y upwards and partly horizontally forwards. That part of the
force which has the upward tendency is neutralized by the weight of the
bird, whilst the horizontal force serves to carry it forward. The stroke
being completed, it moves upon its wings, which, being contracted and
having their edges turned upwards, obviate, in a great measure, the
resistance of the air. When it is sufficiently elevated, it makes a
second stroke downwards, and the impulse of the air again moves it
forward. These successive strokes may be regarded as so many leaps taken
in the air. When the bird desires to direct its course to the right or
the left, it strikes strongly with the opposite wing, which impels it to
the proper side. In the motions of the animal, too, the tail takes a
prominent part, and acts like the rudder of a ship, except that, instead
of sideways, it moves upwards and downwards. If the bird wishes to rise,
it raises its tail; and if to fall, it depresses it; and, whilst in a
horizontal position, it keeps it steady. There are few who have not
observed a pigeon or a crow preserve, for some time, a horizontal flight
without any apparent motion of the wings. This is accomplished by the
bird having already acquired sufficient velocity, and its wings being
parallel to the horizon, meeting with but small resistance from the
atmosphere. If it begins to fall, it can easily steer itself upward by
means of its tail, till the motion it had acquired is nearly spent, when
it must be renewed by a few more strokes of the wings. On alighting, a
bird expands its wings and tail fully against the air, as a ship, in
tacking round, backs her sails, in order that they may meet with all the
resistance possible.

919. IN THE CONSTRUCTION OF THE EYES of birds, there is a peculiarity
necessary to their condition. As they pass a great portion of their
lives among thickets and hedges, they are provided for the defence of
their eyes from external injuries, as well as from the effects of the
light, when flying in opposition to the rays of the sun, with a
nictating or winking membrane, which can, at pleasure, be drawn over the
whole eye like a curtain. This covering is neither opaque nor wholly
pellucid, but is somewhat transparent; and it is by its means that the
eagle is said to be able to gaze at the sun. "In birds," says a writer
on this subject, "we find that the sight is much more piercing,
extensive, and exact, than in the other orders of animals. The ey