1872.] Natural and Artificial Flight. 187 
depend for support and progress by a peculiar reciprocating 
figure-of-8 action. Dr. Pettigrew’s spiral elastic wings and 
flexible elastic screws will be hailed with satisfaction by all 
interested in the navigation of balloons. They possess 
advantages for this purpose which will necessitate their 
being universally adopted. 
Dr. Pettigrew shows how an artificial insect, bat, or 
bird’s wing may be made to vibrate with a wavy, con- 
tinuous, reciprocating motion, devoid of dead points, the wing 
literally floating on the air. He points out how a properly- 
constructed artificial wing will, when elevated and depressed, 
inevitably dart forwards in a series of opposite curves, and 
that by altering the angle of inclination of the wing with 
the horizon, it may be made to fly upwards and forwards, 
horizontally, or downwards and forwards—flight, as he 
explains, being essentially not a vertical but a progressive 
and almost horizontal movement. He likewise gives 
_ directions as to the nature of the forces to be employed in 
driving artificial wings and propellers. The artificial wings 
and propellers, he states, are made to resemble the wing of 
the insect, bat, and bird, or the caudal fin of the fish. They 
are composed of flexible and elastic material, which varies 
in thickness, the thicker portions, which are consequently 
the more rigid parts, corresponding to the root and anterior 
margin of the wing and the root and lateral margins of the 
tail of the fish. When made to vibrate or reciprocate, the 
margins of the propeller formed on the wing and fish-tail 
model are thrown into double or figure-of-8 curves, from the 
fact that the propeller twists and untwists during its action. 
This twisting movement enables the propeller to evade and 
seize the water and air alternately with wonderful rapidity 
and power—the efficiency of the propeller increasing in a 
direct ratio to the velocity with which it is made to vibrate. 
By adding springs which antagonise each other, the propeller 
is lashed about with greater facility and with a more con- 
tinuous play—a similar result being obtained by working the 
steam expansively. 
The subjoined passages and illustrations from Dr. Petti- 
grew’s memoirs will serve to elucidate the figure-of-8 or wave 
theory of flying, and cannot fail to be interesting to the 
reader, the more especially as they are strikingly original. 
“The Wing a Twisted Lever or Helix.i—The wings of 
insects and birds are, as a rule, more or less triangular in 
shape, the base of the triangle being directed towards the 
body, the sides anteriorly and posteriorly. They are also 
