302.
When the smoke from dry wood is seen between the eye of the
spectator and some dark space [or object], it will look blue. Thus
the sky looks blue by reason of the darkness beyond it. And if you
look towards the horizon of the sky, you will see the atmosphere is
not blue, and this is caused by its density. And thus at each
degree, as you raise your eyes above the horizon up to the sky over
your head, you will see the atmosphere look darker [blue] and this
is because a smaller density of air lies between your eye and the
[outer] darkness. And if you go to the top of a high mountain the
sky will look proportionately darker above you as the atmosphere
becomes rarer between you and the [outer] darkness; and this will be
more visible at each degree of increasing height till at last we
should find darkness.
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