PROGRESSION IN OR THROUGH THE AIR. 



195 



176), a form of pinion usually associated with a heavy body, 

 as in the grouse, quail, diver, and grebe, the muscular exer- 

 tion required, and the rapidity with which the wing moves 

 are very great; those birds, from a want of facility in turning, 

 flying either in a straight line or making large curves. They, 

 moreover, rise with difficulty, and alight clumsily and some- 

 what suddenly. Their flight, however, is perfect while it lasts. 



The goose, duck (fig. 107, p. 204), pigeon (fig. 106, p. 

 203) and crow, are intermediate both as regards the form 

 of the wing and the rapidity with which it is moved. 



The heron (fig. 60, p. 126) and humming-bird furnish ex- 

 treme examples in another direction, — the heron having a 

 large wing with a leisurely movement, the humming-bird a 

 comparatively large wing with a greatly accelerated one. 



But I need not multiply examples ; suffice it to say that 

 flight may be 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 he delivered downwards 

 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 has been shown, causing the primary, secondary, and ter- 

 tiary feathers to roll down till they make an angle of 30° or 

 so with the horizon* in order to prepare it for giving the 

 eff'ective stroke, which is delivered, with great rapidity and 

 energy, in a dotvnward and forward direction. I repeat, 

 " downwards and forwards ; " for a careful examination of 

 the relations of the wing in the dead bird, and a close ob- 

 servation of its action in the living one, supplemented by a 

 large number of experiments with natural and artificial 

 wings, have fully convinced me that the stroke is invariably 

 delivered in this direction.^ If the wing did not strike 



1 Prevailing Opinions as to the Direction of the Down Stroke. — Mr. Macgil- 

 livray, in his History of British Birds, published in 1837, states (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 similar opinion. In speaking of the hovering of birds, he asserts that. 



