314 ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 



It was not however until 1841 that the apparatus was modified 

 and improved to such an extent as to give the most satisfactory 

 results. The , experiments with the perfected apparatus were 

 continued till May, 1842, when the result was arrived at that the 

 mean density of the earth is 5.66. From this enumeration of all 

 the experiments which have been made for the determination of 

 the mean density of the earth, it will be evident that the result as 

 given by Baily, is one of the most unequivocally established 

 scientific facts. Not only is there (considering, the different times 

 and circumstances when they were instituted) a surprising coinci- 

 dence in the results obtained by the torsion balance, but iheseare 

 confirmed in the mean by the results obtained from the less 

 accurate methods first described. 



If we compare the mean density of the earth, as found by Baily, 

 with the specific gravities of a few well known minerals, we find 

 that it equals the density of copper glance, and exceeds that of 

 magnetic iron ore, iron pyrites, variegated copper ore, and copper 

 pyrites. If we moreover compare it with the specific gravities of 

 these minerals or rocks which constitute the great bulk of the 

 earth's crust, we find it to possess twice as great a density. The 

 inference is unavoidable that the centre of the earth is much more 

 dense than its crust, and is also possessed of a higher density than 

 that of the earth's whole mass. This conclusion has, nevertheless, 

 been received by many with grave doubts. It has even been sup- 

 pose 1 that the increased density at the earth's centre is attribu- 

 table to the increased density which the substances there existing 

 acquire from the enormous pressure of the superincumbent mass. 

 This explanation rests upon the groundless supposition that 

 solids may be compressed to an indefinite extent. It further ne- 

 glects the very essential circumstance that the attraction exercised 

 on any material point in the interior of the earth is only exerted 

 by that part of the earth which lies within the spherical surface 

 passing through the given point, and that the mass of the earth 

 outside of this surface exercises no attracting influence on it. 

 Since therefore the weight of a body is determined by the sum of 

 the attracting forces acting on it, it follows that the weight of one 

 and the same body must be less in the interior of the earth than 

 on the surface.* Moreover, it is very certain that an extraordi- 

 narily high temperature exists in the interior of our planet, which 



* Naumann, Lehrbuch, i, 40. 



