440 New Oiitrtnes of Chemical Philosophy. 



"From these observations it appears, that a greater diminu- 

 tion of heat frequently takes place near the earth in the night- 

 time, than at any elevation in the atmosphere within tlie limits 

 of Mr. Six's inquiry ; and that the greatest degrees of cold are 

 at such times always found nearest to the surface of the earth ; 

 that this is a constant and regular operation of nature, under 

 certain circumstances and dispositions of the atmosphere, and 

 takes place at all seasons of the year ; that this difference never 

 happens in any considerable degree but when the air is still, 

 and the sky perfe«ilv unclouded ; but when the moistest va- 

 pour, such as dews and fogs, did not, as far as he could perceive, 

 at all impede, but rather increase, the refrigeration. In very 

 severe frosts, when the air frequently deposits a great quantity 

 of frozen vapour, ho generally found it greatest ; but the excess 

 of heat, which in day-time, in the summer season, was foimd 

 at the lower station, in the winter diminished almost to no- 

 thing *." 



The truth of these experiments may be verified by a few fa- 

 miliar observations. 



When the moisture in the air begins to be frozen in the night, 

 hoar-frost is seen upon the bare ground before it is formed upon 

 the grass, trees, and the roofs of houses ; but in the morning it 

 vanishes from the bare ground some time before it ceases to ap- 

 pear upon the grass and higher objects. Hence it is evident, 

 that under certain circumstances, the air is usually some degrees 

 warmer by day and colder by night, close to the ground, than 

 at a few inches above it. 



These variations of local heat are the effects of that universal 

 lavy of nature which causes the ordinary vicissitudes of tempera- 

 ture, that are daily taking place in our atmosphere. 



It has already been advanced in these outlines, that no in- 

 crease of temperature can be produced unless thermogen and 

 photogen be united to matter \. 



In the j)resent instance, the solar influence increases the tem- 

 peratiu'e of tliQ air, and the thermogen that it contains. This 

 element, iiaying its temperature increased, attracts the photogen 

 from the earth ; hence a greater degree of heat is generated upon 

 the surface of the ground than at a few feet above it. 



But as soon as the sun descends below the horizon, the tem- 

 perature of the thermogen contained in the air diminishes, and 

 its attraction for photogen gradually decreases till the attraction 

 of the earth for that element commences, in consequence of 

 'which the photogen returns to the earth, and a diminution of 

 temperature takes place upon the surface of the bare gyoimd, 



* Phil. Trans, abr. \ ol, xvi.p. d04. f Phil. Mag, vol. xliii. p. S40. 



before 



