DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 355 



The same points are illustrated at 1 and 2 of figure 17, page 349, allowance 

 being made in this case for the greater horizontal travel of the body during 

 the down (A C) and up (B D) strokes, the increased horizontal travel of the 

 body, as already pointed out, having the effect of diminishing the angles made 

 by the under surface of the wing with the horizon during its vibrations. 



The Wing acts upon Yielding Fulcra. — -The chief peculiarity of the wing, as 

 has been stated, consists in the fact that it is a twisted flexible lever specially 

 constructed to act upon yielding fulcra (the air). The points of contact of the 

 wing with the air are represented at ab c d efg h ij k I respectively of figures 16 

 and 17, page 349, and the imaginary points of rotation of the wing upon its long 

 and short axes at 1, 2, 3, and 4 of the same figures. The assumed points of 

 rotation advance from 1 to 3, and from 2 to 4 (vide arrows marked r and 

 s, fig. 17). The actual points of rotation correspond to the little loops ab c def 

 g h ij k I of same figure ; the descents of the wing to A and C, and the ascents 

 to B and D. When the wing is in the position represented at g of figures 16 

 and 17, page 349, it is ready to begin a second down stroke, that is, it is 

 screwed in a downward and forward direction. At i the second down stroke 

 (C) is completed ; at i the second up stroke is begun, the posterior margin of 

 the wing being gradually rotated in an upward direction to prepare it for making 

 the return or up stroke (D), as shown at j k I m. A third down stroke (E, fig. 

 16) is commenced at m and completed at o. 



Weight contributes to Horizontal Flight. — That the weight of the body plays 

 an important part in the production of flight may be proved by a very simple 

 experiment. If two quill feathers are fixed into an ordinary cork, as repre- 

 sented at fig. 23, p. 356, and the apparatus is allowed to drop from a height, 

 the cork does not fall vertically downwards, but downwards and forwards in a 

 curve, and for the following reasons. The feathers a b are twisted flexible 

 inclined planes, which arch in an upward direction. They are, in fact, true wings 

 in the sense that an insect wing in one piece is a true wing. When dragged 

 downwards by the cork (c), which would, if left to itself, fall vertically, they 

 have what is virtually a down stroke communicated to them. Under these 

 circumstances they inevitably dart forward ; a struggle ensuing between the cork 

 tending to fall vertically and the feathers tending to travel in a horizontal 

 direction. As a consequence, the apparatus describes the curve d efg before 

 reaching the earth, h i. This is due to the action and reaction of the feathers 

 and air upon each other, and to the influence which gravity exerts upon the 

 cork. The forward travel of the cork and feathers, as compared with the space 

 through which they fall, is very great. Thus, in some instances, they advanced 

 as much as a yard and a half in a descent of three yards. 



When artificial wings constructed on the principle of natural ones (vide fig. 24, 

 p. 357), with stiff roots (c, a), tapering semi-rigid anterior margins (a b, c d), and 



VOL. XXVI. PART IT. 4 Z 



