INCREASE OF THE EARTH'S INTERNAL HEAT. 169 



fused and incandescent, yet the earth may have been built up 

 gradually by the infalling of meteoric matter. Either view gives us 

 a high original temperature, which presumably has been reduced by 

 radiation, so that the interior is now much hotter than the sur- 

 face. It is well known that in mines, and deep wells, and borings, 

 the temperature steadily augments, though the rate of increase varies 

 with the locality; and the conducting power of the rocks is so 

 variable that the increase of temperature beneath the surface is rarely 

 regular. The most celebrated experiment, made at Sperenberg, near 

 Berlin, reaching to a depth of 4172 feet, and passing entirely through 

 rock salt, demonstrates that with increasing depth the distance 

 which has to be passed through to gain an additional degree of 

 temperature, augments. In the first 300 feet the increase was at the 

 rate of i R. for 33 feet and 45 feet; at 400 feet and 900 feet the in- 

 crease was only at the rate of about i R. for 500 feet ; at 2500 feet 

 it was i R. for 166 feet ; and at upwards of 4000 feet 1 R. for 310 

 feet. In the first 1000 feet there was an increase of 11 R., in the 

 second 1000 feet of 7 R., and in the next 2000 feet of 10 R., so that 

 the rate at which the temperature augments steadily diminishes, 1 which 

 may be only another way of saying that radiation is rapid in pro- 

 portion to nearness to the earth's surface. In other localities the 

 temperature at moderate depths increases i F. for 15 feet, and most 

 of the temperatures fall between 30 feet to 60 or 70 feet for a degree. 2 

 In our own country, the average increase is i F. for 51^ feet. 

 Hence we may conclude that an enormous amount of heat has been 

 lost by the earth's surface cooling, that this loss of heat is still in 

 progress, and that there is an immense amount of internal heat un- 

 exhausted. Whether temperature goes on augmenting to the centre 

 of the earth, or whether it soon reaches a constant amount compara- 

 tively near t the surface, is a matter of small moment practically, if 

 ws bear the earth's rigidity in mind ; for then, since the pressure of 

 superincumbent rock increases beneath the surface, side by side with 

 the augmentation of temperature, and since liquefaction of highly 

 heated rock only takes place when the pressure is removed, it is 

 probable that the temperature may never overcome the pressure 

 so as to render the interior liquid. Since we can no longer 

 assume a thin crust for the earth, it is impossible to believe this crust 

 to be blistered by liquid rock boiling beneath it. And we as neces- 

 sarily abandon the ideas that igneous rocks are protrusions of the 

 original fluid substance of the earth, and that volcanoes are chimneys 

 by which such molten rock rises to the earth's surface. Many eminent 

 men of modern times, however, still accept the old idea of internal 

 igneous fusion. 



Hypothesis of Magmas. M. Durocher propounded the hypo- 

 thesis of there being beneath the surface rocks two magmas, from 

 which all erupted rocks were derived. The outermost of these layers 

 was supposed to correspond with the acidic rocks, and to have yielded 



1 See Fisher, "Physics of the Earth's Crust," p. 10. 



2 Mallet, "Report on the Neapolitan Earthquake," vol. ii. p. 310. 



