CONSTITUTION OF THE GLOBE. 21 



The mean density of the earth to that of the moun- 

 tain was afterwards found to be nearly as 9 to 5, or 

 more precisely as 17804 to 9933. Thus it was con- 

 cluded that it must be composed oT substances such, 

 that their mean density must be nearly the double of 

 those rocks which compose Schihallien. For this 

 purpose, it Avas necessary to compare the weight of 

 the matter composing the mass of this hill with that 

 of water, or to ascertain its Specific gravity ; an ele- 

 ment required for determining the mean Specific gra- 

 vity of the terrestrial globe, which was sought. 



It was at first assumed that the Specific gravity of 

 Schihallien was 3 ; geological knowledge not having 

 at that time made much progress in this country. 

 Geologists will be pleased in reflecting, that as the 

 labours of mathematicians and astronomers have con- 

 tributed to throw light on some of the most obscure 

 parts of their science, so, in return, has geology fur- 

 nished the means of completing the data for an astro- 

 nomical problem, which, without its assistance, must 

 have ever remained imperfect, and produced erroneous 

 results. The mineralogical investigations of Mr. Play- 

 fair reduced that specific gravity to 2,7 or 2,8 ; 

 and if 2,75 therefore be assumed as the specific 

 gravity of Schihallien, it will prove that the mean 

 specific gravity of the earth, obtained in this way, is 

 about 5. 



This determination, however, can only be considered 

 as an approximation to the truth. It interests geo- 

 logists, even more than mathematicians, to consider 

 why it is not perfect ; since the doubtful element of 

 the calculation rests with their science. It is neces- 

 sary that the actual weight, or specific gravity, of the 

 spheroid into which the attracting mass of Schihallien 

 is resolved, should be known. Without detracting 



