56 



I have made this special notice of this kind of lever, which is not in any 

 way peculiar to the wins; or connected with the mechanical principles of flight, 

 because in the book which I have referred to already, in pages 158 and 159, the 

 Duke of Argyll speaks of the wing as an implement thiough which the vital 

 force (muscular I suppose) is exerted with immense mechanical advantage for 

 the purpose in view, viz., flight. Now we have seen that the wing in flight does 

 really act as a lever, but that it is a lever which places the power at a dis- 

 advantage, or in other words the power or vital force must be in excess of the 

 weight to be moved ; and again, if the Duke is referring the wing to the third 

 kind of lever, which I hardly think he can be doing, he has missed altogether 

 the ijrinciple of flight, for he has not got any fulcrum on which the lever may 

 act ; and indeed, throughout his argument, it seems to me that he misses the 

 great point of the air being the fulcrum. 



I mention this because I think his use of the word mechanical is apt 

 to mislead, for according to the usual acceptation of the term, there is certainly 

 no mechanical advantage in the kind of lever to which the wing belongs ; and 

 with regard to the third kind of lever to which bones and muscle belong, the 

 advantage is not mechanical, but merely convenience. 



It is very necessary to bear in mind, when speaking of the principles of 

 flight, that the fulci'iim is not altogether a fixed one, but is a compressible fluid 

 which endeavours to escape in every direction, and therefore it is apparent that 

 much of the force of the stroke of the wing is lost owing to the fulcrum giving 

 way, so to speak ; for it is easy to see that the body of the bird is moved upward 

 a very short space in comparison with the space through which the wing moves. 



The same thing is observable in rowing a boat, for the water which is, in 

 that case, the fulcrum, is to some extent displaced, and the boat does not move 

 through so great a portion of water as it would do if the oar acted against a fixed 

 and immoveable fulcr\im. 



This motion of the fulciiim, allowing a corresponding motion to the wing 

 or lever, seems to give the idea that the body or weight is attached to the short 

 arm of the lever, whereas, as far as the mechanical principle is concerned, it is 

 attached to the long end. 



I make these remarks because, although in flight the compressibility of 

 the air is a most important condition, yet no mechanical principle is involved 

 in it ; for in mechanics a moveable fulcrum would be a contradiction. 



While speaking of the compressibilty of the air, I wovild call attention to 

 the concavity of the wing below and its convexity above ; this form confines the 

 air as much as possible, and allows the wing to act firmly upon it in the down 

 stroke before the air escapes ; and in the up stroke the convex form of wing 

 allows the air to roll off with but comparatively little resistance. 



This brings me to speak of the up-stroke ; here the mechanical principle 

 of the stroke is quite altered. 



