H. Skey. — Introdiictiou of the Tension Wheel in Aerial Transit, 175 



tip end is also raised, so that tlie feather becomes less curved longitudinally, 

 and assumes the figure of a thin and slightly bent knife, with the concave 

 side downwards, the screw of the feather unwinding as it were. 



If the feathers and wings of bu-ds were straight when quiescent, and if 

 every transverse section were inclined to the same angle — if, for instance, 

 they were planes when not in action — then, when they came to be acted 

 on by the air, they would, unless perfectly rigid, lose that proper figure 

 essential to buoyancy, for the tips would twist more than the other parts, 

 and the longitudinal section would become convex downwards. Perfect 

 rigidity of a plane would necessitate too much weight. 



We therefore see them formed with a twist or screw ia the reverse 

 direction to that in which the an* itself twists them when in action. Then, 

 when they come into action, they assume of themselves the proper and 

 best figure, so that every part can act at nearly the same angle upon the 

 au'. 



The action of the au' itself confers the requisite rigidity, and the greater 

 the speed or pressure, so much greater is the rigidity ; the wing then assumes 

 a knife-like figure approaching to a plane but shghtly concave downwards. 



This appears to be the figure of the wings of birds while sailing and 

 wheeling, for when they are viewed when the eye is in the same plane as 

 the wing they appear as shown in fig. 3, in which the wuigs are represented 

 as mere lines. It will be noticed in the case of this wheel that this is also 

 the figure which the tensile radii assume when in action. 



The method here adopted allows of the very important advantage of 

 using extremely thin anterior edges for the planes, whereby the air is cut, so 

 to speak, for the nearer they approach to a mathematical line the better, as 

 the resistance to the horizontal motion of the radii is surprisingly lessened. 



When these planes are revolved with the same velocity as the wind, it is 

 clear that' even in a gale the wind cannot give any pressure against them 

 except on the under and elevating sides. 



Sailiog birds first acquire a great initial velocity generally by flapping 

 their wings. Naturalists and observers differ as to the du'ection of the 

 vibratory motion of the wing in this kind of flight, probably on account of 

 different bh'ds being selected as examples, and omitting in some cases to 

 mention whether the forward motion of the bird itself is included in the 

 asserted dhection of the stroke. For instance, a buxl fastened by a string, 

 or one which may be rising vertically, or, better still, a hovering bkd, 

 wliich we will assume to strike vertically downwards with its ^^dngs, would 

 not really when in transit move them vertically downwards, although the 

 stroke or attempt might be so ; for the downward stroke compounds with 

 the horizontal motion of the bhd producing a forward and downwards or 



