CONDUCTION AND KADIATION. 145 



from observation or from tlieoiy, must possess great interest and im- 

 portance. 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 interior 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 succession 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 endeavored 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 thermometers deep in the earth, while their stems appeared 

 above the surface. In 1813, '16, and '17, he observed thus the tempera- 

 tures 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 range of oscillation was twenty-five degrees (Fahrenheit) ; 

 at two feet it was twenty degrees ; at four feet it was fifteen degrees ; 

 at eight feet it was only nine degrees and a half. And the time at 

 which the heat was greatest was later and later in proceeding to the 

 lower points. At one foot, the maximum and minimum were three 

 weeks after the solstice of summer and of winter ; at two feet, they 

 were four or five weeks ; at four feet, they were two months ; and at 

 eight feet, three months. The mean temperature of all the thermome- 

 ters was nearly the same. Similar results were obtained by Ott at 

 Zurich in 1762, and by Hcrrenschneider at Strasburg in 1821, '2, '3. 10 



These results had already been explained by Fourier's theory of 

 conduction. He had shown 11 that when the surface of a sphere is 

 affectedby a periodical heat, certain alternations of heat travel uniformly 

 into the interior, but that the extent of the alternation diminishes in 

 geometrical progression in this descent. This conclusion applies to 

 the effect of days and years on the temperature of the earth, and shows 

 that such facts as those observed by Leslie are both exemplifications of 



9 Leslie, art. Climate, Supp. Enc. Brit. 179. 10 Pouillet, Mctiorol. t. ii. p. G-1R 

 J1 J/t'm. Inst. for 1821 (published 1826), p. 162. 

 VOL. II. 10. 



