June, 1905.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



121 



(b.) If the primary feathers be examined carefully it 

 will be seen that each one differs from its fellows, and 

 that they differ in a graduated series. The quill is 

 curved, r.nd the feathered portion or penna is set 

 round this in a helicoidal curve. Here, again, the 

 portion anterior to the quill is stiff compared to the 

 portion behind it. 



(iT.) Another characteristic of a bird's wing is that 

 a fore and aft vertical section through the body of the 

 wing discloses a curve somewhat of this shape. 



This curve is more pronounced about mid-way be- 

 tween the wrist and the shoulder joint, t.e., in the 

 region of the elbow. When the wing is in the ex- 

 tended position for flight, this joint is distinctly behind 

 the front edge of the wing. (Mr. Hargrave, of New 

 South Wales, has devoted study to this curved por- 

 tion, and it may, perhaps, be convenient to describe 

 the curve as the Hargrave curve.) 



Mr. E. P. Frost, of West Wratting, a well-known 

 member of the Council of the Aeronautical Society of 

 Great Britain, as a result of careful observation of the 

 structure of natural wings of all kinds, and of the 

 movements of wings in flight, came to the conclusion 

 more than 20 years ago that for ordinary flight a wing 

 is merely beaten up and down. 



It is obvious that, owing to the yielding elastic 

 posterior edge of a bird's wing, on the wing being 

 beaten downwards both a lift and a drive is obtained. 



It is also obvious that on the wing being elevated, a 

 forward and downward resistance is evoked. (But 

 the wing is so shaped that the down stroke must en- 

 counter greater air resistance than the up stroke, apart 

 from considerations of the amount of energy put into 



the up and into the down stroke. Also the arrange- 

 ment of the wing feathers causes a valvular action. 

 Air passes through the body of the wing on the up 

 stroke.) 



Mr. Frost has contended that the result of the 

 arrangement of the primaries must be that on being 

 struck downward in the air, their ends travel forwards 

 and upwards. In flight the wing tips of a bird {e.g., 

 a rook) can be seen to be curved upwards. If a shed 

 primary feather be taken and held in its natural 

 orientation and struck smartly down in the air the tip 

 can be seen to spring markedly forward. Then the 

 posterior edge of the penna becomes tense. But when 

 the feather is not so stressed the posterior edge is 

 sinuous and has a fullness. Other (normal) movements 

 have been described, notably the so-called " figure of 

 eight " curve generated by the movements of the wing 

 tips; but Mr. Frost has contended that the movements 

 of the wing tips in what may be considered normal 

 steady flight are the automatic results of the peculiar 

 construction of the wing and of its being beaten up and 

 down against the air. 



If during the down stroke the primary feathers are 

 strained forward and upward within their elastic limits, 

 it is obvious that energy is stored in them; and its 

 restoration may in part occur even on the up stroke. 



The curve (Fig. A) is taken from Marev's " Le vol 

 des oiseaux," and the following description extracted 

 from the text. It was photographically obtained, and 

 shows the movements of a piece of white paper fixed to 

 the tip of the first primary of a black crow. The crow- 

 was caused to fly in front of a dark screen, and the 

 lens exposed during five beats of the wings. The 

 curve shows only the trajectory of the white paper; 

 and Marey directs attention to the increase of distance 

 between successive loops due to the increasing mean 

 velocity of the bird. 



The recurving at the bottom of the loops would seem 



Dirt 

 Fig. B. 



.tion of Flight. < 



—Trajectory of Bird's Wing, 



