rth at
its surface and at depths of 500 or 600 feet; raising, of course, the
temperature of all springs which have their source at even moderate
depths, and entirely securing them from the effects of frost, which, it
is well known, cannot penetrate the earth to a greater depth than 3 or 4
ft.

275. Many instances might be given of the strong effect of this interior
heat. The glaciers of the Alps, for instance, frequently cover an extent
of three or four square leagues, with a mass of ice 400, 500, or even
600 feet deep, thus entirely preventing the access of exterior heat to
the soil; yet the radiation of heat from the ground itself is so
powerful as to dissolve the ice very rapidly, and to occasion streams of
no inconsiderable size beneath the ice, whose temperature, in summer,
is, I believe, as far as can be ascertained, not many degrees below that
of streams exposed to the air; and the radiation of heat from the water
of these streams forms vaults under the ice, which are frequently 40 ft.
or 50 ft. above the water; and which are formed, as a glance will show,
not by the force of the stream, which would only tear itself a broken
cave sufficient for its passage, but by the heat which radiates from it,
and gives the arch its immense height, and beautifully regular form.

These streams continue to flow in winter as well as in summer, although
in less quantity; and it is this process which chiefly prevents the
glacier from increasing in size; for the melting at the surface is, in
comparison, very inconsiderable, even in summer, the wind being cold,
the sun having little power, and slight frosts being frequent during the
night. It is also this melting beneath the ice (subglacial, suppose we
call it) which loosens the ice from the ground, and occasions, or rather
permits, the perpetual downward movement, with which

"The glacier's cold and restless mass
Moves onward day by day."

276. But more forcible and striking evidence is afforded by experiments
made in mines of great depth. Between 60 ft. and 80 ft. down, the
temperature of the earth is, I believe, the same at all times and in all
places; and below this depth it gradually increases. Near Bex, in the
Valais, there is a perpendicular shaft 677 ft. deep, or about 732 ft.
English, with water at the bottom, the temperature of which was
ascertained by Saussure. He does not tell us whether he used Reaumur's
or the centesimal thermometer; but the result of his experi