14 On the relative Heating of the Soil and of the Air, 



soil at a few centimetres below the surface is still more heated on 

 the mountain than it is in the plain. 



This immense heating of the soil, compared with that of the 

 air on high mountains, is the more remarkable, since during the 

 nights the cooling by radiation is there much greater than in the 

 plain. I have already had the honour of communicating to the 

 Academy (on the 6th of June, 1859) the results obtained by 

 MM. Peltier, Bravais, and myself, by means of Pouillet's swans- 

 down actinometer, on the summit of the Faulhorn, at an altitude 

 of 2683 metres, and on the Grand Plateau below Mont Blanc, at 

 3930 metres above the level of the sea. I afterwards developed 

 the same subject in a memoir on the causes of the low tempera- 

 tures on high mountains*. Two corresponding observations of 

 the minima of the air and of the soil at Bagneres and at the Pic 

 du Midi during the clear nights from the 7th to the 8th, and 

 from the 8th to the 9th of September, will show still more stri- 

 kingly how much more considerable is the cooling of the soil by 

 radiation in the rarefied air of high mountains than in the denser 

 strata of plains. The willow soil being, of all those which I 

 tested, the one whose emissive power is greatest, the low tempe- 

 ratures given in the sixth column of the following Table, con- 

 taining the results we obtained, will cause no surprise. 



Lowest Temperatures of the Air and of the Soil during the Night. 



Dates, 

 Septem- 

 ber 

 1864. 



Bagner.es, altitude 551 

 metres. 



Pic du Midi, altitude 2877 

 metres. 



Remarks. 



Lowest temperature 



Lowest temperature 



Of the 

 air. 



Of the 

 dept 



o-oo 



metre. 



soil at a 

 hof 



0-25 

 metre. 



Of the 

 air. 



Of the 

 dept 



000 

 metre. 



soil at a 

 hof 



0-25 

 metre. 



8 

 9 



14-4 

 14-9 



10a 

 12-9 



13-8 

 13-9 



o 



1-3 



-2-8 

 01 



2 6 | 

 2-1 



Hoar-frost on willow soil- 

 Dewon the soil of the Pic. 



It will be seen that the difference between the lowest tempe- 

 ratures at the surfaces of the soil was 13°* 3 during the night 

 from the 7th to the 8th of September, and 12°*8 during 

 that from the 8th to the 9th, the excess being in favour of the 

 plain. When the sun rises, his oblique rays impart heat, very 

 slowly at first, to the frozen soil of high mountains : thus, 

 although his rays had already struck the summit of the Pic at 

 5.30 a.m., it was not until 7 a.m. that the surface of the soil 

 reached a mean temperature of 9°*3; at 11.30 a.m. this tempe- 

 * Annales de Chimie et de Physique, 3 ser. vol. lviii. (1860). 



