398 Questions Jbr Soluiimi relating to Meteorology^ 



It has been known for half a century, that a thermometer 

 placed under a clear sky, on the grass of a meadow, indicates 

 11°, 12i% or even 14° Fahr. less than a thermometer, in every 

 respect similar, suspended in the air, at a few feet from the 

 ground. But it is only a few years since an explanation of this 

 phenomenon was given ; for it was only in 1817 that Wells esta- 

 blished the fact by means of important experiments, and in a 

 thousand different ways, that this inequality of temperature is 

 caused by the feeble radiating poiver of a clear slty. 



A screen placed between certain solid bodies and the sky pre- 

 vents them from cooling, because the screen intercepts their ra- 

 diating communications with the colder regions of the atmo- 

 sphere. The clouds act in the same manner; they take the 

 place of the screen. But if we distinguish every vapour which 

 intercepts the solar rays coming from above, or the calorific rays 

 ascending from the earth towards the sky, by the name of a 

 cloud, it cannot be said that the atmosphere is ever entirely 

 free from them. The only difference is their greater or less 

 density. 



These differences, however slight they may be, may be indi- 

 cated by the degree of cold to which solid bodies are reduced in 

 the night ; and this accompanying peculiarity is worthy of obser- 

 vation, that the transparency measured in this manner, is the 7/iean 

 transparency of the entire firmament, and not that alone of the 

 circumscribed region which may be occupied by a single star. 



In order to make these experiments under the most favour- 

 able conditions, it is obvious that we must choose bodies which 

 cool most by radiation. According to the researches of Wells, 

 swan-down is the substance that ought to be selected. A ther- 

 mometer, having its bulb surrounded with this down, should be 

 placed on a table of painted wood supported by slender feet, in 

 a situation where nothing intercepts the view to the horizon. 

 A second thermometer, with the bulb naked, should be sus- 

 pended in the air at some height above the ground. With re- 

 gard to the latter, a screen will secure it from all radiation towards 

 the sky. In England, Wells obtained a difference of 15° Fahr. 

 between the indications of two thermometers placed in the 

 manner described. It would certainly be strange, if less import- 

 ant differences were to result from them in equinoctial coun- 

 tries, which have been so much praised for the purity of their 



