Such stones closely joyned and compacted together compose a
whole mountain, and that a very firm one; though in summer-time the
country-people have observed it to burst asunder with great cracking,
thunder-like.']
[Footnote 210: See the woodcut illustrating Professor Tyndall's remarks
in the 148th volume of the _Philosophical Transactions_ (1858, p. 214).]
* * * * *
CHAPTER XIX.
ON THE MEAN TEMPERATURE OF THE REGIONS IN WHICH THE GLACIERES OCCUR.
Many interesting experiments have for long been carried on with a view
to determine the mean temperature at various depths below the surface of
the earth. The construction of Artesian wells has afforded useful
opportunities for increasing the amount of our knowledge on this
subject; and the well at Pregny, near Geneva,[211] and the Monk Wearmouth
coal-mines, as observed by Professor Phillips while a fresh shaft was
being sunk,[212] have supplied most valuable facts. Without entering
into any detail, which would be an unnecessary trouble, it may be stated
generally, that, under ordinary circumstances, 1 deg.
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