186 Natural and Artificial Flight. [April, 
power is necessary in flight, the wing being at all times 
thoroughly under control; that the wing in the bird descends 
as a long lever and ascends asa short one, the tip of the wing 
describing an ellipse as it does so; that the wing forms a 
parachute from which the body is suspended both during the 
down and up strokes; that the wing of the bird opens and 
closes as it rises and falls, and has a valvular action; that 
all wings are drawn towards the body and partly elevated by 
the action of elastic ligaments, &c. 
Dr. Pettigrew’s researches are dual in character. He first 
carefully describes and figures what is found in nature, after 
which he proceeds to demonstrate that the structures and 
movements which he has described and figured may be re- 
produced artificially. He takes, e.g.,a fish’s fin or tail, and 
shows that during its a¢tionit lashes from side toside figure-of-8 
fashion—the margins of the fin or tail throwing themselves 
into double or figure-of-8 curves as it does so. He then 
takes the wing of an inse¢t, bat, or bird, and by placing the 
creature in certain positions the spectator can clearly trace 
the figure of 8 made by the tip and margins of the pinion. 
He, however, goes further. He points out how the tail of 
the fish and the wing of the bird may be imitated both as 
regards its structure and function. He, in fa¢t, proves 
experimentally that the fish-tail and the wing have many 
features in common, and that propellers formed on the fish- 
tail and wing model are the most effective that can be 
devised, whether for navigating the water or the air. To 
operate efficiently on fluid, yielding media, the propeller 
itself must yield. Dr. Pettigrew has made this point very 
clear; and in this, we think, he has made a valuable dis- 
covery, for there can be little doubt that the propeller at 
present employed in navigation is faulty both in principle 
and application. 
In the concluding part of his second memoir, Dr. Pettigrew 
explains that the inclined planes hitherto employed for water 
and air are vigid, whereas they ought to be flexible and 
elastic ; that the old rigid inclined planes are made to attack 
the water and air at one angle and at a uniform speed, 
whereas they ought to strike at a great variety of angles 
and at a variable speed ; that the inclined planes at present 
in vogue either advance in a straight line or revolve in one 
direction, whereas they ought to reverse and reciprocate 
to form vibrating amine; that the inclined planes at present 
employed draw a current after them, which, being never 
met, is consequently never utilised; that the artificial fish- 
tail and wing create the currents on which they mainly 
