CONDUCTION AND RADIATION. 531 



range of oscillation was twenty-five degrees (Fah- 

 renheit) ; 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 thermometers was 

 nearly the same. Similar results were obtained by 

 Ott at Zurich in 1762, and by Herrenschneider 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 affected by a 

 periodical heat, certain alternations of heat travel 

 uniformly into the interior, but that the extent of 

 the alternation diminishes in geometrical progres- 

 sion 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 the 

 general circumstances of the earth, and are per- 

 fectly in accordance with the principles on which 

 Fourier's theory rests. 



2. Climate. The term climate, which means 



10 Pouillet, MeteoroL t. ii. p. 643. 



11 Mem. Inst. for 1821 ( pl ihlished 1826), p. 162. 



MM2 



