THE EARTH'S INTERNAL HEAT. 1 3 



low this variable surface layer, though the rate of 

 increase differs with the kinds of rock passed 

 through, and with the locality. It averages one 

 degree Fahrenheit for every 55 feet of depth. 



In the famous Artesian well at (irenelle near 

 Paris, the water rose from a depth of 1794 English 

 feet, with a temperature of nearly 82 F. The 

 deep boring at Sperenberg near Berlin appears to 

 show an increase of i° F. in 42 feet at the depth 

 of 1000 feet; t° F. in 57 feet, at 2000 feet; and i° 

 F. in 95 feet, at ;>ooo and 4000 feet. From these 

 facts the inference has been made that tempera- 

 ture does not augment appreciably below a mod- 

 erate external thickness of rock. 



The difference between the surface tempera- 

 ture and the interior temperature, results from the 

 loss of the earth's internal heat by radiation. On 

 this circumstance attempts have been made to 

 timate the duration of geological time. By mt 

 uring the amount of heat which the earth radiates 

 from its surface in a year, I rd Kelvin has con- 

 cluded that in a period of 20,000 millions of years, 

 more than enough heat would have been lost to 

 melt the entire bulk of the earth, if the rate of 

 loss had been always what it is now, and if the 

 earth had consisted throughout of the same ma- 

 terials as its surface rocks. This is the time which 

 the physicist conceives as possible for the earth's 

 origin and history. Sir John Herschel had doubt- 

 ed Xhe primitive fluidity of the earth. It is per- 

 haps possible that the heat which the earth loses 

 may not be the original heat of an igneous fusion, 

 but the result of strain due to its rigid state. It 

 rotates so that its surface experiences the lifting 

 influence of tidal attraction which reduces the 

 pressure, although the amount is too small to dis- 



