NEWTON S PRINCIPIA. 333 



the experiments of Cavendish and Maskelyne have revealed 

 to us the mean density of the globe ; and, supposing the 

 density of the superficial stratum to be the same as that of 

 granite, we have two equations to determine A and q. 

 Working out these numerical calculations, we find that 



sm 



where D is the density of the superficial stratum. 



There are two results which will serve to verify this 

 law. 



1 . We know that there is a precession of the equinoxes, 

 because the resultants of the attractions of the sun and 

 moon do not pass through the centre of the earth. The 

 position of the lines of action of these resultants mani 

 festly depend on the law according to which the density of 

 the several strata varies. This calculation has been made, 

 and it is found that, taking the above law of density, the 

 precession should amount to 51&quot; -3566. The observed 

 precession is 50&quot; 1. 



2. The ellipticity of the earth is caused by its rotation, 

 and depends also on the law of density of its strata. The 

 calculated result is s = The result of geodetic 



measures s e = 



It is upon the remarkable agreement of our supposition 

 with observation in these two cases that our belief in the 

 law is founded. 



7. But though we have thus investigated the law of den 

 sity of the strata, it is not to be therefore concluded that 

 the earth is solid throughout ; on the contrary, the rate at 

 which the temperature increases with the depth is such 

 that if continued for 25 miles, the heat would be sufficient, 

 under a pressure of one atmosphere, to melt a stratum of 

 granite. Is, then, the interior of the earth a vast mass of 

 molten strata whose densities obey the law already inves 

 tigated ? and if so, what is the thickness of the crust ? 



