FLIGHT 



26s 



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 Avith 

 the head retracted, biit which, Avhen 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 



Fig. 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, 

 and tail, thereby 

 relatively advancing the centre of gravity of the bird. Change 

 of direction, upward or downward, can in this way be obtained 



^ 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 



Fio. 4. (From Marey.) 



