266 DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



presented is, with few exceptions, broken up to diminisli the resistance, by permitting the 

 air to escape, llus peculiarity in the wing of the bird is occasioned by each primary, secon- 

 dary, and tertiary feather rotating upon its axis to procure the maximum of resistance 

 during extension, and the minimum of displacement during flexion, as happens in the 

 win^r of the insect. The wing of the bird is consequently an advance upon those of the 

 insect and bat, since it combines the advantages peculiar to each. The wing of the 

 bird, in virtue of its shape and conformation, acts as a twisted inclined plane, in other 

 words, as a helix or screw — the mere extension of the wing, because of the spiral 

 arrangement of the joints, causing it to rotate from its plane of least resistance till it 

 makes an angle approximating to 30° with the horizon, the reverse of this occurring 

 during flexion. In this respect it intimately agrees with the wing of both the insect and 

 bat, thus proving that, however the instrument of flight may be modified, the principle 

 involved is the same in all. 



VIII. All wings act as levers of the third order, a very slight movement at the root 

 of the pinion being followed by an immense sweep of its tip or extremity. This lar 

 sweep is necessary for compressing the vast quantity of air which is to supply the 



o 



fulcrum for elevating, propelling, and sustaining. 



1 V. All wings are twisted upon themselves naturally. They, moveover, twist upon 

 themselves during their action; so that the course described by them, or, what is the 

 same thing, the blur or impression produced on the eye by their action is essentially 

 spiral in its nature. It, in fact, closely resembles in its appearance the blade of a screw 

 propeller — the velocity with which the wing is driven causing the blur to assume a more 

 or less solid aspect. 



X. The wing exceeds, in elevating-, propelling-, and sustaining-power, every form of 

 screw yet devised, the rare combination of rigidity, elasticity, and mobility which it dis- 

 plays enabling it to strike the air at various degrees of obliquity during the down and up 

 strokes, and this in a manner which insures a maximum result with a minimum expen- 

 diture of power. The effect is such as could not be produced by wholly rigid materials, 

 however fashioned and applied. 



X L The wings, during the down and up strokes, describe a wave-track, the termination 

 of the np and the beginning of the down stroke corresponding to the crest of the wave, 

 the down stroke to the slope or side of the wave which is inclined forwards, or in the 

 direction in which the wave is supposed to travel, the termination of the down and the 

 1) ginning of the up stroke coinciding with the hollow of the wave, and the up or return 

 stroke with that slope or side of the wave which is inclined backwards, L e. in a con- 

 trary direction to the plane of progression. The down and up strokes may consequently 

 be conveniently divided into four stages, the first two consisting of preparation for the 

 down stroke and the down stroke itself, the remaining two to preparation for the up 

 stroke and the up or return stroke. The body may thus be said to glide along by a 



of gentle undulations or gradient 



& ilV^ «* v ~ & 



XI i. The wings, while they are describing the wave-tracks referred to during the 

 down and up strokes, are in addition rotating upon their long axes, to increase the angle 

 which they make with the horizon during their descent, and to dimmish it during their 



