198 Natural and Artificial Flight. (April, 
considerable rotation of the posterior margin (h7jkl). This 
figure also shows that the individual primary, secondary, 
and tertiary feathers of the bird’s wing have each what is. 
equivalent to a long and a short axis. Thus the primary 
and secondary feathers marked h2j7k/ are capable of rota- 
ting on their long axes (vy s) and upon their short axes (mn). 
The feathers rotate upon their long axes in a direction from 
below upwards during the down stroke, to make the wing 
impervious to air; and from above downwards during the up 
Fig. 13. 
SS 
a Ae 
stroke, to enable the air to pass through. The primary, 
secondary, and tertiary feathers have thus a distinétly val- 
vular action. They rotate upon their short axes (mn) 
during the descent and ascent of the wing, the tip of the 
feathers rising slightly during the descent of the pinion and 
falling during its ascent.” 
“* The Wing during its Vibration produces a Cross Pulsation.— 
The oscillation of the wing on two separate axes—the one 
running parallel with the body of the bird, the other at right 
angles to it—is well worthy of attention, as showing that 
the wing attacks the air on which it operates in every 
direction, and at almost the same moment, viz., from within 
outwards, and from above downwards, during the down or 
effective stroke ; and from without inwards, and from below 
upwards, during the up or return stroke. As a corollary to 
the foregoing, the wing may be said to agitate the air in two 
principal directions, viz., from within outwards, or the 
reverse, and from behind forwards, or the reverse, the agita- 
tion in question producing two powerful pulsations—a longi- 
tudinal and a lateral; the longitudinal running in the 
direction of the length of the wing, the lateral in the 
direction of its breadth. As, however, the curves of the 
