384 DR PETTIGREW ON THE PHYSIOLOGY OF WINGS. 



gannet is extended the angle which its under surface makes with the horizon, 

 especially the portion opposite the elbow joint (q of fig. 16, Plate XIII.), is 

 much greater than one would anticipate — indeed, it is little short of 45°. The tip 

 of the wing (op of fig. 16, Plate XIII.,) does not, however, make an angle of more 

 than 25° or 30°. This is a most interesting point, as it shows that the different 

 portions of the wing in extension make different angles with the horizon — that 

 made by the tip of the wing being the least, and that made by the root of the wing 

 the greatest. The inclined surfaces are no doubt adapted to suit the travel of 

 the wing, and to produce a uniform result as far as buoyancy is concerned. 

 Thus the wing acts with a gradually decreasing angle from the root towards 

 the tip — the speed of the wing increasing in the direction of its extremity. This 

 is important, as a surface with a small angle travelling at a high speed sup- 

 plies the same amount of buoying power as a surface with a greater angle mov- 

 ing at a lower speed. Indeed, on making a careful examination of the gannet's 

 wing I have had no difficulty in determining that the different parts of the wing 

 not only make various angles of inclination with the horizon in an antero- 

 posterior direction at every stage of extension flexion in the down and up strokes, 

 but that they also make various angles of inclination with the horizon in a direc- 

 tion from within outwards. In other words, I find that in extension the wing 

 attacks the air from behind forwards and from within outwards at one and the 

 same instant — the different parts of the pinion tacking upon the air kite fashion, 

 precisely as a sailing vessel would. The same thing happens in the wing of the 

 insect. Here, as I have already pointed out, the posterior margin twists upon and 

 partially rotates round the anterior margin, so as to convert the wing into a 

 screw which moves in all its parts. This twisting and untwisting has the effect 

 of alternately producing a surface which attacks the air (at various angles of 

 inclination) from within outwards, and from behind forwards, and from without 

 inwards and from before backwards. Curiously enough, the inclined surfaces 

 formed by the different portions of the insect's wing with the horizon vary to 

 accommodate themselves to the velocity acquired by its different parts — the 

 surfaces being least inclined where the speed is highest, and vice versa. This, 

 therefore, is a fundamental point in the construction and application of all 

 wings, and affords the only rational solution of the involved problem of flight. 

 The various angles of inclination made by the wing with the horizon from 

 within outwards and the reverse, and from behind forwards and the reverse, are 

 all necessary to produce a perfect buoyancy. 



When the wing of the gannet is fully extended it is also rendered more or 

 less rigid. The joints, however, even the metacarpal ones, are free to move, 

 which shows that the wing, to be effective during the down stroke, must be 

 thoroughly under the control of the muscular and ligamentary system. This 

 is all the more necessary, as the roots of the primary and secondary feathers 



