On Solar Radiation. 187 



inches above the surface of the earth ; and because it has 

 been proved by experiment by Mr. Foggo, (No. 27, Edin- 

 burgh Phil. Journal,) that a thermometer so laid upon a grass 

 plat, shewed only 119°, while another not in contact with the 

 earth, rose to 150°. 



This objection is not however a valid one, for it must be 

 remembered, that in England the soil is always wet and 

 damp, and the evaporation of moisture from its surface 

 would alone be sufficient to account for the depression of 

 temperature. 



In the case of Captain Sabine's experiments, it is probable 

 the soil was perfectly dry, not having perhaps for six months 

 been moistened with a shower, and it is well known in the 

 tropics that the soil is always hotter than the air, and there- 

 fore that the experiments, if erroneous at all, must have shewn 

 a result on excess of the true power of solar radiation. 



Mr. Daniel's experiments shew a maximum result of 

 65° for solar radiation, the blackened thermometer being at 

 140°, while in the shade the temperature was only 75°. Mr. 

 Foggo's experiments instead of disproving Daniel's theory, 

 serve very satisfactorily to confirm it ; for he says that the 

 blackened thermometer rose to 15° in the sun's rays near 

 Edinburgh ; while in India no such high temperature has 

 ever been observed. At Chicacole in October 1836, in very 

 dry hot weather, I made some experiments with thermo- 

 meters prepared with every care, with black wool, Indian 

 ink, black varnish, and the highest result I ever obtained 

 was 127° in the open air, while the thermometer in the shade 

 was 84°, or 43° only for the maximum effect of the solar 

 radiation, in latitude 18° 15' N. at the time of year of 

 the observation ; and Dr. Baikie has shewn that on the Niel- 

 gharry hills in January, the maximum radiation is 24° only. 



The best instrument now recommended for observations 

 on solar radiation is Herschell's actinometer, an instrument 

 which I have not had an opportunity of observing with. 



