li. B. Oldham — The Interior of the Earth. 19 



that there was much to be said against this view, for, though 

 mathematics is the court of appeal, it can only decide on the facts 

 placed before it by the sciences of observation, and so the discussion 

 seems profitably prefaced by a statement of the leading facts which 

 have been collected, and those conclusions which are so directly 

 derived from them as to have almost the validity of observation. 



The fii'st, and still one of the most important, of these observations 

 is tbat the temperature rises regularly to the greatest depth yet 

 penetrated into the earth, the rate of increase being on the average 

 about 1° C. for every 25 m. of depth ; we also know that at depths 

 from which volcanic eruptions take place the temperature reaches at 

 least 1000° C. So long as the nebular hypothesis held undisputed 

 sway, it was natural to suppose that the increase of temperature was 

 continuous to the centre of the earth, and, as long ago as 1793, 1 we 

 find Benjamin Franklin indicating the necessary corollary from this, 

 that below a certain depth the material, of which the earth is 

 composed, must necessarily be in a molten, and, at a still greater 

 depth, be converted into a gaseous, condition, an hypothesis very 

 similar to that associated in recent times with the name of Arrhenius. 



The fundamental assumption on which this deduction is based has 

 been sapped by the discovery that the radium content of the outer 

 layers of the earth is amply sufficient to account for the whole of the 

 temperature gradient observed in its superficial portion, and, other 

 considerations being ignored, the hypothesis which has been seriously 

 proposed, that the innermost parts of the earth may be intensely 

 cold, approaching the absolute zero temperature of space, is a possible 

 one. We have also the demonstration that the earth as a whole is 

 something like twice as rigid as an equal-sized globe of solid granite, 

 which precludes the assumption that the interior can be in a fluid or 

 gaseous condition in any ordinary sense of the word, but here we 

 must allow for the effect of pressure. When it is remembered that 

 at a depth of but 4 km., or about 2J miles, the pressure is already 

 greater than the crushing strength of the strongest known rock, and 

 that at the centre it is about three thousand times as great as this, it 

 becomes evident that the properties of matter ma)' be very greatly 

 modified and that the terms used to describe the three states, as we 

 know them under surface conditions, may need to be used in a very 

 esoteric sense when transferred to the interior of the earth. For the 

 present all that need be said is that material, which can be shown to 

 be from twice to six times as rigid as strong granite, can only be 

 described as fluid or gas in a very Pickwickian sense of the word, 

 and it is possibly a mere accident that the threefold division of the 

 interior, outlined by Benjamin Franklin, comes so near that which 

 I propose to show is the deduction to be drawn from the present 

 state of knowledge. 



Taking the outer layers of the earth first, we find that the rocks 

 which are exposed at the surface consist in part of material which 

 has been disintegrated by the processes of surface denudation, trans- 

 ported, deposited, and resolidified, and partly of rock which has not 



1 Trans. Amer. Phil. Soc, iii, pp. 1-5, 1793. 



