FLIGHT OF ANIMALS — GRAY 293 



wings makes with its own direction of motion. The speed of the glide, 

 on the other hand, depends partly on the weight of the bird and partly 

 on the size of the wings. A heavy bird with small wings for its size 

 glides rapidly ; a light bird with large wings glides slowly. 



The distance that can be traveled by a bird when it is gliding 

 through still air under its own weight is, of course, limited by the 

 height from which the bird starts — sooner or later it must reach 

 earth and the glide will come to an end. But if the air is not still, 

 but is itself moving over the earth, the story is very different. The 

 lift and drag forces exerted by the wings depend solely on the partic- 

 ular motion of the wings through air; if the air itself is moving, the 

 motion of the bird relative to the earth is the resultant of these two 

 motions: the motion of the bird through the air, and the motion of 

 the air itself over the ground. For example, if a bird is gliding down 

 an air slope losing vertical height at a rate of 5 feet per second, and the 

 air itself at the same time is rising from the ground at 5 feet per second, 

 then the bird will glide along level above the ground, although it is 

 all the time traveling downward through the rising air (fig. 5). And 

 if the air is rising from the ground faster than the bird is losing 

 height through the air, the bird will keep on getting higher above the 

 ground although it makes no wing movement to do so. A horizontal 

 wind cannot affect the rate at which a bird loses height above the 

 earth — such a wind affects only the rate at which the bird moves 

 along horizontally over the ground during its fall. What will happen 

 then, when the wind is blowing backward and upward at exactly the 

 same speeds as a bird is moving forward and downward along its 

 cushion of air? Seen from the ground then, the bird remains fixed 

 in space (fig. 6) . In such conditions a bird is not unlike a man who 

 is walking down a moving stairway at the same speed as the stairway 

 itself is moving upward. 



The extent to which upward currents of air may account for the 

 behavior of gliding birds can be judged by watching birds at flight in 

 regions where upward air currents are known to exist. There are 

 two main causes of upward air currents : first, when a horizontal wind 

 meets an obstruction and is deflected upward ; and second, where air, 

 warmed by the surface of the earth, moves upward and is replaced 

 by colder air from high levels of the atmosphere. Typical cases of 

 gliding on upcurrents caused by obstructions may be seen when swifts 

 glide along the eaves on the windward side of a building ; or when gulls 

 glide up and down a line of cliffs when the wind is blowing onshore; 

 or when eagles and buzzards glide on the windward side of mountains. 

 When a strong headwind is deflected upward by a mountainside, 

 buzzards may hang on the air, fixed in space high above the earth for 

 surprising lengths of time. 



