PETTIGREW ON THE MECHANISM OF FLIGHT. 2 



But I need not multiply examples ; suffice it to say that, unless in the case of the 

 sailing birds, which are exceptional, flight may he attained within certain limits by every 

 size and form of wing, if the number of its oscillations be increased in proportion to the 

 weight to be raised. 



Reasons why the Effective Stroke should be delivered Downward* and Forwards* — 

 The wings of all birds, whatever their form, act by alternately presenting oblique and 

 comparatively non-oblique surfaces to the air, — the mere extension of the pinion, as lias 

 been shown, causing the primary, secondary, and tertiary feathers to roll down till they 

 make an angle of 30° or so with the horizon, in order to prepare it for giving the effec- 

 tive stroke, which is delivered, with great rapidity and energy, in & downward had fortoa r< / 

 direction. I repeat, " downward and forward ;" for a careful examination of the relations 

 of the wing in the dead bird, and a close observation of its action in the living one, have 

 fully convinced me that the stroke is invariably delivered in this direct ion*. If the wing 

 did not strike downwards and forwards, it would act at a manifest di advantage : 



1st. Because it would present the back or convex surface of the wing to the air — a 

 convex surface dispersing or dissipating the air, while a concave surface gathers it toge- 

 ther or focuses it. 



2nd. In order to strike backwards effectually, the concavity of the wing would also 

 require to be turned backwards ; and this would involve the depression of the anterior or 

 thick margin of the pinion, and the elevation of the posterior or thin one, during the 



down stroke, which never happens. 



3rd. The strain to which the pinion is subjected in flight would, if the wing struck 

 baekwards, fall, not on the anterior or strong margin of the pinion formed by the bones 

 and muscles, but on the posterior or weak margin formed by the tips of the primary, 

 secondary, and tertiary feathers— which is not in accordance with the structure of the 

 parts. 



Mr. Macgillivray, in his * History of British 

 Bfa^''^UM^ He says (p. 34) that in flexion the wing is drawn upwards, 



forwards, and inwards, but that during extension, when the effective stroke is given, it is made to strike outwards, 

 downwards, and backwards. The Duke of Argyll holds a very similar opinion. In speaking of the hovering 

 of birds, he states that, « if a bird, by altering the axis of its own body, can direct its wing-stroke in some degree 

 forwards, it will have the effect of .topping instead of promoting progression;" and that, "Except for the purpose 



of gravity."— Good Words, Feb. 18G5, p. 132. . 



Mr. Bishop, in the - Cyc. of Anat. & Phys.' vol. Hi. p. 425, says, - In consequence of the planes of the wmgs 

 being disposed either perpendicularly or obliquely backwards to the direction of their mot.cn. a corresponding impulse 

 is given to their ceufre of gravity " Professor Owen, in like manner, avers that « a downward stroke would on y 

 tend to raise the bird in the air ; to carry it forwards, the wings require to be moved m an oblique plane, so as to 

 strike backwards as well as downwards."— Cb»p. Anat. and Phys. and Vertebrates, vol. n. p. 15. 



Tl™ e n • • *v * • uirpi ;«;« .— '< When a bird is about to depress its wing, this is a little 



ine following is the account given by 3L. h. mais. wuena u . , , . j i „ M nj *~ 



inclined from before backward, When the descending movement commences, the wmg does no, £"*P"^ 



itself in a direction from before backwards , bat the movement is accompaaied by a rotaMon of severa degrees ound 



n. . . . . . i> .. „u_ KoLmrs ami the descending movement u transferred 



* * * * * 



-- ~, „ — 



more mid more backwards. ,.*— 



lower than at the commencement of the movement. 

 **. 3rd series, p. 156. 



/< 



>j 



of Birds and Insects, Annals of Nat 



2 M 2 



