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 deg. F. is lost with every 300 feet of ascent. 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 glacieres 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 Glacieres of S. Georges, S. Livres, Monthezy, and the Schafloch. If we take as a starting point the mean temperature of the town of Geneva, which has been determined at 49 deg..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 deg..22 Fahr.
S. Livres (Lower) .... 38 deg..55”
Schafloch .... 33 deg..88”
Monthezy .... 41 deg..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 deg..8 Fahr. S. Livres .... 40 deg..1” Schafloch .... 35 deg..6” Monthezy .... 42 deg..5”
If any certain information could be obtained of the elevation of the Abbey of Grace-Dieu, I am sure that a result more surprising than that in the case of the Glaciere of Monthezy would appear. The elevation of the floor of the church in the citadel of Besancon is 367.7 metres, and the plateau on the north side of the town of Baume-les-Dames is 531.9 metres. 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 glaciere occurs.