1872.) Natural and Artificial Flight. 185 
arrived at these results from a careful study of the extremities 
and travelling surfaces of a large number of animals fitted 
by nature for moving in water and air, and from numerous 
experiments with artificial fish tails, fins, and wings, which 
he made to vibrate with steam by a dire¢t piston action. 
Continuing his researches, Dr. Pettigrew presented a second 
memoir on the subject to the Royal Society of Edinburgh, 
on the znd of August, 1870. This is published in vol. xxvi. 
of the ‘‘ Transations”’ of that Society, and in it he gives the 
details on which the conclusions arrived at in his first memoir 
were based.* Dr. Pettigrew shows that the wing acts as a 
kite both during the down and up strokes, and that it elevates 
and propels in either case—the rising and falling movements 
gliding by insensible degrees into each other to form one 
pulsation; that when the wing rises the body falls, and 
vice versa; that the wing, when the body of the flying 
animal is advancing in space, describes a waved track, the 
body describing a similar but smaller wave; that the wing 
is twisted upon itself when at rest and when in motion; 
that the blur or impression produced on the eye by it when 
made to vibrate rapidly is concavo-convex and twisted; that 
the under or concave surface of the wing, in virtue of its 
being carried obliquely forward against the air by the body, 
is effective during both the down and up strokes; that the 
wing rotates in the direction of its length and breadth as it 
rises and falls; that it reverses its planes more or less com- 
pletely at every stroke; that it produces during the one 
stroke the currents by which it is elevated during the suc- 
ceeding stroke—the wing literally rising on a whirlwind of 
its own forming ; that the wing is movable and flexible as 
well as elastic, and capable of change of form in all its 
parts; that it is forced into waves during its action, and 
impinges upon the air as an ordinary sound does; that it 
produces a cross pulsation, the pulsatile waves running in 
the direction of the length of the wing and across it; that 
during its vibration it moves on the surface of an imaginary 
sphere ; that the natural wing when elevated and depressed 
must move forwards; that the movements of the wing are 
comparatively slow at its root, but very rapid at its tip; that 
balancing is in a great measure effected by purely mechanical 
arrangements which operate independently of the will of 
the animal; that weight is necessary to horizontal flight ; 
that the wing acts upon yielding fulcra; that a regulating 
* These memoirs extend to some 220 pages quarto, and are illustrated by 
nearly 200 original engravings and woodcuts. 
VOL. 11. (N.S.) 2B 
