530 HISTORY OF THERMOTICS. 



principles. Still, such knowledge, whether obtained 

 from observation or from theory, must possess great 

 interest and importance. The doctrines of this 

 kind which we have to notice refer principally to 

 the effect of the sun's heat on the earth, the laws 

 of climate, the thermotical condition of the in- 

 terior of the earth, and that of the planetary 

 spaces. 



1. Effect of Solar Heat on the Earth. That 

 the sun's heat passes into the interior of the earth 

 in a variable manner, depending upon the succes- 

 sion of days and nights, summers and winters, is 

 an obvious consequence of our first notions on this 

 subject. The mode in which it proceeds into the 

 interior, after descending below the surface, re- 

 mained to be gathered, either from the phenomena, 

 or from reasoning. Both methods were employed 9 . 

 Saussure endeavoured to trace its course by digging, 

 in 1785, and thus found that at the depth of about 

 thirty-one feet, the annual variation of temperature 

 is about 1-1 2th what it is at the surface. Leslie 

 adopted a better method, sinking the bulbs of ther- 

 mometers deep in the earth, while their stems 

 appeared above the surface. In 1815, 16, and 17, 

 he observed thus the temperatures at the depths 

 of one, two, four, and eight feet, at Abbotshall, in 

 Fifeshire. The results showed that the extreme 

 annual oscillations of the temperature diminish as 

 we descend. At the depth of one foot, the yearly 



9 Leslie, art. Climate, Supp. Enc. Brit. 179. 



