of the Soil and of the Air, on a Mountain and in a Plain, 13 



the Spanish side, and, driven by a slight breeze from the south, 

 gradually enveloped the Pic. Nevertheless during the three days 

 I was able to select twenty corresponding and perfectly com- 

 parable observations, made alike under a pure sky, in full sun- 

 light and in calm air : these coincidences occurred between 7 in 

 the morning and 2 in the afternoon. 



With reference, first, to the heating of the superficial soil, the 

 mean of the twenty observations on the temperature of the air in 

 the shade was 22°*3 at Bagneres, and only 10 o, l at the Pic du 

 Midi. The mean temperature of the surface of the soil was 36°*1 

 at Bagneres, and 33°'8 at the Pic. The mean excess of the 

 temperature of the soil above that of the air at Bagneres had 

 consequently to the like excess at the Pic the ratio of 10 to 17; 

 in other words, it was nearly double on the mountain. Further, 

 although the mean temperature of the soil in the plain was 2°'3 

 above that of the soil on the mountain; still, on the 10th of 

 September, between 11 and 11.30 a.m. (after which latter time 

 the Pic became enveloped in clouds), the temperature of the soil 

 at its summit was 6°*9 higher than that of the soil at Bagneres — 

 the mean temperature of the air at the latter station being 

 23°*2, and at the former 1 3°*8. The absolute maxima observed at 

 the surface of the soil in the twenty corresponding observations 

 were 50 o, 3 at Bagneres (2 p.m., September 9, temperature of air in 

 the shade 27°'l), and 52°*3 on the Pic du Midi (11.30 a.m., 

 September 10, temperature of air in the shade 13°*2). These 

 experiments place beyond doubt the fact of the greater calorific 

 power of the sun on the mountain than in the plain. 



I proceed, secondly, to the heating of the soil at a depth of 

 5 centimetres. I have already stated that the soil resulting 

 from the decomposition of the wood of the willow was penetrated 

 with greater difficulty b}^ solar heat than was any one of the 

 other soils I examined, and besides this that it was the best radiator. 

 The physicist will not be at all surprised at this. The greater 

 relative heating power of the sun on the mountain is shown by 

 the thermometers whose bulbs were at a depth of 0*05 of a 

 metre, as well as by those whose bulbs rested on the surface. 

 Thus the mean temperature of the soil at a depth of 0*05 of a 

 metre was 25°*5 at Bagneres, that is to say, only 3°*2 above the 

 temperature of the air, whilst on the Pic du Midi the first tem- 

 perature was 17° *1, higher consequently by 7 degrees than the 

 temperature of the air surrounding this elevated summit. The 

 mean thermic excess, at Bagneres and at the Pic, of the soil at 

 a depth of 0*05 of a metre, is therefore as 10 : 22 ; that is to 

 say, on the mountain the excess in question is more than 

 double that in the plain. On the surface of the soil the ratio, 

 as we have seen, was as 10 to 17. Relatively, therefore, the 



