FLIGHT OF ANIMALS — GRAY 287 



Tests witli various sizes and shapes of wings, in winds of various 

 speeds, have shown that the lifting effect of the air depends on four 

 main factors : the shape of the wing ; its size ; the angle at which it is 

 inclined to the direction of the wind; and the speed at which the 

 wind is traveling past it. 



The mechanical efficiency of a wing depends largely on its ability 

 to develop a large lifting force for a relatively small increase in back- 

 ward drag; but, from a practical point of view, the ability to fly de- 

 pends on the extent to which the exact balance can be maintained 

 between the lift and drag forces of the air on the one hand, and the pull 

 of gravity on the other. Stable, continuous flight can happen only 

 when the wings are so arranged that any slight accidental disturbance 

 of the airflow is automatically compensated. 



We can see these principles in action when a thin sheet of light wood 

 or a card is released in the air. A uniform sheet glides for a short 

 time toward one side, then changes its direction of motion and falls 

 to the ground like a falling leaf. But if a small lead weight is at- 

 tached to the leading edge, the sheet glides smoothly downward when 

 the size of the weight is so adjusted as to bring the center of gravity 

 of the whole wing to a point about twice as far from the trailing edge 

 as it is from the leading edge. We can, with a little practice, get the 

 same result — a smooth glide downward — by shaping the card itself 

 into the outline of a bird with outstretched wings and tail. 



We have now reached a point at which we can say that an animal 

 can fly, provided it keeps its wings moving through the air in such 

 a way as to ensure that the lift and drag forces exerted by the air 

 against the body all combine to form an upward force equal to the 

 animal's weight and acting through the center of gravity of its body- 

 Now let us look at some of Nature's gliders. 



Broadly speaking, gliding animals may be separated into two 

 classes: those that get themselves the necessary motion through the 

 air by their own muscular efforts beforehand, and those that keep 

 themselves moving by falling under gravity. A flying fish {Exocoetus 

 cypsilurus) is a good example of the first kind. Wliat is noticeable 

 about a flying fish is the enormous size of its pectoral fins. When 

 a flying fish is swimming it keeps those fins furled against its sides, 

 but spreads them wide as it leaves the water. Before the takeoff the 

 fish drives itself slantingly upward to the surface by vigorously swim- 

 ming with its tail ; as it leaves the water its pectoral fins (and sometimes 

 also the pelvic fins) are spread, and the fish finds itself airborne just 

 above the surface of the sea (pi. 1). 



According to most observers, flight seldom lasts for more than 1 

 or 2 seconds and the distance traveled in that time varies from 10-50 

 meters. From time to time the fish may regain flying speed by beating 



