238 Aerial Locomotion. (April, 
placed. Thisisa glaring inaccuracy on the part of Professor 
Marey. 
He has in the first place reversed the direction of the 
arrows in Dr. Pettigrew’s figure, and in the second place he 
makes the half of the figure represent the whole. In Dr. 
Pettigrew’s original figure the arrows are pointing from left 
to right ; whereas in Professor Marey’s copy of it, they are 
pointing from right to left. 
In the description given of Dr. Pettigrew’s figure, it is 
distinctly stated that 7 extension the arrows of the figure-of-8 
are directed from left to right, but that 7 flexion they are 
directed from right to left.* 
In one complete revolution of the wing, therefore, accord- 
ing to Dr. Pettigrew, the arrows are directed alternately 
from left to right, and from right to left, and this is precisely 
what happens in every figure-of-8 delineated by Professor 
Marey. 
Dr. Pettigrew, when speaking of the change of plane 
occurring during the down and up strokes of the wing of the 
insect, states that :— 
‘“‘ A fioure-of-8 compressed laterally, and placed obliquely with its long axis 
running from left to right of the spectator, represents the movement in question. 
“The down and up strokes, as will be seen from this account, cross each 
other, the wing smiting the air during its descent from above, as in the bird and 
bat, and during its ascent from below, as in the flying fish and boy’s kite.” + 
A little further on, and on the same page of his 1867 
memoir, in which the figure-of-8 and waved tracks made by 
the wing in stationary and progressive flight are delineated, 
Dr. Pettigrew says :— 
“The figure-of-8 action of the wing explains how an inse¢t or bird may fix 
itself in the air, the backward-and-forward reciprocating action of the pinion 
affording support, but no propulsion. In these instances the backward and 
forward strokes are made to counterbalance each other. ... . Although the 
figure-of-8 represents with considerable fidelity the twisting of the wing upon 
its axis during extension and flexion, when the inse¢t is playing its wings 
before an objed, or still better when it is artificially fixed ; it is otherwise when 
the down stroke is added, and the inseé is fairly on the wing, and progressing 
rapidly. 
‘In this case the wing, in virtue of its being carried forward by the body in 
motion, describes an undulating or spiral course.” { 
The figure-of-8 and undulating wave movements originally 
described and figured by Dr. Pettigrew, in March and June, 
* According to Dr. Pettigrew extension in the insect signifies ‘‘ the carrying 
of the wing in a forward direction, away from the body; flexion meaning the 
reverse, or the drawing of the wing from before, backwards towards the body.” — 
(Trans. Linn. Soc., vol. xxvi., p. 226). 
+ On the Mechanical Appliances by which Flight is Attained in the Animal 
Kingdom. p 
t Trans. Linn. Soc., vol. xxvi., p. 233- 
