142 Transactions. 
the gull are so slow that the eye can easily follow them. An attentive 
examination will convince anyone that the wings are moved from the shoulder 
straight up and down, or very nearly so, that the elbow joint is not 
appreciably bent during either stroke, but that during the down stroke the 
wrist joint, which bears the primary feathers, is bent back, and expanded 
again during the up stroke. While, therefore, the movement of the main 
part of the wing from the shoulder is nearly vertical, the tips, by having also 
a horizontal movement, do not describe a simple “wave track” in the air, but 
a cycloidal curve as stated by Professor Marey. 
I should, not, however, omit to mention that Dr. Pettigrew, who also says 
that flexion occurs principally at the wrist joint, states that while watching 
rooks he has, over and over again, satisfied himself that the wings are flexed 
during the up stroke. The rook, however, cannot be compared to the gull in 
‘affording facilities for observation. It cannot be seen so near ; it moves its 
wings faster, and it never occurs under those circumstances just mentioned, 
when the bird, although flying through the air, appears to be stationary, 
sometimes for more than a minute at a time. Still, I must allow that confir- 
matory evidence is necessary to others before they can accept my statement as 
correct, while at the same time such evidence would be very satisfactory to me. 
If, however, I am correct in stating that this backward, or rowing motion 
of the primaries, is delivered during the down stroke, it is obvious that it is 
this that drives the bird forward, easily, therefore, explaining the results 
arrived at in the previously mentioned experiments, viz.—that when the 
_ primaries are cut flight is stopped, but that when left intact it is but little 
impeded, although the secondaries are cut off. 
It is also obvious that, in order to preserve a steady line of flight, it will 
be necessary to expose a greater surface of the wing to the air while it is 
being raised than while it is being depressed, in order that it may support the 
bird by its kite-like action, as I have explained in my previous paper on the 
flight of the albatros (Trans. N.Z. Inst., II., 230). The truth of this has been 
proved by the experiments of Professor Marey, who has shown that during 
each complete vibration of the wings, a bird rises and falls twice successively, 
but that these oscillations are unequal in extent, the greater corresponding to 
the depression of the wings, and the lesser to their elevation ; this latter being 
caused by their kite-like action just described. 
From an anatomical examination of the wing, Dr. Pettigrew states that 
“during flexion the anterior margin is slightly directed downwards, and in 
extension decidedly directed upwards.” This is just what we should expect 
= flexion takes place during the down stroke, and it will then agree with 
-Professor — experiments ; and it is, I think, entirely from supposing 
‘that flexion must eens occur during the up stroke that has led Dr. 
