46 



NATURE 



[Feb. 29, 1872 



been taken for granted on the strength of the soundness 

 of this doctrine, as we find no allusion to the size of the 

 bulb in M. Soret's account of his observations of solar 

 intensity on Mont Blanc ; nor does Mr. Waterston, who 

 employed a similar instrument during his observations in 

 India, advert to the dimensions of the bulb of the ther- 



mometer exposed to the sun. These physicists apparently 

 overlook the fact that, while the entire convex area of the 

 bulb is exposed to what may be considered the cold radia- 

 tion from the enclosure, only one half receives radiant 

 heat from the sun. This circumstance would be unim- 

 portant if the heat thus received were instantly trans- 



mitted to every part ; but the bulb and its contents are 

 slow conductors, while the conducting power diminishes 

 nearly in the inverse ratio of the square of the depth. 

 Co isequently, by increasing the diameter, the parts of the 

 bulb opposiie to the sun will receive considerably less 

 heat in a given time than if the diameter be diminished. 



T.\BLE A, showini; the result of observ'ations made with Secchi's 

 ' Solar Intensity Apparatus," manufactured by Casella. 



September 6, iS?!, 



T.4BLE B, showing the result of employing differtnt thermo- 

 meters. 



Diameter of Bulb 0*30 in. 



