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 (Lower)....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.

APPENDIX.

M. Thury's observations during his winter visit to the Glacière of S. Georges are so curious and valuable, that I give the principal results of them here.

It will be remembered that this glacière consists of a roomy cave, 110 feet long and 60 feet high, with two orifices in the higher part of the roof, one of which is kept covered with the trunks of trees to shut out the direct radiation of the sun. A little thought suggested to M. Thury that the cold in the cave in mid-winter would most probably be greater than the external cold of the day, and less than that of the night; so that there should be a time in the later evening when a column of colder and heavier air would begin, to descend through the hole in the roof. To test the correctness of this supposition, he took up his abode in the cavern for the evening of the 10th January, 1858, with a lighted candle. The flame burned steadily for some time; but at 7.16 P.M. it began to flicker, and soon inclined downwards through an angle of about 45°; and when M. Thury placed himself under the principal opening, the flame was forced into an almost horizontal position. At 8 P.M. the current of air had all but disappeared. This violent and temporary disturbance of equilibrium was a matter of much surprise to M. Thury; for he had naturally expected a quiet current downwards, continuing through the greater part of the night.

At 7.16 P.M. the external temperature was 23·9° F., and the temperature of the atmosphere in the cave at the same time was 30°·88 F.;[[217]] so that there is no wonder the current of air should be strong. It is very difficult to say, however, why it did not commence much earlier, considering that the external air must have been heavier than that in the cave long before 7 o'clock. M. Thury refers to the mirage as a somewhat similar instance, that phenomenon being explained by the supposition that atmospheric layers of different temperatures lie one above another in clearly-defined strata. He suggests, also, that as the heavier air tends to pass down into the cave, the less cold air already in the cave tends to pass out; and the narrow entrance confining the struggle between the opposing tendencies to a very small area, the weaker initial current is able for a time to hold its own against the intruder. On this supposition, it is easy to see that when the rupture does occur it will be violent.

The next day, M. Thury arrived at the glacière at 9.50 A.M. He had determined, in the summer, that the temperature of the cave was invariable, at any rate through the 3-1/2 hours of his visit (from 7.30 to 11 A.M.); but his winter experience was very different. The following are the results of his observations.

In the cave:--

January9,at7.16 P.M.[[218]]...30°·884Fahr.
""7.20 "...29°·75"
""7.27 "...27°·5"
""7.50 "...26°·834"
January10,at10.12 A.M....23°·684"
""10.30 "...23°·9"
""11.20 "...24°·022"
""12.14 "...24°·134"
""1.30 "...24°·35"
""2.30 "...24°·584"
""3.14 "...24°·8"
""4.0 "...25°·142"

Supposing the weather to have been much the same on the 9th and 10th of January, as M. Thury's account seems to say, there is something very strange in the great difference between the temperatures registered at 4 P.M. on the one day, and at 7.16 P.M. on the other.

The external temperatures at the mouth of the cave were as follows:--

January10,at10.53 A.M....25°·934Fahr.
""11.14 "...26°·384"
""11.45 "...28°·04"
""12.32 P.M....27°·944"
""1.12...30°·644"
""3.3...26°·834"
""3.56...25°·7"
""4.26...25°·25"

The minimum temperature of the external air during the night of January 10-11 was 18°·392 F., and that of the glacière 19°·76 F.[[219]] During the preceding night, the minimum in the cave was 22°·442 F., which may throw some light upon the difference between the temperatures at 7.16 P.M. on the 9th, and at 4 P.M. on the 10th.

M. Thury bored a hole, of about 10 inches in depth, in the flooring of ice, and placed a thermometer in it, at 12.25 P.M., closing it up with cotton. At 2.55 P.M., and at 4.7. P.M., the thermometer marked the same temperature, namely, 26°·24 F.

M. Thury's views on glacières in general, based upon the details of the three which he has visited, are much the same as those which I have expressed. He has, however, more belief than I in 'cold currents.'