PROGllESSION IN OR THROUGH THE AIR. 157 



forcibly elevated or depressed, even in a strictly vertical 

 direction, they inevitably dart forward. This is well shown 

 in fig. 81. 



If, for example, the wing is suddenly depressed in a vertical 

 direction, as represented at a h, it at once darts downwards 

 and forwards in a curve to thus converting the vertical 

 down stroke into a doivn oblique forward stroke. If, again, the 

 wing be suddenly elevated in a strictly vertical direction, as 

 at c d, the wing as certainly darts upwards and forwards in 

 a curve to e, thus converting the vertical up stroke into an 

 upivard oblique fonvard stroke. The same thing happens when 

 the wing is depressed from e to /, and elevated from g to h. 

 In both cases the wing describes a waved track, as shown at 

 e g, g ^, which clearly proves that the wing strikes downwards 

 and forwards during the down stroke, and upwards and foncards 

 during the up stroke. The wing, in fact, is always advancing ; 

 its under surface attacking the air like a boy's kite. If, on 

 the other hand, the wing be forcibly depressed, as indicated 

 by the heavy waved line a c, and left to itself, it will as surely 

 rise again and describe a waved track, as shown at c e. This 

 it does by rotating on its long axis, and in virtue of its flexi- 

 bility and elasticity, aided by the recoil obtained from the 

 air. In other words, it is not necessary to elevate the wing 

 forcibly in the direction c d to obtain the upward and forward 

 movement c e. One single impulse communicated at a causes 

 the wing to travel to e, and a second impulse communicated 

 at e causes it to travel to i. It follows from this that a series 

 of vigorous down impulses would, if a certain interval tvere 

 cdlowed to elapse between them, beget a corresponding series of 

 up impulses, in accordance with the law of action and re- 

 action ; the wing and the air under these circumstances being 

 alternately active and passive. I say if a certain interval 

 were allowed to elapse between every two down strokes, but 



