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.

_Why do the Heavens appear blue?_

Because of our looking at the dark vacuity beyond our atmosphere
through an illuminated medium. Were there no atmosphere, it is
universally admitted the appearance would be perfectly black, except
in the particular direction of the sun, or some other of the heavenly
bodies, and since the atmosphere is transparent, this blackness (if
such an expression may be used) must be seen through it, only somewhat
modified by the rays of light reflected by the atmosphere to the eye,
from the direction in which we look. For this reason, the clearer or
more transparent the atmosphere is, the darker is the appearance of
the heavens, there being then less light reflected by the atmosphere
to the eye. In the zenith, the appearance is always darker than nearer
the horizon; and from the tops of high mountains, the heavens in
the zenith appear nearly black.--_Mr. B. Hallowell, in the American
Journal of Science and Arts._

_Why does the heat of temperature of different parts of the earth
vary?_

Because of the position of the place with respect to the equator, or
rather to the ecliptic, or, more strictly still, with respect to the
plane in which the earth revolves around the