FLIGHT 265 
for the same reason as a sheet of paper which is allowed to fall 
tends to describe curves with their concavities upwards, so also 
does a gliding bird tend to rise in the air whenever its velocity is 
increased. This tendency the bird can counteract in various ways. 
It can, in the first place, change the position of its centre of 
gravity forwards in relation to the position of its centre of support 
by the outstretched wings. It can do this in the case of such a 
bird as a Heron by extending its neck, which is usually bent with 
the head retracted, but which, when the bird strives to fly fast, is 
stretched out forward to the full extent. In the case of most 
birds, however, the short neck does not allow of this means of 
moving forward or backward of the centre of gravity, and what the 
bird does is to move forward or backward the extended wings. 
As a matter of fact, it had been noted long before the true reason 
Fic. 3. (From Marey.) 
was understood that birds which glide slowly (Fig. 3) had their 
wings much further forward than the partially flexed wings of 
birds which — glide 
rapidly (Fig. 4) 
through the air.? 
A similar effect is 
obtained by spread- 
ing out the tail- 
feathers, which 
moves back the cen- 
tre of the plane of 
suspension formed 
by the wings, body, Fic. 4. (From Marey.) 
and tail, thereby 
relatively advancing the centre of gravity of the bird. Change 
of direction, upward or downward, can in this way be obtained 
1 This fact (that the point of maximum resistance is moved forward when a 
flat surface strikes a fluid, with at the same time a movement parallel with the 
plane of the surface) seems to us well fitted to explain why it is that the shafts of 
the primary wing-feathers, which during extension of the wing make a great angle 
