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 
H 
