266 FLIGHT 
by moving backwards or forwards the centre of gravity in rela- 
tion to the centre of suspension, and this does not involve any 
change in the angle which the under surface of the wings or 
tail makes with the line of flight. It involves, moreover, only 
slight muscular exertion, and therefore comparatively little ex- 
penditure of energy on the part of the bird in guiding its motion. 
By raising or lowering the expanded tail the direction can also be 
changed, but either of these movements tends to destroy equilibrium, 
and they do not seem to be used in ordinary gliding. In order 
to turn to the right or left a gliding bird has only to move its 
centre of gravity to one or other side of its centre of suspension. 
This it can do in a variety of ways ; for instance, by turning its head 
to one side, when, as was observed by Leonardo da Vinci, the 
course of the gliding turns in the same direction. A still more 
effective way is to change the centre of suspension by partially 
flexing one wing, which causes the bird to turn towards the same 
side. A similar effect is produced by raising one side of the 
extended tail and lowering the other. Any or all of these move- 
ments give the bird power to turn to either side. 
We should not have considered it necessary to say so much 
about the gliding flight of birds had it not been that what we have 
said on this mode of aerial locomotion is equally true when the 
bird moves by active strokes of the wings or by sailing. 
with the line of flight, are placed so much nearer the anterior margin of the 
feathers than is the case with the secondary feathers, which lie in a line more 
Fia. 5. (From Marey.) 
nearly parallel with the axis of flight, as is shewn in the annexed figure (Fig. 5) 
of a fully extended eagle’s wing. Were it otherwise, the outer or anterior vane 
would bend upwards more than the posterior vane and make forward movement, 
as a result either of the passive extension in gliding or the active down stroke, 
an impossibility. This explanation of the different shape of the primary and 
secondary feathers of the wing seems to have been hitherto overlooked. 
