of Edinburgh, Session 1870 - 71 . 
349 
elastic properties of the wing and the resistance which it experi- 
ences from the air, as well as to the fact that the tip and posterior 
part of the wing travel at a much higher speed than the root and 
anterior part. The small angle made by the tip, as compared with 
the root of the wing, equalises its action, a large angle urged at a 
low speed giving the same amount of buoyancy and propelling 
power as a smaller angle urged at a higher speed. 
The artificial wing, because of its elasticity and by the aid of 
certain springs, can be made to slow and reverse of its own accord 
at the end of the down and up strokes in precisely the same 
manner as the natural wing. It can likewise be made to change 
its course without halt or dead point, so as to give continuity of 
motion and continued buoyancy. 
If the artificial wing be moved figure-of-8 fashion in a more or 
less horizontal direction, it can be made to create and utilise its 
own currents, the stroke from right to left producing the currents 
on which the wing rises in its passage from left to right, and the 
reverse. It can also be made to utilise and evade natural currents. 
If the tip of a properly constructed artificial aerial wing be 
turned downwards, and the wing be made to move from side to side 
figure-of-8 fashion like the tail of a fish, it forms a very excellent 
aerial propeller. 
The artificial wing, to be effective, must rotate about two separate 
axes, the one corresponding to its root (short axis), the other to its 
anterior margin (long axis). 
If two artificial wings, similar to those described, be placed end 
to end, inclined at a certain upward angle, and made to revolve, 
they form a most powerful aerial screw. This form of screw is 
propelled with comparatively little force, and its working is 
attended with quite a nominal amount of slip. 
The aerial screw here recommended is elastic and capable of change 
of form in all its parts, and so constructed that its angles vary to 
adapt themselves to the speed attained by the different portions of 
the blades at any given time. Thus the angles made by the blades 
are greatest when the speed at which the screw is driven is least, and 
vice versa ; the angles made by those portions of the blades which 
are nearest the axis of rotation being always greater than those 
made by the portions nearer the tips of the blades. This form of 
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VOL. VII. 
