Because of the inertia of the atmosphere, which gives effect to their
wings. Were it possible for a bird to live without respiration, and in
a space void of air, it would no longer have the power of flight. The
plumage of the wings being spread, and acting with a broad surface
on the atmosphere beneath them, is resisted by the inertia of the
atmosphere, so that the air forms a falcrum, as it were, on which the
bird rises, by the leverage of its wings.

_Why is air generally considered to be invisible?_

Because, though a coloured fluid, and naturally blue, its colour
acquires intensity only, or, in other words, becomes visible only,
from the depth of the transparent mass. According to rigid Newtonians,
air is transparent, or, rather, invisible; and the azure colour of the
atmosphere arises from the greater refrangibility of the blue rays of
light. Other philosophers imagine that the blue tint is inherent in
air; that is, that the particles of air have the property of producing
a blue colour, in their combination with light.

_Why are the most distant objects in a prospect of a blue tinge?_

Because their colours are always tinted by the deepening hues of the
interjacent atmosphere. Again, the blending of the atmospheric azure
with the colours of the solar rays, produces those compound and
sometimes remarkable tints, with which the sky and clouds are
emblazoned. Hence, the mountains appear blue, not because that is
their colour, but because it is the colour of the medium through which
they are seen.