. Any one will allow
that the length of a shadow cast by the sun depends upon two things:
firstly, upon the height of the object which causes the shadow, and
secondly, upon the elevation of the sun at the moment in the sky. The
most casual observer of nature upon our earth can scarcely have failed
to notice that shadows are shortest at noonday, when the sun is at its
highest in the sky; and that they lengthen out as the sun declines
towards its setting. Here, then, we have the clue. To ascertain,
therefore, the height of a lunar mountain, we have first to consider at
what elevation the sun is at that moment above the horizon of the place
where the mountain in question is situated. Then, having measured the
actual length in miles of the shadow extended before us, all that is
left is to ask ourselves the question: "What height must an object be
whose shadow cast by the sun, when at that elevation in the sky, will
extend to this length?"

There is no trace whatever of water upon the moon. The opinion, indeed,
which seems generally held, is that water has never existed upon its
surface. Erosions, sedimentary deposits, and all those marks which point
to a former occupation by water are notably absent.

Similarly there appears to be no atmosphere on the moon; or, at any
rate, such an excessively rare one, as to be quite inappreciable. Of
this there are several proofs. For instance, in a solar eclipse the
moon's disc always stands out quite clear-cut against that of the sun.
Again during occultations, stars disappear behind the moon with a
suddenness, which could not be the case were there any appreciable
atmosphere. Lastly, we see no traces of twilight upon the lunar surface,
nor any softening at the edges of shadows; both which effects would be
apparent if there were an atmosphere.

The moon's surface is rough and rocky, and displays no marks of the
"weathering" that would necessarily follow, had it possessed anything of
an atmosphere in the past. This makes us rather inclined to doubt that
it ever had one at all. Supposing, however, that it did possess an
atmosphere in the past, it is interesting to inquire what may have
become of it. In the first place it might have gradually disappeared, in
consequence of the gases which composed it uniting chemically with the
materials of which the lunar body is constructed; or, again, its
constituent gases may have escaped into space, in accordance with the
principles of that kinetic