AVIATORS 181 



The pelican has a regular air-pouch in his breast, 

 and every bone in his body is filled with air. In 

 many birds, however, only two bones are thus filled 

 with air. The air passed through the bones facili- 

 tates respiration under varied circumstances and 

 thus aids the aviator in flight. 



The aviator's wings are made in a wonderful 

 way: the feathers are arranged like shingles on a 

 roof, one overlapping the other. They are so 

 arched as to give a convex form to the upper part of 

 the wing. Dr. Brehm thus describes the bird's 

 movements: "By raising the pinion, the air is al- 

 lowed to pass between the feathers, while in its 

 descent they offer an insuperable resistance. This 

 partially explains the fact that a bird always either 

 rises with each stroke of the wing, or keeps at the 

 same level, and is never in the least depressed by 

 it. The forward movement is attributable to the 

 fact that all strokes of the pinion do not fall in a 

 perpendicular direction, but slope obliquely down- 

 ward from the front toward the back. By this 

 means the wing is so canted as not to present its 

 surface horizontally to the air on rising, but rather 

 to cut through with its edge. Moreover, the pres- 

 sure of the pinion downward is quite equal to four 

 times that of the upward stroke : this is proved by a 

 simple examination of the respective muscles. The 



