220 The Realm of Nature CHAP. 



the Calton Hill at Edinburgh during the eight years 1880- 

 1887 were, at the depth of 2 feet, 39-4 in February and 

 53-o in July, an annual range of i3-6, with the minimum 

 and maximum in winter and summer respectively ; and at 

 the depth of 20 feet, 46-9 in January and 45-4 in July, a 

 range of only i-5, with the maximum temperature in 

 winter and the minimum in summer, showing that it 

 requires six months for the conduction of heat from the 

 surface to the depth of 20 feet (compare 229). A 

 zone of invariable temperature lies beyond the reach of 

 solar heat and is found at different depths in different 

 places, being deeper in regions of great annual range of 

 temperature. Beneath the invariable zone temperature 

 increases with depth in all parts of the world. In deep 

 mines the air is always oppressively hot, and the water from 

 deep Artesian wells is warm in proportion to their depth. 

 The Underground Temperature Committee of the British 

 Association, after collecting all the observations of tempera- 

 ture at great depths which have been taken in mines and 

 deep borings in all parts of the world, concluded that the 

 rate at which temperature increases downward averages i 

 in each 5 5 feet ; Professor Prestwich, after a full discussion, 

 puts it at i in 45 feet. 1 In some instances the increase is 

 more rapid, in others less so, according to the conducting 

 power of the rocks. The temperature I mile beneath the 

 surface must be about 100 higher than that of the invari- 

 able layer, and at the depth of 30 miles the temperature 

 must be high enough to melt all known substances. At 

 greater depths than this the rate of increase of temperature 

 must diminish, in accordance with calculations from the 

 condition of small heated bodies. Professor Tait calculates, 

 from the gradient of temperature and the conductivity of 

 rocks, that through every square- foot of surface the interior 

 of the Earth is losing heat at the rate of 230 units (65) 

 per annum, or sufficient to warm I J Ibs. of water from the 

 freezing to the boiling point. 



292. Interior of the Lithosphere. Surface rocks have 

 an average density of 2-5, and the deep-seated igneous 

 rocks a density of about 3-0, while the mean density of the 



