52 
counteract the force of gravity ; and in proportion to the increasing 
strength of the wind, the amount of vane which must he exposed 
to it becomes less and less. I have seen a kestrel stand suspended 
in a half-gale, with the wings folded close to the body, and with 
no visible muscular motion whatever. And so nice is the adjust- 
ment of position which is requisite to produce this exact balance of 
all the forces bearing on the bird, that the change in that position 
which again instantly results in a forward motion, is very often 
almost insensible to the eye. It is generally a slight expansion of 
the wings, and a very slight change in the axis of the body.”* 
In order to turn when on the wing, a bird does not shorten the 
stroke of one wing and exercise the other with greater violence, as 
we should do with the oars of a boat, to produce the same effect, 
nor does it use its tail as a rudder, or the swift, which has a very 
small tail, but great facility of evohition, would be unable to 
pursue its circling flight — but it appears to be the result of an 
involuntary effort of the bird, as we turn or incline to the right ox- 
left in walking ; the wing on the inside of the curve is depressed, 
and that on the outside elevated, to a corresponding degree, and in 
proportion to the sharpness of the curve. Captain Hutton says, 
the albatross turns in this way when soaring, the wings remaining 
rigid the whole time, and sometimes depressed to such an extent 
as to be almost perpendicular to the sea ; and this inclination of 
the wings is ahoays seen when the bird is turning, but at no other 
time. The same may be constantly observed in the swift. The 
tail having no lateral motion, could have little or nothing to do 
with the change of direction. The chief uses of the tail appear to 
be, the additional surface it enables the bird to present for its 
support in the air — to assist it in rising or falling by its upward ox- 
downward deflection ; it also, doxxbtless, assists the bird in stopping 
or slowing itself, and in maintaining its general equilibrium. 
The rapidity with wlxich the wing strokes follow each other ixx a 
strong flying bird is enormoixs ; so rapid are they, that ixx many 
birds it is impossible to follow them with the eye. The Duke of 
Argyll says, the heron, Avhich is remarkable fox- its slow aixd heavy 
flight, flaps its wings no less than 120 to 150 times ixx a xninute ; 
counting both the up and dowix strokes, there arc 240 to 300 
separate movements per minute. But this is as nothing conxpared 
* “ Reigu of Law,” pp. 160—2. 
