[79] The above observation was made at Carlow on the Barrow in October 1867, and the account of it is taken from my note-book. _The Natural Wing, when elevated and depressed, must move forwards._--It is a condition of natural wings, and of artificial 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. 81. [Illustration: FIG. 81.] If, for example, the wing is suddenly depressed in _a vertical direction_, as represented at _a b_, it at once darts downwards and forwards in a curve to _c_, thus converting the vertical down stroke into _a down 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 _upward oblique forward stroke_. The same thing happens when the wing is depressed from _e_ to _f_, and elevated from _g_ to _h_. In both cases the wing describes a waved track, as shown at _e g_, _g i_, which clearly proves that the wing strikes _downwards 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 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 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 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 were allowed to elapse between them_, beget a corresponding series of up impulses, in accordance with the law of action and reaction; 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 this is practically impossible, as the wing is driven with such velocity that there is positively no time to waste in waiting for the purely mechanical ascent of the wing. That the ascent of the pinion is not, and ought not to be entirely due to the reaction of the air, is proved by the fact that in flying creatures (certainly in the bat and bird) there are distinct elevator muscles and elastic ligaments delegated to the performance of this function. The reaction of the air is therefore only one of the forces employed in elevating the wing; the others, as I shall show presently, are vital and vito-mechanical in their nature.