1872.] Natural and Artificial Flight. 183 
Guided by these indications, he has especially directed 
his attention to the twisting flaiJ-like movements of the wings 
of insects, of the flippers and tails of sea mammals, and 
of the fins and tails of fishes. These he finds all act upon 
the air and water by curved surfaces, the curved surfaces 
reversing, recriprocating, and engendering a wave pressure, 
which can be continued indefinitely at the will of the animal. 
In order to prove that sea mammals and fishes swim, and 
insects, bats, and birds fly, by the aid of curved figure-of-8 
surfaces, which exert an intermittent wave pressure, Dr. 
Pettigrew constructed artificial fins, flippers, and wings, 
which curved and tapered in every direction, and which were 
flexible and elastic, particularly towards the tips and posterior 
margins. These fins, flippers, and wings were slightly 
twisted upon themselves, and when applied to the water and 
air by a sculling or figure-of-8 motion, curiously enough not 
only reproduced the curved surfaces referred to, but all the 
other movements peculiar to the fins and tail of the fish 
when swimming, and to the wings of the insect, bat, and bird 
when flying. 
To Dr. Pettigrew is due the discovery of the celebrated 
figure-of-8 or wave theory of flight which has been exciting 
so much attention on the Continent and in America. As 
early as 1867 Dr. Pettigrew gave his novel theory to the 
world in an evening lecture, delivered at the Royal Institution 
of Great Britain. On that occasion (vide Proc. Roy. Inst. 
of Great Britain, March 22, 1867) he pointed out the in- 
teresting fact that the wing was a screw structurally and 
functionally ; in other words, that the wing when at rest was 
twisted upon itself, and that when it was made to vibrate or 
reciprocate it twisted and untwisted figure-of-8 fashion. The 
wing was shown to be as effective in water as in air, and the 
tail of the fish was represented as lashing from side to side, 
after the manner of an oar in sculling. In June of the same 
year (1867) he read a memoir on the subject to the Linnean 
Society of London (Trans. Linn. Soc., vol. xxvi.), in which 
he described, figured, and compared the movements made 
by the fins and tail of the fish and the wing of the bird in 
flying and diving. These movements he showed were re- 
ciprocating movements, produced by helicoidal surfaces, which 
were mobile and plastic, and acted at a great variety of 
angles, so as to obtain a maximum result with a minimum 
of power, and, what is not less important, with a minimum 
of slip. The fish was represented as throwing its body into 
figure-of-8 curves in swimming, and the wing of the bird 
into similar curves in flying and diving—the figure of 8, 
