404 The Outline of Science 



resistance of the air, and of the consequent upward tendency im- 

 parted to a body moving horizontally and having a flat inclined 

 under-surface. 



When we come to active flight a difference is at once obvious : 

 the aeroplane propellers supply a motive force independently of 

 the planes, while in the bird the wings are both propellers and 

 planes at the same time. There is, indeed, a further difference 

 in that the aeroplane's propellers, during level flight at least, 

 exert force purely in a horizontal direction, the lifting force being 

 wholly due, as in gliding, to air resistance. In the bird the wing- 

 strokes themselves supply part of the lifting power, as well as 

 propelling the body forwards. Nor must we forget the bird's 

 tail, which plays a part in steering and balancing as in the case of 

 the aeroplane rudder; it is also often used as a brake, without 

 which many a swiftly pouncing bird of prey would be apt to dash 

 itself to destruction on the ground. 



Some of the larger birds are adepts at soaring, and can re- 

 main in the air for a long time with motionless wings, and can even 

 rise in slow spiral ascent to a great height. The late Mr. F. W. 

 Headley, a keen and exact student of the flight of birds, came to 

 the conclusion that this feat was inexplicable except on the sup- 

 position that advantage was taken of up-currents in the air, the 

 bird's actual motion being merely a gliding one. He pointed out 

 that gulls are adepts at this when flying above the edge of a cliff, 

 but that they cannot do it at sea, where, as aviators and air 

 travellers know, there are not the vertical disturbances caused by 

 the varying ground-level temperature and by the changing 

 elevation of dry land. Another feat, namely hovering, is 

 familiar in the hunting methods of the Kestrel, which maintains 

 a stationary position for an appreciable time. Against a 

 strong wind it would be easy to maintain a ground speed of nil, 

 and it would be possible even with motionless wings. In still air, 

 however, the ordinary gliding basis of flight is in abeyance, and 

 altitude must be maintained by sheer vertical force of wing-stroke, 



