This last phenomenon, since Saussure's immortal work on the
Alps, has received much light, in recent times, from the labors of Venetz,
Charpentier, and the intrepid and persevering observer Agassiz.
We know only the 'lower', and not the 'upper' limit of perpetual snow; for
the mountains of the earth do not attain to those ethereal regions of the
rarefied and dry strata of air, in which we may suppose, with Bouguer, that
the vesicles of aqueous vapor are converted into crystals of ice, and thus
rendered perceptible to our organs of sight. The lower limit of snow is
not, however, a mere function of geographical latitude or of mean annual
temperature; nor is it at the equator, or
p 330
even, in the region of the tropics, that this limit attains its greatest
elevation above the level of the sea. The phenomenon of which we are
treating is extremely complicated, depending on the general relations of
temperature and humidity, and on the form of the mountains. On submitting
these relations to the test of special analysis, as we may be permitted to
do from the number of determinations that have recently been made,* we shall
find that the controlling causes are the differences in the temperature of
different seasons of the year; the direction of the prevailing winds and
their relations to this land and sea; the degree of dryness or humitidy in
the upper strata of the air; the absolute thickness of the accumulated
masses of fallen snow; the relation of the s-line to the total height of the
mountain; the relative position of the latter in the chain to which it
belongs, and the steepness of its declivity; the vicinity of either summits
likewise perpetually covered with show; the expansion, position, and
elevation of the plains from which the snow mountain rises as an isolated
peak or as a portion of a chain; whether this plain be part of the
sea-coast, or of the interior of a continent; whether it be covered with
wood or waving grass; and whether, finally, it consist of a dry and rocky
soil, or of a wet and marshy bottom.
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