57 



The air is no longer the fulcrum but the weight, tlie shoulder joint is 

 the fulcrum, and the deltoid and lesser pectorals are the power : this is a lever 

 of the third kind, where the power and the weight act in opposite directions but 

 on the same side of fulcrum, the power being inside, or next to the fulcrum. 



It is to be noticed, also, that in the up stroke the feathers of the wing 

 fold over one another, and the arm bones ai-e drawn in, so that a comparatively 

 small surface is presented to the air : this is very essential, for if it were not 

 80 the bird would lose during the up stroke what it had gained in the down 

 stroke. 



Having now looked into the laws of the strokes of the wing, I will briefly 

 direct attention to the manner in which progression is effected. 



Birds, when flying straight forward horizontally, keep the body nearly 

 horizontal, and flap their wings nearly perpendicularly to the horizontal line. 



When in that position tlie motion of the wings has the tendency to send 

 them forward as well as sustain them ; and the cause of this is a very beautiful 

 contrivance in the structure of the wing. 



It will be remembered that I called attention to the fact that the shaft 

 of the wing feathers was not set in the centre of the vane, but considerably 

 forward. Now the whole wing is constructed on the same principle, namely, 

 that the ridge which meets the air is stiff and inflexible, and all the feathers 

 which cover the wing have their stiffer side presented to the wind, and their 

 more flexible portion turned backward. The effect of that is that the air, when 

 compressed by the down stroke of the wing, and trying to escaiJe in every 

 direction, finds the ends of the feathers offer but little resistance to it, and it 

 therefore bends them up, and that resistance or force sends the bird on in a 

 horizontal position. The up stroke, also, must have some effect in this way too. 



In proof of the above statements I would call attention to the position of 

 a bird when soaring or hovering, which is always more or less inclined to the 

 horizon : the reason for the bird adopting this position is to alter the direction 

 of the force of the air upon the flexible ends of the feathers, so that it shall act 

 as a raising and not a propelling force. 



When there is a considerable breeze less inclination is neces-sary for the 

 resultant direction of the forces of the wind and the down stroke is upward. 



Some birds rarely hover or soar except when there is a wind. 



There is no motion which requires more complete command over the 

 wings, and greater muscular power, than soaring and hovering ; but, indeed, it 

 is quite impossible to watch any of the motions of a bird without being struck 

 with admiration at their ease and elegance, and with the perfect command which 

 birds have over their wings, being able to adjust them to the very nicest balance. 



