of Edinburgh, Session 1870 - 71 . 
141 
The wing of the insect is, in some cases (the wasp, for instance), 
folded upon itself during the back stroke to avoid the resistance of 
the air ; in other cases, when two pairs of wings are present (the 
butterfly, for example), the first pair of wings is made to overlap 
the second pair for a similar purpose. 
When the wing is in one piece, and made to vibrate in a more or 
less horizontal direction, it is followed in its passage from right to left 
by a current which the wing meets in its passage from left to right. 
When the wing passes from left to right it is followed by a current 
which the wing meets in its passage from right to left, and so on. 
The wing has therefore the power of creating the current on which 
it rises. 
When the wing is in several pieces, and made to vibrate more or 
less vertically, one portion of the pinion (during the acts of exten- 
sion and flexion) makes a current which another portion utilises. 
Thus the tip and root of the wing (hand and arm) make a current 
during extension on which the middle part of the wing (fore-arm) 
acts during flexion, and the reverse. This arrangement begets a 
cross pulsation, and extends in the bird even to the primary and 
secondary feathers. The wing may thus be said to rise upon a 
whirlwind of its own forming. 
The wing has the power of producing artificial currents, and of 
utilising and avoiding natural currents, so that it is equally adapted 
for flying in a calm and in a storm. As the wing (or parts of the 
wing) strikes in opposite directions, it in this manner reciprocates, 
the down stroke running into and contributing indirectly to the 
efficacy of the up stroke, and the reverse. The down and up strokes 
consequently form one continuous act, and neither is complete 
without the other. The down stroke produces the current on 
which the wing operates during the up stroke, and vice versa. 
The reciprocation of the wing is most perfect when the animal 
is fixed in one spot, and least perfect when it is flying at a high 
horizontal speed. It is, however, a matter of indifference whether 
the wing attacks the air or the air attacks the wing, so long as a 
sufficient quantity of air is worked up under the wing in any given 
time. 
The wing of the bat and bird are drawn towards the body and 
flexed at the termination of the down stroke to destroy the 
3 A 
VOL. VII. 
