THEORIES REGARDING THE EARTh's INTERIOR. 235 



physical condition of the interior of the earth by means of 

 vibrations which travel through it. When an earthquake 

 originates at any one place there is sent forth a vibration which 

 affects the outer siliceous surface only; the latter heaves as 

 an ice-t^oe would upon the surface of an agitated ocean. Be- 

 sides this surface quake there are vibrations which travel by 

 the brachistochronic or the shortest possible paths right 

 through substance of the earth's interior to a spot on the other 

 side, where the vibration can be recorded. Now, when a shock 

 is communicated to a body, there are two kinds of waves 

 that are set up ; there is the normal or compressional vibration, 

 which can be propagated in any medium solid, liquid or gas; 

 but if the medium is solid, there is a further transverse wave, 

 which, on account of its being distortional, cannot be pro- 

 pagated in a liquid or gas. The velocities of these two waves 

 are very different, and when we find on the recording Seismo- 

 graph two distinct vibrations separated by a definite time 

 interval, which is proportional to the distance traversed in the 

 earth's interior, then we can state that the earth's interior is 

 solid throughout. Indeed, we can say more than that. The 

 difference in the rates of the two waves is such as would be 

 experienced in a medium twice as rigid as steel. Arrhenius 

 has maintained that a gas might be so compressed that it 

 becomes as compact as a solid, but then it could not transmit 

 distortional waves, which can only be propagated in rigid 

 substances, and Arrhenius' theory of the gaseous interior of 

 the earth must fall to the ground on that fact. 



Tliere is another curious fact connected with earthquakes 

 which bears on our subject. For direct paths between two 

 points in the earth's crust less than i,ooo miles apart, the one 

 being that at which the earthquake occurs and the other being 

 that at which it is recorded, the speed of transmission is such 

 as is found in waves propagated through ordinary rocky 

 material ; but if this straight path penetrates more than 30 

 miles below the surface, the waves are accelerated. There is, 

 as it were, a globe of high elasticity, twice as rigid as steel, 

 immediately below 30 miles in the earth's crust, whereas all 

 above is not materially different in physical condition from the 

 rocks exposed on the surface. If the earth's interior were 

 molten, let alone gaseous, it would be impossible from the laws 

 of diffusion for this sharp demiarcation between crust and 

 interior to remain in existence. 



Having established the fact that the earth's interior is a 

 solid, let us now examine the temperatures observed in the 

 earth's crust. 



As we go downwards in the earth's crust there is a distinct 

 increase of temperature below the variable zone affected by 

 the climatic and seasonal conditions. This increment is on 

 an average about 1° F. for every 60 feet. But what is this 

 average calculated upon? It is obtained by massing all the 

 figures from various rock systems and dividing through. If now 

 we take the older and younger rock systems we shall find that 

 the former, nearer the earth's centro-'sphere, have a tempera- 



