THE CRUST OF THE EARTH. 



direct observation and measurement, mathematicians had already 

 ascertained by computation what ought to be its form consequent 

 upon its rotation upon its polar axis, and the time in which it is 

 actually known to rotate, and the result of their computations was 

 afterwards found to agree with the utmost possible numerical 

 precision with the actual form which the earth was proved to have 

 by immediate observatiofl and measurement. 



11. It is therefore inferred from this that the earth, in its 

 original state, and before it assumed its present condition, was a 

 fluid mass, and that while in this fluid state, it received the 

 diurnal rotation by which it is now affected, and which produces 

 the vicissitudes of day and night. It was, therefore, after having 

 assumed this spheroidal form, that its superficial parts hardened 

 and solidified, and after undergoing a certain succession of changes 

 assumed their present condition. 



12. It has been also shown in our Tracts on " Terrestrial Heat," 

 and on " Earthquakes and Volcanoes," that when we penetrate 

 into the crust of the earth by mines, boring, or other artificial 

 means, the temperature is found to undergo a gradual and regular 

 augmentation, and this augmentation being continuous, so far as 

 direct observation has been carried, may be assumed by analogy 

 to increase to a still greater depth in the same proportion a con- 

 clusion which also is verified by the phenomenon of hot springs 

 rising from a considerable depth, and by various volcanic effects. 

 Assuming this gradual increase of temperature to go on indefinitely 

 in descending towards the centre, it has been shown that at a 

 certain depth the temperature must be such that even the most 

 refractory constituents of the globe would be reduced by it to a 

 state of fusion. 



13. The increase of temperature being at the rate of about a 

 hundred thermometric degrees per mile, it would follow that at 

 the depth of about 40 miles, or about the 100th part of the entire 

 distance from the surface to the centre, we should arrive at a 

 temperature of 4000, at which it is quite certain that no part of 

 the matter composing the earth could remain solid. It is therefore 

 inferred, without the necessity for extreme numerical precision, 

 that the earth is a spherical shell, the superficial part only being 

 solid all the central part being in a state of igneous fusion the 

 thickness of the solid crust being, as just stated, about the 200th 

 part of the entire diameter. A section of it would be such as is 

 represented in fig. 1. 



If, therefore, as we have already stated in our Tract on 

 11 Earthquakes and Volcanoes," the egg of a fowl be imagined to 

 represent the earth, its shell would be much too thick to represent 

 its solid crust. 

 36 



