145 
of Edinburgh, Session 1870 - 71 . 
tively slow movement communicated to the root of the wing is at 
once converted into a very rapid one at the tip. 
If an artificial wing be constructed in strict accordance with 
any of the natural wings (insect, bat, or bird), and applied by a 
sculling figure-of-8 movement to the air, it will be found to supply 
a steady buoying and propelling power, similar in all respects to 
that supplied by the living wing. 
In order to secure this result, the artificial wing should be 
concavo-convex, and slightly twisted upon itself, i.e., it should be 
finely arched in every direction. It should be mobile as well as 
elastic,* and be applied to the air at different angles and at different 
degrees of speed, in such a manner that the wing and air may be 
active and passive by turns. 
The artificial wing , like the natural one, must be more or less 
triangular in shape. It must taper from the root towards the tip, 
and from the anterior margin in the direction of the posterior 
margin. It should be capable of change of form, and elastic 
throughout, the flexibility being greatest at the tip and posterior 
margin of the wing, and least at the root and along the anterior 
margin. It must move in all its parts at different periods of 
time, as in this way the air is alternately seized and dis- 
missed, dead points avoided, and a continuous reciprocating 
movement secured. In producing a continuous vibration of 
the artificial wing, much assistance is obtained by employing a 
ball-and-socket joint at its root, with a system of elastic springs 
of different strengths. The principal springs should be ar- 
ranged at right angles to each other, the superior and posterior 
springs being stronger than the inferior and anterior ones. 
Oblique springs may be added, and the whole, because of their 
different strengths and their peculiar directions and insertions, 
can be made to give the wing any amount of torsion in the direc- 
tion of its length during every portion of either the up or down 
stroke. The muscles and elastic ligaments of insects, bats, and 
birds, perform a similar function. A ball-and-socket joint, or 
what is equivalent thereto, is necessary at the root of the wing, 
* Borelli (1668), Durkheim, and Marey state that an artificial wing should 
be composed of a rigid rod in front and a flexible sail behind, but experiment 
lias convinced the author that no part of the wing should be absolutely rigid. 
