Nature of the Earth's Interior. 165 



man, increases in direct ratio as we descend towards its centre. 

 Other observations on the temperature of the water from deep-seated 

 and hot springs, and from Artesian wells, fully confirm the ex- 

 periments made in mines, and show that the temperature of the 

 water furnished by them also becomes more elevated in proportion 

 to the depth of the source from which it is derived. 



As might naturally be expected, the interference of local circum- 

 stances renders it a matter of considerable difficulty to determine the 

 true mean rate of such increase in temperature of the earth's sub- 

 stance downwards ; still, in the main, observers all agree in placing 

 it at somewhere between 1\° and 2^° Farenheit for every hundred 

 feet in depth downwards, so that we shall not be far wrong if for 

 our present object we estimate it at 2° Farenheit for every hundred 

 feet, a rate which will be equivalent to 121° for each geographical 

 mile nearer the earth's centre. Since no facts are at the present 

 time known which can in any way invalidate the supposition that 

 this or a somewhat similar rate of increase in temperature holds 

 good at still greater depths, it appears to be perfectly correct and 

 justifiable reasoning to assume that such is actually the case; whence 

 it follows by a very simple calculation that, at a depth of about 

 twenty-five geographical miles from the surface downwards, a 

 temperature of about 3000° Farenheit should be attained, which 

 would represent a heat at which iron melts, or one which is suf- 

 ficient to keep lava in a state of perfect fusion at the surface of the 

 earth. As it must be remembered, however, that at this depth the 

 substance of the earth would be exposed to the pressure of the 

 superincumbent mass, and as it has been demonstrated by experi- 

 ment that many substances become more refractory, i.e., require a 

 greater heat to melt them or keep them in the molten state when 

 exposed to pressure, the above calculation will have to be modified 

 considerably in order to meet this condition of things. Unfortu- 

 nately, we have not as yet sufficient data at command to enable us to 

 settle the true ratio in which the melting points of rocks would become 

 elevated by increased pressure ; yet we may safely take it for granted, 

 after allowing far more than the maximum rate of increase found in 

 the experiments of Bunsen and Hopkins, that we should not require 

 a distance as deep again in order to reach an internal temperature 

 fully sufficient to keep such substances in a state of fusion, or in other 

 words, to necessitate the inference that the solid rock crust of our 

 earth cannot, at the utmost, be more than fifty miles in thickness. 



If now we reason from the above data as our premises, it will 

 follow as a natural consequence, that our globe must in reality be a 

 sphere of molten matter surrounded by an external shell or crust of 

 solid matter of very insignificant thickness when compared to the 

 diameter of the entire globe itself, and that in point of fact this 

 deduction represents exactly such a state of things as would be 

 brought about in the event of a sphere of molten matter becoming 

 consolidated on its exterior by the cooling action of the surrounding 

 atmosphere ; and the figure of the earth itself, which is an ellipsoid 

 .of revolution, i.e., a sphere somewhat flattened at the poles, but 



