340 Proceedings of the Royal Society 
curves, the body describing a series of smaller and opposite curves, 
the wing always rising when the body falls, and vice versa. The 
descent of the wing in this manner necessitates the elevation of the 
body, and the descent of the body contributes to the elevation of 
the wing. 
The wing elevates the body when it descends, and the body, 
when elevated, falls forwards in a curve, and so contributes to the 
elevation of the wing. This arrangement draws the wing forward 
upon the air during the up stroke, and opposes the direct down- 
ward action of gravity by presenting the concave or biting surface 
obliquely to the air in the direction of the travel of the body. 
The under surface of the wing is thus made to act as a true kite 
during the up stroke. 
The wing is urged at different velocities, the power applied being 
much greater during the down stroke than during the up one. 
The power is also greater at the beginning of the down and up 
strokes than towards the termination of those acts. The variation 
in the intensity of the driving power is necessary to slow the wing 
towards the termination of the down stroke, to prepare it for the 
up stroke, and to afford the air an opportunity of reacting on the 
under surface of the W'ing, to the elevation of which it contributes. 
The wing is elevated more slowly than it is depressed, and allows 
the body time to fall downwards, the fall of the body assisting in 
elevating the wing relatively to the bird. The wing, the air, and the 
weight of the body, are consequently active and passive by turns. 
The wing is depressed by voluntary muscular efforts. It is 
elevated by vital, and mechanical acts, viz., by the contraction of 
the elevator muscles and elastic ligaments, by the reaction of tbe 
air called into play by the fall and forward travel of the body. 
If the wing is in one piece, it is made to vibrate figure-of-8 
fashion in a more or less horizontal direction . It thus attacks the 
air by a series of zig-zag movements, very similar to those per- 
formed by an overloaded dray-horse when ascending a hill. If the 
wing is in more than one piece, it is made to oscillate in a more 
or less vertical direction ; the wing, under these circumstances, being 
usually closed during the up stroke and opened out during the down 
stroke. The wing is closed and its area diminished during the 
up stroke, expressly to avoid the resistance of the air. 
