of Edinburgh, Session 1870 - 71 . 
339 
root corresponding to the short axis of rotation. (The long axis of 
rotation runs along the anterior margin of the wing.) 
The angles which the wing makes with the horizon are increased 
during the down stroke, and decreased during the up stroke, the 
posterior margin of the wing being screwed down upon the air 
during the down stroke to increase the elevating and propelling 
power of the wing, and unscrewed or withdrawn from the air during 
the up stroke to afford support, and assist in propulsion. 
The wing, in virtue of the variations of inclination of different 
parts of its surface, acts as a true kite during both the down and up 
strokes, i.e ., it flies down and up alternately in such a manner as to 
keep its ventral concave or biting surface always closely applied 
to the air. The wing is, therefore, effective during both the down 
and up strokes , so that it is a mistake to regard the down stroke as 
alone contributing to flight. In reality the down and up strokes 
are parts of one movement, the wing describing first a looped and 
then a wave track. 
The tip of the wing in especial acts as a kite during the up 
stroke, the kite being inclined upwards, forwards, and outwards. 
The kite formed by the wing differs from the boy’s kite in 
being capable of change of form in all its parts. The change of 
form of the wing is rendered necessary by the fact, that the wing is 
articulated or hinged at its root (short axis), its different parts, as a 
consequence, travelling at various degrees of speed in proportion 
as they are removed from the axis of rotation. It is also practi- 
cally hinged along its anterior margin (long axis), so that the tip 
travels at a higher speed than the root, and the posterior margin 
than the anterior. The compound rotation and varying degree of 
speed attained by the different parts of the wing has the effect 
of twisting the wing upon its long axis, and producing a variety of 
kite-like surfaces calculated to operate effectually upon the air, 
whatever the position of the wing may be. 
The wing, when the flying animal is fixed or hovering steadily 
before an object, describes a figure-of-8 wave track in space, — the 
figure-of-8, when the animal flies in a horizontal direction, being 
opened out or unravelled to form first a looped and thqn a waved 
track. 
In horizontal flight the wing describes a series of large waves or 
