PROGRESSION IN OR THROUGH THE AIR. 199 



vantage is taken of the weight of the body and the shape of 

 the pinion to utilize the air as a supporting medium. In 

 these the pinion acts as a long lever,^ and is wielded with 

 great precision and power, particularly at the shoulder. 



The Flight of the Albatross compared to the Movements of a 

 Compass set upon Gimbals. — A careful examination of the 

 movements in skimming birds has led me to conclude that 

 by a judicious twisting or screw-like action of the wings at 

 the shoulder, in which the pinions are alternately advanced 

 towards and withdrawn from the head in a manner analogous 

 to what occurs at the loins in skating without lifting the 

 feet, birds of this order can not only maintain the motion 

 which they secure by a few energetic flappings, but, if neces- 

 sary, actually increase it, and that without either bending the 

 wing or beating the air. 



The forward and backward screwing action of the pinion 

 referred to, in no way interferes, I may remark, with the rota- 

 tion of the wing on its long axis, the pinion being advanced 

 and screwed down upon the wind, and retracted and un- 

 screwed alternately. As the movements described enable 

 the sailing bird to tilt its body from before backwards, or 



1 Advantages possessed hy long Pinions. — The long narrow wings are most 

 effective as elevators and propellers, from the fact (pointed out by Mr. Wen- 

 Jiam) that at high speeds, with very oblique incidences, the supporting effect 

 becomes transferred to the front edge of the pinion. It is in this way "that 

 the effective propelling area of the two-bladed screw is tantamount to its 

 entire circle of revolution." A similar principle was announced by Sir George 

 Cayley upwards of fifty years ago. " In very acute angles with the current, it 

 appears that the centre of resistance in the sail does not coincide with the 

 centre of its surface, hut is considerably in front of it. As the obliquity of 

 the current decreases, these centres approach, and coincide when the current 

 becomes perpendicular to the plane ; hence any heel of the machine backwards 

 or forwards removes the centre of support behind or before the point of sus- 

 pension." — Nicholson's Journal, vol. xxv. p. 83. When the speed attained 

 by the bird is greatly accelerated, and the stratum of air passed over in any given 

 time enormously increased, the support afforded by the air to the inclined 

 I'lanes formed by the wings is likewise augmented. This is proved by the 

 rapid flight of skimming or sailing birds when the wings are moved at long 

 intervals and very leisurely. The same j)rinciple supports the skater as he 

 rushes impetuously over insecure ice, and the thin flat stone projected along 

 the surface of still water. The velocity of the movement in either case pre- 

 vents sinking by not giving the supporting particles time to separate. 



