MODES OF FLIGHT. Broadly speaking, there are two modes of flight: (1) By repeated strokes of the wings; and (2) by gliding or sailing with outstretched, almost motionless, wings; but al most every intermediate condition is to be met with. The humming-bird and albatross represent the extremes of these methods; the former moves the wings so rapidly that they are invisible, while the latter sails for long periods without a movement of its pinions. The common chim ney-swift is an excellent and familiar example of an intermediate state, vibrating its wings rapid ly for a few strokes and then sailing.
The first method calls for a rather short and strongly built wing and large muscles; the sec ond for a long wing and very little expenditure of muscular energy. The wings of the albatross are from 10 to 12 feet from tip to tip, and but nine inches broad, while the muscular power of the bird is so slight that it cannot rise from the water when gorged with food. It is of interest to note that the long, narrow wing is the one which, in theory as well as in practice. is best for sail ing. Contrary to what might be supposed, the support of the wing is stronger in sailing birds than in others, reaching the maximum of strength in the frigate-bird, which, though noted for its power of long-sustained flight, has very small muscles. Birds of prey exhibit a combination of the two methods of flapping and sailing, and their wing-museles are so powerful that they are able to carry away birds as heavy as themselves.
Birds that sail must obtain their initial veloc ity either by darting from some elevated place, by strokes of the wings, or by strokes of the wings aided by the 'wind, which they face when taking flight. (See below.) The albatross can
not rise from the (leek of a vessel, and when taking wing on a calm (lay is obliged to flap along the water for a considerable distance. Once in the air the sailing bird utilizes to the utmost the momentum first acquired, and it is probable that the ever-changing force and direction of the wind is an important factor in sailing, the bird instinctively making use of every variation in the currents of air. As bearing on this it may be said that the system of muscles by whieh the movements of the outer parts of the wing are controlled is mud] more complex in sailing than in flapping birds.
Insects resemble humming-birds in the com parative lack of flexibility in their wings and mode of flight, while, owing to the small size of the wings and the slight contractions of the muscles that drive them, they are moved with almost inconceivable rapidity, the smaller the wing the more frequent being its strokes. 'Thus, while the large moths and butterflies flutter slowly, the wings of the house-fly, according to Marcy, vibrate 330 times per sieond.
Bats, the only group of flying mammals, also fly entirely by means of wing-strokes, the fre quency of time stroke being in an inverse ratio to the size of the species, the small bats flutter ing, the large fruit-bats flapping their wings somewhat like a crow. The length and narrow ness of the wings of pterodactyls, and the small surface for muscular attachment, indicate that these creatures sailed or flew with infrequent wing-strokes, after the fashion of a chimney swift.