Chapter XIX

ON THE MEAN TEMPERATURE OF THE REGIONS IN WHICH THE GLACIÈRES 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° F. of temperature is gained for every 50 or 60 feet of vertical descent into the interior of the earth. I have only met with one account of an experiment made in a horizontal direction, and it is curious that the law of the increase of temperature then observed seemed to be very much the same as that determined by the mean of the vertical observations. Boussingault[213] found several horizontal adits in a precipitous face of porphyritic syenite among the mountains of Marmato. In one of these adits--a gallery called Cruzada, at an elevation of 1,460 mètres--he found an increase of 1° C. of mean temperature for every 33 mètres of horizontal penetration, or, approximately, 1° F. for 60 feet.[214]

Again, observations have been made, in various latitudes, of the decrease of temperature consequent upon gradual rising from the general surface of the earth; as, for instance, in the ascent of mountains. Speaking without any very great precision, but with sufficient accuracy for ordinary purposes, 1° F. is lost with every 300 feet of ascent.[215] It is evident that this decrease will be less rapid where the slope of ascent is gradual, from such considerations as the angle at which the sun's rays strike the slope, and the larger amount of surface which is in contact with a stratum of atmosphere of any given thickness.

With these data, it is easy to arrive at some idea of the probable mean temperature of the rock containing several of the glacières I have described. The elevation of some of them has not been determined with sufficient accuracy to make the results of any calculation trustworthy; but four cases may be taken where the elevation is known--namely, the Glacières of S. Georges, S. Livres, Monthézy, and the Schafloch. If we take as a starting point the mean temperature of the town of Geneva, which has been determined at 49°·55 F., the elevation of that town being nearly 1,200 feet, we obtain the following approximate results for the mean temperature of the surface at the points in question:--

S. Georges .... 40°·22 Fahr.
S. Livres (Lower) .... 38°·55"
Schafloch .... 33°·88"
Monthézy .... 41°·55"

The law of decrease of temperature enunciated by M. Thury gives a higher mean temperature for the surface of the earth in these places, as in the following table:--

S. Georges .... 41°·8 Fahr.
S. Livres .... 40°·1"
Schafloch .... 35°·6"
Monthézy .... 42°·5"

If any certain information could be obtained of the elevation of the Abbey of Grâce-Dieu, I am sure that a result more surprising than that in the case of the Glacière of Monthézy would appear. The elevation of the floor of the church in the citadel of Besançon is 367·7 mètres, and the plateau on the north side of the town of Baume-les-Dames is 531·9 mètres. I am inclined to think, from the look of the country, that the latter possesses much the same elevation as the valley in which the Abbey lies; and in that case we should have comparatively a very high mean temperature for the surface in the neighbourhood where the glacière occurs.

But if these are the mean temperatures of the surface, the natural temperatures of the caves themselves should be still higher, on account of the allowance to be made for increase of temperature with descent into the interior of the earth. This element will very materially affect our calculations in such a case as the lower part of the ice in the Glacière of the Pré de S. Livres, and the strange suggestive beginning of a new ice-cave 190 feet below the surface, on the Montagne de l'Eau, near Annecy. In any open pit or cave, the ordinary atmospheric influences find such easy access, that the temperature cannot be expected to follow the law observed when perforations of small bore are made in the earth, as in the case of the preliminary boring before commencing to dig a well;[216] but the two glacières mentioned above are so completely protected in their lowest parts, that they may be treated as if they were isolated from external influence of all ordinary kinds; and it may fairly be said that the mean temperature there ought to be considerably higher than at the surface.

It is not very likely that the results of the above calculations are strictly in accordance with what a careful series of observations on the spot might show. The distance between Geneva and the Glacières of S. Georges and S. Livres is sufficiently small to make it probable that the reality is not very far different from the calculated temperature; but the other two caves are comparatively so far off, that the temperature and elevation of Geneva are not very safe data to build upon.

FOOTNOTES:

[Footnote 211: Bischof, _Physical Researches_, 189.]

[Footnote 212: _Philosophical Magazine_, v. 446 (1834).]

[Footnote 213: _Annules de Chimie et de Physique_, liii. 2-10. See also Bischof, 136.]

[Footnote 214: The English edition of Bischof affords here a proof of the danger of frequent changes from one scale to another. Bischof in the first instance rendered Boussingault into degrees Réaumur, and this was in turn reduced to degrees Fahrenheit; the result being that the authorised English edition of his book gives 2°·25 F. for 127·5 feet, which does not come within 10 feet of Boussingault's statement.]

[Footnote 215: M. Thury calculates a decrease of 1° C. for every 174 mètres between Geneva and S. Bernard, which is less than the decrease given in the text. He arrives at this conclusion by correcting the mean temperature of Geneva from 8°·9 C., the observed mean of eighteen years, to 9°·9 C., in consequence of supposed local causes, which unduly depress the temperature of Geneva. With the mean 8°·9 C. a result nearly in accordance with that of the text is obtained.]

[Footnote 216: Professor Phillips found, in the course of his investigations in the Monk Wearmouth mines, some hundreds of yards below the sea, that when a new face of rock was exposed, its temperature was considerably higher than that of the gallery or shaft in which it lay. In some cases the difference amounted to 9 and 10 degrees. The rock soon cooled down to an agreement with the surrounding temperature.]

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