90 CONDUCTORS OF HEAT. 



he fatal to them. Hair is a better conductor than wool : 

 hence, by nature's alchemy, hair changed into wool in 

 the animals of some countries on the approach of winter, 

 and feathers into down. 



It is therefore evident that the rate at which solar 

 heat is conducted into the crust of the earth must alter 

 with the condition of the surface upon which it falls. 

 The conducting power of all the rocks which have been 

 examined is found to vary in some degree.* 



It follows, as a natural consequence of the position of 

 the sun to the earth, that the parts near the equator 

 become more heated than those remote from it. As this 

 heat is conducted into the interior of the mass, it has a 

 tendency to move to the colder portions of it, and thus 

 the heat absorbed at the equator flows towards the poles, 

 and from these parts is carried off by the atmosphere, or 

 radiated into space. Owing to this, there is a certain 

 depth beneath the surface of our globe at which an equal 

 temperature prevails, the depth increasing as we travel 

 north or south from the equator, and conforming to the 

 contour of the earth's surface, the line sinking under 

 the valleys and rising under the hills. t 



* On this subject consult Robert Were Fox, On the Temperature 

 of the Mines of Cornwall. Cornwall Geological Transactions, 

 vol. ii. ; W. J. Henwood, on the same subject. Ib. vol. v. ; Reports 

 of the British Association, 1840, p. 315; Edinburgh New Philoso- 

 phical Journal, vol. xxiv. p. 140. 



f On the causes of the temperature of Hot and Thermal Springs ; 

 find on the bearings of this subject as connected with the general 

 question regarding the internal temperature of tha Earth : by 

 Professor Gustav Bischoff, of Bonn. Edinburgh New Philoso- 

 phical Journal, vol. xx. p. 376 ; vol. xxiii. p. 330. Some interest- 

 ing information on the temperature of the ground will be found in 

 Erman's Travels in Siberia, translated by W. D. Cooley, vol. i. p. 

 J:W; vol. ii. p. 300. Sur la Profondeur a laquelle se trouve la 

 couche de Temperature invariable entre les Tropiques, by Boussin- 

 gault : Annales de Chimie et de Physique, 1833, p. 225. 

 Reference may also be made to Humboldt's Cosmos, Otto's 

 translation ; , and to the excellent article on Meteorology, by 



