
So much for the, general structure of flying creatures. ' Turning now to the principles of flight we have as the main agencies (1) the mus cillar propulsion on the part of the bird; (2) support offered to the °sail by the air; (3) the force of the wind, both in raising and in propelling the bird. Mechanical and Physi cal•problems are involved in the study of flight which it will be impossible to consider here, but one ortwo principles may be mentioned which atle of fundamental and the appli cation of which may be seen in all forms of flight. We know that the resistance (that is, support) of the air to a flat surface is greatly increased When the surface 'is traveling rapidly through the air in its own plane, and it follows that when a bird is once on the wing the buoy aney of the air relative to it is greatly height ened; and having gained a certain momentum be possible for the bird' to sail some dis tance without loss of altitude. Again when a flat surface is sailing diagonally through the air the maximum pressure (that is, support) of the 'air is not under the centre of the surface, but under the forward' edge. It is on this prin ciple•that we 'find a tendency to force up the anterior edge of the 'wings of a sailing bird and in order to counteract it the bird is com pelled to shift the centre of gravity of its body farther forward. To do this it may draw the wings• back, spread the tail or perhaps extend the head and neck On the same principle to turn the direction of flight it is 'merely neces sary to flex or raise one wing or one side of the tail. Flight proper may be of two sorts, (1) by flapping the wings, and (2) by soaring. The former is the most familiar and is practised by all birds, while the latter is possible only for birds of large expanse of wing, such as vul tures, eagles, gulls, etc.