196 Natural and Artificial Flight. [April, 
being alternately active and passive: I say if a certain in- 
terval were allowed to elapse between every two down 
strokes; but this is praétically impossible, as the wing is 
driven with such energy that there is positively no time to 
waste in waiting for the purely mechanical ascent of the 
wing. That the ascent of the pinion is not, and ought not 
to be, entirely due to the reaction of the air, is proved by the 
fact that in flying creatures (certainly in the bat and bird) 
there are distinct elevator muscles and elastic ligaments, 
delegated to the performance of this function. The reaction 
of the air is therefore only one of the forces employed in 
elevating the wing; the others are vito-mechanical in their 
nature.” 
“The Body and Wings move in Opposite Curves.—I have 
stated that the wing advances in a waved line, as shown at 
ace giof Fig. 11; and the same remark holds true, within 
certain limits, of the body, as indicated at I, 2, 3, 4, and 5 of 
the same figure. Thus, when the wing descends in the curve 
Fic. 12. 
a e, it elevates the body in a corresponding but minor curve, 
as shown at I, 2; when, on the other hand, the wing ascends 
in the curve ce, the body falls or descends in a corresponding 
but smaller curve (2, 3), and so on ad infinitum. ‘The undu- 
lations made by the body are so trifling when compared with 
those made by the wing that they are apt to be overlooked. 
They are, however, deserving of attention, as they exercise 
an important influence on the undulations made by the wing, 
the body and wing swinging forward in space alternately, 
the one rising when the other is falling, and vice versa. 
Flight may be regarded as the resultant of three forces :-— 
the muscular and elastic force, residing in the wing, which 
causes the pinion to a¢t as a true kite, both during the down 
and up strokes; the weight of the body, which becomes a force 
the instant the trunk is lifted from the ground, from its ten- 
dency to fall downwards and forwards; and the recoil obtained 
from the airy by the rapid aétion of the wing. These three 
forces may be said to be active and passive by turns.” 
“The Wing acts as a true Kite both during the Down and Up 
Strokes.—If, as I have endeavoured to explain, the wing, even 
when elevated and depressed in a striCly vertical direction, 
