160 TERRESTRIAL PHENOMENA. 



Beich, and give for their result 5-44 to 1 as the ratio of the 

 mean density of the Earth to that of distilled water. Now, we 

 know from the general nature of the rocks and strata which 

 form the dry or continental portion of the Earth's surface, 

 that their density can hardly amount to 2 '7, or that 

 of the land and sea surfaces taken together to 1*6; it 

 follows, therefore, that, either by pressure, or from the 

 heterogeneity of the substances, the elliptical strata in the 

 interior must undergo a great increase of density towards the 

 center of the Earth. Here, again, the horizontal (as before 

 the vertical) pendulum, shews itself to be justly entitled to 

 the name of a geological instrument. 



The results thus obtained have led physical philosophers 

 of celebrity to form to themselves, according to the different 

 hypotheses from which they proceeded, wholly opposite views 

 respecting the nature of the interior of the globe. It has 

 been computed at what depths liquid and even gaseous 

 substances, from the pressure of their own superimposed 

 strata, would attain a density exceeding that of platinum, 

 or of iridium ; and in order to bring the actual degree of 

 ellipticity, which was known within very narrow limits, into 

 harmony with the hypothesis of the infinite compressibility 

 of matter, Leslie conceived the interior of the Earth to be 

 a hollow sphere, filled with " an imponderable fluid of enor- 

 mous expansive force." Such rash and arbitrary conjectures 

 have given rise, in wholly unscientific circles, to still more 

 fantastic notions. The hollow sphere has been peopled with 

 plants and animals, on which two small subterranean revolv- 

 ing planets, Pluto and Proserpine, were supposed to shed a 

 mild light. A constantly uniform temperature is supposed 

 to prevail in these inner regions, and the air being rendered 



