Hence we find, that every cave, crevice, place, and
vessel, having communication with the atmosphere, if it be not filled
with something else, is filled with air; against which it is no
argument that we do not see it, as it is perfectly transparent, and
consequently invisible.
_Why do birds fly?_
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.
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