1872.] Natural and Artificial Flight. 195 
wings constructed on the principle of living wings, that when 
forcibly elevated or depressed, even in a strictly vertical 
direction, they inevitably dart forward. This is well shown 
in Fig. 11.” 
If, for example, the wing is suddenly depressed in a vertical 
direction, as represented at a6, it at once darts downwards and 
forwards in acurve toc, thus conne¢ting the vertical down 
Fic. Io. 
stroke into adown oblique forward stroke. If, again, the wing be 
suddenly elevated in a strictly vertical direction, as at cd, 
the wing as certainly darts upwards and forwards in a curve 
to e, thus connecting the vertical up stroke into an upward 
oblique forward stroke. The same thing happens when the 
wing is depressed from e¢ to f, and elevated from g toh. In 
both cases the wing describes a waved track, as shown at 
é g, g 1, which clearly shows that the wing strikes downwards 
Fic. 11. 
and forwards during the down stroke, and upwards and 
forwards 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 wave line ac, and left to itself, it 
will as surely rise again, and describe a waved track, as 
shown at ce. This it does in virtue of its flexibility 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 move- 
ment c e. One impulse communicated at a causes the 
wing to travel to e, and a second impulse communicated 
at e causes it to travel to 7. It follows from this that a 
series of vigorous down impulses would, if a@ certain interval 
were allowed to elapse between them, beget a corresponding 
series of upimpulses, in accordance withthelaw of aétion and 
reaction, the wing and the air under these circumstances 
