THE BIRD IN THE AIR. 87 



Like the swimmer he advances by a series of undulations, 

 a long incline upward (Fig. 19) when his wings press the air, a 

 little drop downward as he raises them to get them in position 

 once more. But the bird's stroke is different from the swim- 

 mer's. As soon as his wings are at their highest point, they 

 begin to turn forward and downward with a strong, even 

 sweep that lifts the body and carries it ahead. 



The air is driven backward less by the direction in which 

 the wing is moved than by the curvature of the under surface, 

 by its general shape, and by the rotary motion at the shoulder 

 joint. When it is necessary to recover for the next stroke, 

 see by the picture how neatly it is done. The wing bends at 

 the joint, leaving only half as great a resisting surface, the 



FIG. 19. GULLS FLYING 50 IMAGES PER SECOND. (AFTER MAREY.) 



(The line shows the centre of gravity in successive positions.) 



secondaries roll on edge, removing still more pressure; the 

 body drops a little by its own weight, and up flies the wing 

 into place so quickly that the camera can get but two pictures, 

 though it takes four of the descending stroke. Please notice 

 carefully that the wing-beat is a forward motion ; the tip of 

 the wing never drags far back ; even when it is ready to be 

 raised it is still on a line with the eye. The bird is always 

 reaching ahead to cut into air not yet disturbed by his own 

 movement. 



We know that the bird rises by the resistance of the air, 

 using his wings as levers and the air as a fulcrum. But how 



