VARIETIES OF WING AND OF FLIGHT 97 



wings with their rich red coverts, looks like a butter- 

 fly, and a butterfly of marvellous beauty. 



The reason that birds of larger build do not fly in 

 this style is, probably, as I have pointed out above, 

 that their rate of stroke being slower they lose 

 altitude while raising their wings, unless they have 

 considerable momentum. Their best policy, there- 

 fore, is to attain and maintain great pace, rather 

 than to gain altitude and then indulge in a slightly 

 downward glide. Moreover, if a big, bulky bird 

 were to set his body at a suitable incline for rising 

 almost vertically, as a Pigeon sometimes rises, his 

 wings, turning stiffly as they do at the shoulder- 

 joint, would not be able to beat in the right direction 

 and would drive him backward rather than lift him. 

 Some rather big birds do a good deal of gliding, but 

 it is a very different performance from that of the 

 small bird. They first gain altitude, not, however, 

 by only two or three strokes, as the small bird does, 

 but by a number. In fact they get up momentum 

 and then are lifted, as an aeroplane set at a slight 

 incline to the horizon is lifted when it is driven 

 rapidly forward. They will then glide onward with 

 wings outstretched to the full, so as to lose as little 

 altitude as possible. A wide spread of wing is the 

 thing needed, for, since it is the front part of the wing 

 on which the air mainly acts, the greater the front 

 presented the greater the supporting power. The 

 big bird's gliding is very unlike that of the small bird 

 that knows how easily by three or four strokes he 

 can recover elevation, and therefore quickens his 

 glide by the sacrifice of some of that which he 

 has already gained. Let us take as examples of 



