INTERIOR HEAT OF THE EARTH. 85 



rocks. Hence it has been confidently concluded by many that the 

 earth, beneath a comparatively thin crust of thirty miles, must be liquid. 

 A crust of thirty miles on our globe is equivalent to a crust of less than 

 one tenth of an inch in a globe two feet in diameter. There are, how- 

 ever, many objections to this conclusion. The question of the interior 

 constitution of the earth is one of extreme difficulty and complexity, 

 and science is not yet in a position to solve it completely. Neverthe- 

 less, it can be proved that the solid crust must be much thicker than is 

 usually supposed, if, indeed, there be any general interior fluid at all. 



The argument for the interior fluidity of the earth, beneath a crust 

 of only thirty miles, proceeds upon two suppositions, viz : 1. That the 

 interior temperature increases at the same rate for all depths ; and, 2. 

 That the fusing -point of rocks is the same for all depths. Now, neither 

 of these can be true. 



1. Rate of Increase not uniform. — Although we have spoken of 1° 

 for every 30 feet or 50 feet or 90 feet, yet it must not be supposed 

 that observation gives a uniform rate of increase at any place. On the 

 contrary, the rate is sometimes faster and sometimes slower, depending 

 on the conductivity of the rock penetrated, and on other causes little 

 understood. The rate given is always an average. In other words, 

 observation gives the fact of increase, but not the law. We are thus 

 thrown back on general reasoning. 



If two bars, one a good conductor, like metal, and the other a bad 

 conductor, like charcoal, be heated red hot at one end, and the rate of 

 decreasing temperature — fall of heat — toward the other be observed, it 

 will be found that the rate is very rajrid in the case of the charcoal, so 

 that a temperature of 60° is reached at the distance of two or three 

 inches ; while in the case of the metal the rate of decrease is much 

 slower, and 60° is only reached at a distance of several feet. Con- 

 versely, the rate of increase, or rise, in passing toward a source of heat, 

 is rapid in the case of the bad conductor, and slow in the case of the 

 good conductor. Now, the average density of materials at the surface 

 of the earth is about 2*5, but the average density of the whole earth is 

 more than 5*5 ; therefore the density of the central portions must be 

 much more than 5*5. It has been estimated at 16-27.* There can be 

 no doubt, therefore, that the density of the earth increases toward the 

 center ; and as this increase is probably largely the result of pressure, 

 it is probably somewhat regular. Whatever be the cause, the effect 

 would be to increase the conductivity for heat, and therefore to diminish. 

 the rate of increasing temperature. Thus it follows that, though in 

 a homogeneous globe the melting-point of rocks (3,000°) would be 

 reached at the depth of thirty miles, yet, in a globe increasing in 



* Cosmos, vol. iv, p. 33. 



