W * the Interior of the Earth. H J887 



prove that they were much more numerous at periods antece- 

 dent to the present geological epoch. This circumstance is ex- 

 plained by the greater thinness of the earth's crust at that time, 

 and the more rapid progress of refrigeration. 



22. If we judge by the appearance of lava, the fluidity of 

 the incandescent matter which constitutes the interior of the 

 earth is very great, and its density in the regions, situated at a 

 distance from the centre, (for example, at a distance equal to 

 Jgth of the radius,) is much inferior to the mean density of the 

 whole globe. These two data are not in opposition to the in- 

 fluence which must be accorded to the enormous and increasing 

 pressure which is due to the action of the central forces. It is 

 to be observed, in the first place, that fluids can be very little 

 compressed, that their compressibility in this case must have a 

 limit, and that its effects may be balanced by excessive heat. 

 Moreover, the present lavas have, after their consoh'dation, a 

 mean specific weight, greater than that of the primordial rocks 

 taken together ; from which it may be concluded, independently 

 of every other consideration, that the density of the central mat- 

 ters depends much more upon their nature than upon pressure ; 

 they have been originally placed in the order of their specific 

 gravities. The existence of gold and platina proves that sub- 

 stances having, from their nature, an extreme density, may oc- 

 cur at the centre of the earth. 



23. The preceding statement shews, that there is some proba- 

 bility in the hypothesis of Halley, who attributed the magnetic 

 actions to the existence of a mass composed in a great measure 

 of metallic iron, of irregular form, and possessing a particular 

 revolving motion, situated at the centre of the earth. Two 

 kinds of phenomena, of which Halley had no knowletlge, add to 

 this probability. On the one hand, the rotation of the ring of 

 Saturn round that planet may be brought forwai'd as furnishing 

 a sort of analogy ; and, on the other, the nature of the stones 

 that have fallen from the sky, and the existence of meteoric iron, 

 prove that iron in the metallic state, and alloyed with nickel, 

 may enter abundantly into the composition of the planetary 

 masses. 



24. If Halley's hypothesis be admissible, it furnishes the li- 

 mit of the internal temperature of the earth. This limit is 



