The Flight of Animals' 



By James Gray 



Fellow of King's College 

 Professor of Zoology in the University of Cambridge 



[With 10 plates] 



If a kangaroo, say, or a frog, or a flea, or any land-living animal is 

 to lift its body off the gi'ound at all and stay in the air even for a very 

 short time, we know that it has to exert an intense muscular effort to do 

 so. Then how can a bird rise so easily from the ground and stay in the 

 air in flight for hours at a time ? 



Flight depends on wings. A wing, we might say, is a limb whose 

 movement through the air produces forces that can counteract the 

 downward pull of gravity and can also drive the body forward 

 through the air. It has long been known that wings can do these 

 things, and from time to time adventurous people have tried to design 

 mechanical wings capable of lifting a man and carrying him along 

 through the air. All these experiments failed; but in the end, for 

 good or ill, they led on to the invention of airplanes. Men tried for a 

 long time, by watching the birds, to learn how a man could fly ; today, 

 quite the opposite, we are trying to understand the flight of birds by 

 applying principles which have emerged during the design of air- 

 planes. The movements of birds' or insects' wings are extremely 

 complicated, and it is easier to feel our way into the very difficult 

 problem of animal flight by drawing a distinction between two kinds 

 of flight — active flapping flight, and passive gliding flight. 



We can start our inquiry, then, by comparing the motion of a soaring 

 eagle with that of a "glider" airplane ; in both, the wings are used as 

 fixed and rigid surfaces, and neither glider nor eagle uses an internal 

 engine or source of power. From the very start of our study we must 

 realize that all flight — whether active or gliding — depends on forces 

 set up between the wing and the surrounding air. In a vacuum, an 

 airplane or a bird would fall to the ground just as rapidly as a stone. 



* Reprinted by permission from "How Animals Move" (The Royal Institution 

 Christmas Lectures 1951), published by the Cambridge University Press, Great 

 Britain. 



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