347 
of Edinburgh, Session 1870 - 71 . 
produced.* Thus if the wing be made to vibrate at its root, a double 
wave is produced, the one wave running in the direction of the length 
of the wing, the other in the direction of its breadth. The wing 
further twists and untwists figure-of-8 fashion during the down and 
up strokes. There is, moreover, a continuous play of the wing, 
the down stroke gliding into the up one, and vice versa , by a 
system of continuous and opposite curves, which clearly shows 
that the down and up strokes are parts of one whole, and that 
neither is perfect without the other. This form of wing is endowed 
with the very remarkable property that it will fly in any direc¬ 
tion, demonstrating more or less conclusively that flight is essen¬ 
tially a progressive wave movement . Thus if the anterior or thick 
margin of the wing be directed upwards, and the angle which the 
under surface of the wing makes with the horizon be something less 
than 45 degrees, the wing will, when made to vibrate, fly with an un- 
dulatory motion in an upward direction , like a pigeon to its dove-cot. 
If the under surface of the wing make no angle, or a very small 
angle with the horizon, it will dart forward in a series of curves in 
a horizontal direction, like a crow in rapid horizontal flight. If the 
angle made by the under surface of the wing be reversed, so that 
the anterior or thick margin of the wing be directed downwards, 
the wing will describe a wave track and fly downwards , as a 
sparrow from the top of a house or tree. In all those move¬ 
ments progression is a necessity; the movements are continuous 
gliding forward movements ; there is no halt or pause between the 
strokes, and if the angle which the wing makes with the horizon 
be sufficiently great, the amount of steady, tractile, and buoying 
power developed is truly astonishing. This form of wing elevates 
and propels both during the down and up strokes, and its working is 
accompanied with little or no slip. Its movements may be regarded 
as the literal realisation of the figure-of-8 hypothesis of flight. 
* The author has made a great variety of artificial wings. Of these some 
are in one piece, with a continuous covering; others in a single piece, with 
the cover broken up into a large number of small valves ; others in several 
pieces, with a continuous covering, and others jointed, with the cover broken 
up into a number of valvular segments. In all cases the frames of the wings 
are composed of elastic material, such as steel tubes, bamboo and other canes, 
osier twigs, whalebone, gutta percha, &c., &c. ; the covers of the wings are 
made of india-rubber cloth, tracing cloth, argentine, linen, silk, &c., &c.; the 
springs of the wings of steel, caoutchouc, &c., &c. 
