12 M. C. Martins on the relative Heating } by Solar Radiation, 



all comparable with each other had one thermometer been placed 

 at the surface of the natural soil of the mountain, whilst the 

 other reposed on the soil of M. Costallat's garden; for, as I 

 have elsewhere shown*, superficial soils of different natures are 

 very unequally heated under the influence of solar radiation. In 

 order that the experiments might admit of comparison, it was 

 necessary to observe the heating effect on the same soil at both 

 stations. For this purpose I chose the soil resulting from the 

 decomposition of the wood found in old hollow willow trees ; 

 it is a vegetable soil, since plants, such as briars, honeysuckles, 

 elders, &c, grow thereon very vigorously ; it is moreover homo- 

 geneous, comparable with like soils, and easy to procure in all 

 countries. Its absorbing power is a mean between those of 

 seven other kinds of soil with which I have compared it; its 

 conducting-power for heat is inferior, however, to that of any of 

 the seven, whilst its emissive power is greater. 



The experiments were made in the following manner : — A 

 wooden box, whose length and breadth were equal to 20 centi- 

 metres, and whose depth was 10 centimetres, was filled with the 

 above soil, and a thermometer with a cylindrical bulb was laid 

 upon its surface — the bulb and the tube, up to zero, being covered 

 with a thin layer of soil. Another thermometer, which had a 

 bent tube, was placed so that the bulb was 5 centimetres below 

 the surface of the willow soil. This box was placed on an iso- 

 lated hillock in M. Costallat's garden, and another exactly simi- 

 lar one was placed on the summit of the Pic du Midi by my 

 assistant, M. Pierre Roudier, who for twelve years has assisted 

 me in my meteorological researches with great zeal and intelli- 

 gence. Both boxes were buried so that their surfaces were on 

 a level with the natural soils at the two stations. The stratum 

 of atmospheric air between the two boxes was, as already stated, 

 2326 metres in thickness. 



Our experiments commenced on the morning of the 8th 

 of September 1864, the sky being admirably pure and the air 

 perfectly calm. Every hour from sunrise to 10 in the morning, 

 and every half hour from the latter period to 3 in the afternoon, 

 we observed, first, the thermometer at the surface of the willow 

 soil; secondly, the thermometer which was imbedded therein; 

 thirdly, a thermometer with a small bulb placed in the shade and 

 previously well swung (tourne en fronde) ; and lastly a psychro- 

 meter, also placed in the shade. The sky at Bagneres remained 

 constantly pure, and I was enabled to make all the observations 

 agreed upon. It was otherwise on the Pic du Midi ; the mornings 

 were there magnificent, but towards noon cirro-cumuli rose from 



* Annuaire de la Societe Meteorologique de France, 1863, vol. xi. p. 129 ; 

 and Memoires de VAcad. des Sciences de Montpellier, 1863, vol. v. p. 374. 



