_The Body and Wings move in opposite Curves._--I have stated that the wing advances in a waved line, as shown at _a c e g i_ of fig. 81, p. 157; and similar remarks are to be made of the body as indicated at 1, 2, 3, 4, 5 of that figure. Thus, when the wing descends in the curved line _a c_, it elevates the body in a corresponding but minor curved line, as at 1, 2; when, on the other hand, the wing ascends in the curved line _c e_, the body descends in a corresponding but smaller curved line (2, 3), and so on _ad infinitum_. The undulations made by the body are so trifling when compared with those made by the wing, that they are apt to be overlooked. They are, however, deserving of attention, as they exercise an important influence on the undulations made by the wing; the body and wing swinging forward alternately, the one rising when the other is falling, and _vice versâ_. Flight may be regarded as the resultant of three forces:--the _muscular and elastic force_, residing in the wing, which causes the pinion to act as a true kite, both during the down and up strokes; the _weight of the body_, which becomes a force the instant the trunk is lifted from the ground, from its tendency to fall downwards and forwards; and _the recoil obtained from the air_ by the rapid action of the wing. These three forces may be said to be active and passive by turns. When a bird rises from the ground it runs for a short distance, or throws its body into the air by a sudden leap, the wings being simultaneously elevated. When the body is fairly off the ground, the wings are made to descend with great vigour, and by their action to continue the upward impulse secured by the preliminary run or leap. The body then falls in a curve downwards and forwards; the wings, partly by the fall of the body, partly by the reaction of the air on their under surface, and partly by the shortening of the elevator muscles and elastic ligaments, being placed above and to some extent behind the bird--in other words, elevated. The second down stroke is now given, and the wings again elevated as explained, and so on in endless succession; the body falling when the wings are being elevated, and _vice versâ_, (fig. 81, p. 157). When a long-winged oceanic bird rises from the sea, it uses the tips of its wings as levers for forcing the body up; the points of the pinions suffering no injury from being brought violently in contact with the water. A bird cannot be said to be flying until the trunk is swinging forward in space and taking part in the movement. The hawk, when fixed in the air over its quarry, is simply supporting itself. To fly, in the proper acceptation of the term, implies to support and propel. This constitutes the difference between a bird and a balloon. The bird can elevate _and carry itself forward_, the balloon can simply elevate itself, and must rise and fall in a straight line in the absence of currents. When the gannet throws