XXIV] MECHANISM OF FLIGHT 611 



arising from the ulna, secondaries (8, Fig. 305, C); a few arising 

 from the upper arm are called tertiaries; any air which might 

 escape between the bases of the long feathers is stopped by an 

 upper layer of shorter feathers, called coverts (1, 2, 3, 5 and 7, 

 Fig. 305, C and D). Air is prevented from escaping in front by the 

 hand, which is stretched out in a vertical plane, and by two folds of 

 skin, one in the angle between fore-arm and upper arm, the other 

 between the upper arm and the body. Each of these folds is 

 termed a patagium. The name bastard wing is given to a tuft 

 of feathers borne by the thumb (4, Fig. 305, C and D). 



The full mechanical explanation how the down-stroke of the 

 wing not only prevents a bird from falling but urges 

 it onwards is not completely understood, and much 

 of what is generally accepted is too complicated for an elementary 

 text- book, but the broad principles involved may be simply set 

 forth. A bird when it is in the air, like any other heavy body, 

 is continually falling : the blow of the wing has therefore not only 

 to effect a forward impulse, but also an upward one sufficient 

 to compensate for the distance the bird has fallen between two 

 strokes. These impulses are derived from the elastic reaction of 

 the air compressed by the down-stroke of the wing. When the 

 wing is expanded, it is slightly convex above and concave beneath. 

 This arises from the fact that the quill feathers are attached to 

 the upper edge of the webbed limb and project gently downwards 

 and backwards, so that there is a space left which is bounded 

 behind by the quills and in front by the bones and the patagia. 

 Now if this space had a symmetrical shape the air would be com- 

 pressed in such a way that the resultant impulse would be 

 directly upwards ; but it is not symmetrical, for its roof has a very 

 steep slope in front and a very gentle one behind, and the air is 

 compressed in such a way that an oblique reaction results, a 

 reaction which we can resolve by the parallelogram of forces into 

 an upward and an onward one. So much for the flight of a bird in 

 still air. The air is, however, very rarely still, and the currents 

 which exist are never quite horizontal, but generally inclined 

 slightly upwards, since the lowest layer of air is checked by friction 

 against' the ground, and birds which are good flyers can, by in- 

 clining their wings at the proper angle, obtain quite sufficient 

 support from the play of the current against the wing without 

 exerting themselves to any great extent. This is called soaring, 

 and can be seen beautifully in the flight of the Gannet. In this 



392 



