FLIGHT OF ANIMALS — GRAY 291 



stretching the membrane out into a taut and effective wing (pi. 4, 

 fig. 2, and text fig. 3). All these animals live in trees; they climb a 

 tree, and from there, launching themselves into the air, glide grace- 

 fully down, often rising toward the end of their flight and landing 

 on a neighboring tree trunk or branch. 



All the flying animals we have examined so far have solved the 

 problem of gliding flight only to the extent of being able to stay in 

 the air for a few seconds. How do some birds glide or soar as they 

 do for many minutes or even hours at a time ? 



Let us start with the type of glide seen when a pheasant or a gull 

 is approaching the ground; the wings are stiffly stretched out and 

 the bird is slowly losing height all the time. Its speed through the 

 air is being maintained by the accelerating effect of gravity, and its 

 motion can be likened to that of a sled traveling down a slope. On 

 a slope, the weight of a sled acts partly down along the slope, and 

 partly downward at right angles to the slope; the force acting down- 

 ward at right angles is met by an equal but opposite reaction from the 

 slope itself, but the force acting down along the slope speeds up the 

 sled and overcomes the friction between the ruimers and the slope. 

 Call the force acting down the slope the driving force, and the force 

 acting at right angles to the slope the sinking force. How much 

 bigger or smaller one of these two forces is than the other depends, of 

 course, on the angle of the slope. 



We can apply this picture to a gliding pheasant if we imagine each 

 of its wings to rest on a smooth rigid runner sloping downward toward 

 the earth (fig. 4) ; as with the sled, the weight of the bird would give a 

 driving component and a sinking component. As the bird began to 

 slide down the runners, the motion of its stiff wings through the air 

 would induce lifting forces against the wings, and because of this tiie 

 passive reaction from the rigid runners would decrease ; at the same 

 tune, the wings and body would be subjected to a drag force acting 

 along the line of the runners in a direction opposite to that of the 

 driving force of gravity. As the speed of glide increased, the lift 

 would also increase, until a time would come when the lift is equal to 

 the sinking force of gravity, and the drag force is equal to the driving 

 component. At this moment we might take away the rigid runners 

 without making any difference at all to the bird: it would go on glid- 

 ing through the air at constant speed. 



A good glider travels a long way horizontally with the smallest pos- 

 sible loss of height, and you can judge how good it is as a glider by 

 measuring the angle between the track of its motion and the level 

 horizon. It is very important to know that this angle does not depend 

 on the weight of the bird ; it depends solely on the ratio of the forces — 

 the lift force to the drag force — exerted by the air ; that is, it depends 

 on the shape of the wings and on the angle which the surface of the 



