[Footnote 202:  Branch Physique, article Glace]

[Footnote 203:  P. 146 (an. 1853).]

[Footnote 204:  Dr. Lister experimented on sea-water in December 1684 (Ph.  Trans, xiv. 836), and found that though it took two nights to freeze, it was much harder when once frozen than common ice, lasting for three-quarters of an hour under a heat which melted 100 times its bulk of common ice at once.  It was marked with oblong squares, and had a salt taste.  Ice formed from water with an admixture of sulphuric acid is said to assume a crystalline appearance.]

[Footnote 205:  See also a pamphlet entitled Das unterirdische Eisfeld bei der Dornburg am Suedlichen Fusse des Westerwaldes, by Thomae of Wiesbaden (32 pages, with a map of the district), published in 1841.]

[Footnote 206:  But see page 262.]

[Footnote 207:  lv. (an 1842), 472.]

[Footnote 208:  Journal de Physique, xxvi. (an 1785), 34.]

[Footnote 209:  In looking through some early volumes of the Philosophical Transactions, I found an ’Extract of a letter written by Mr. Muraltus of Zurich (September 1668), concerning the Icy and Chrystallin Mountains of Helvetia, called the Gletscher, English’d out of Latin’ (Phil.  Trans. iv. 982), which at first looked something like an assertion of the prismatic structure of ice on a large scale.  The English version is as follows:—­’The snow melted by the heat of the summer, other snow being faln within a little while after, and hardened into ice, which by little and little in a long tract of time depurating itself turns into a stone, not yielding in hardness and clearness to chrystall.  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.  F. of temperature is gained for every 50 or 60 feet of vertical descent into the interior