of Edinburgh, Session 1870 - 71 . 
343 
level with it. When above the body, the latter is suspended from 
the wings as from a parachute. When beneath the body, it is 
suspended from the top of a cone formed by the wings, and when 
on a level with the body, the latter is placed in the centre of a 
circle described by the rapidly oscillating wings. The body 
is suspended from the wings very much as a compass set upon 
gimbals is suspended. 
The wing balances the body in consequence of its travelling at 
such a speed as enables it to convert the area mapped out by its 
vibrations into what is practically a solid basis of support. 
The wing, whether in one piece or in many, rotates upon two 
centres, the one centre corresponding to the root of the wing (short 
axis), the other to the anterior margin (long axis). The rowing 
feathers have a similar compound motion. This mode of action of the 
wing is intimately associated with the power it enjoys of alter¬ 
nately seizing and evading the air, of producing artificial currents, 
and of utilising artificial and natural currents. 
The wing is cranked slightly forwards, a small degree of rotation 
of the anterior margin being followed by a very considerable sweep 
of the posterior margin. 
The wing area is greatly in excess of what is absolutely neces¬ 
sary, and as much as four-sixths may be removed in certain 
insects (the common blow-fly, e.gf without destroying the power 
of flight. The wing area may also be considerably reduced in 
birds without in any way impairing flight. This shows that 
elaborate calculations of wing area, in relation to weight of trunk, 
must prove futile, unless the rapidity with which the wing 
vibrates and the state of the air are also taken into account. 
Weight is necessary to the flight of the insect, bat, and bird, as 
at present constructed. If flying creatures were lighter than 
the air, the wing would require to be twisted completely round as in 
the auks and penguins, so that the under ventral or concave surface 
would strike from below upwards, instead of from above downwards. 
In aerial flight the under or concave surface of the wing is 
applied from above , whereas in subaquatic flight it is applied from 
below. The scull, like the subaquatic wing, is applied from below, 
so that the analogy between the aerial wing and the oar as employed 
in sculling is more apparent than real. 
