of the Soil and of the Air, on a Mountain and in a Plain. 11 



heat of the sun's rays and of the ground, when compared with 

 the temperature of the air in the shade, or with that of the soil 

 at night. As early as 1842, MM. Peltier and Bravais made, 

 from the 10th to the 18th of August, a series of bi-hourly 

 observations on the temperatures of the air and of the soil 

 on the summit of the Faulhorn, a Swiss mountain which rises 

 to a height of 2680 metres above the level of the sea. Experi- 

 ments of a similar kind were made by Bravais and myself, two 

 years afterwards, on the same mountain, between the 21st of 

 September and the 2nd of October. The 125 observations 

 comprised in these two series, and which were continued from 

 six in the morning to six at night, both in fair weather and in 

 foul, under cloudy as well as cloudless skies, gave, notwithstand- 

 ing, a mean temperature of 11°- 75 for the soil during the day, 

 that of the air being only 5 o, 40. It thus became evident that 

 during the day the soil was heated by the sun twice as much as 

 was the air. We did not know, however, what, during the same 

 period, had been the relative heating of the soil and the air in 

 the Swiss plains. For a long time I was anxious to remove this 

 incompleteness, by determining the relative heating of the same 

 kind of soil, at the same moment, on a lofty mountain and on an 

 opeu plain, when the sky was pure and the air calm. Bagneres- 

 de-Bigorre and the Pic du Midi appeared to me to possess all 

 the conditions desirable for experiments of this nature. The 

 horizontal distance between the two places, as measured on the 

 new map of the Etat-Major, is not more than 14,450 metres, 

 and the two points are under the same meridian. The Pic du 

 Midi, perfectly isolated from the principal chain of the Pyrenees, 

 rises to a height of 2877 metres above the sea — an altitude 

 which merits all confidence, since the Pic du Midi was selected 

 as one of the principal points of the triangulation upon which 

 the new map of France is based. I was moreover able to con- 

 nect, by a single sight through a level, my point of observation 

 in the garden of my friend Dr. Costallat, at Bagneres, with the 

 system of levels connected with the railways of France. The point 

 in question was thus ascertained to be 551 metres above the level 

 of the ocean, giving a difference of 2326 metres between the alti- 

 tudes of my two stations. Besides this, the valley of Bagneres has 

 the advantage of not being one of those narrow ones where the 

 reflexion of the sun's rays increases the temperature; its breadth, 

 in fact, taken from crest to crest of the hills by which it is 

 bounded on the east and west, amounts to 2800 metres. It will 

 be admitted, therefore, that it would be difficult to find in the 

 Alps or in the Pyrenees two stations more favourably situated 

 for the success of the comparative observations which I had in 

 view. These observations, however, would not have been at 



