20 TRANSACTIONS OF THE 
hereto annexed are copied from his drawings. The first repre- 
sents one of the primary feathers, and shows its peculiar emar- 
gination along parts of both the upper and lower edges. ‘The 
next cut exhibits the interlocked primaries as they appeared in 
the wing of a red-shouldered hawk (Luteo lineatus). In this 
figure it will be seen that the cutting of the lower margin of 
each primary extends from the tip of the feather, a distance cor- 
responding to the cutting of the upper margin of the feather 
below it. In this particular case the cuttings extend about half 
the length of the feathers. 
The same figure shows how each of the primaries, beginning 
with the outer one, can, for a distance, corresponding to these 
emarginations, be thrown behind the succeeding primary, cross- 
ing it in such a way that the effect is to interlock the primaries 
in an open or extended position. 
The muscular action by which this locking and unlocking 
can be accomplished at will, appear to be as follows: Referring 
again to the second figure, which represents the wing as it would 
be presented to an observer standing on the ground and looking 
upwards ata bird soaring above, the primaries corresponding 
to the fingers of the human hand now interlocked, are capable 
of being extended and opened by means of a muscle, the tendon 
of which lies along the bones which form the hand. This tendon 
is shown in the figure, detached from the muscle at one end and 
separated from the bone. A force exerted along this tendon 
opens the primaries by acting upon a lever of the third order, 
but with great disadvantage as far as the leverage is concerned. 
If the muscle be relaxed while the feathers are extended, the 
latter will fall together like the leaves of a fan, as they are al- 
ways seen when the wing is folded. This is the process of unlock- 
ing. If, however, the bird wishes to lock the feathers, they are 
first extended by the muscle referred to, and a rotary muscle 
rotates the hand slightly in such a way that if the extensor 
muscle is suddenly relaxed the primaries may fall into their 
interlocked position, especially if the tips of the feathers are 
bent backward by a slight stroke of the wing at the same in- 
stant. 
The operation of locking and unlocking may thus be per- 
formed almost instantaneously and at will. 
This discovery throws much light on the mechanics of flight; 
especially of soaring flight. 
Some birds remain soaring for hours at a time, far above the 
earth, without flapping their wings. During this time, it is 
hardly possible that the primary feathers could be kept extended 
solely by the continued exertion of the comparatively small and 
weak extensor muscle, which has been referred to. ‘The re- 
