30 



On the Constitution and Structure of the 



respects we know so little, could be interrogated respecting 

 the structure of our globe in those deep recesses to which we, 

 who live on its surface, cannot find access — ay, and should be 

 able to return us distinct and instructive responses 1 Such is 

 the case, as Mr Hopkins has shewn in these very learned papers. 



Mr Hopkins's conclusions rest entirely on the effects of the 

 sun and the moon's attraction, as indicated by the phenomena 

 of Precession and Nutation. Any analysis of his elaborate 

 calculations would be out of place here. What I propose, is 

 to give a general idea of the basis and form of his argument, 

 and an abstract of his conclusions. 



It is known that the sun itself, and all the planets and 

 secondaries of the solar system, attract each other with forces 

 directly proportioned to their masses (that is their weights), 

 and inversely proportioned to the squares of their relative dis- 

 tances. It is known also that the earth is not a perfect 

 sphere, but an oblate or flattened spheroid, the equatorial dia- 

 meter of which exceeds the polar by 1 -300th part. To 

 express it in another form, the equatorial parts are thirteen 

 miles farther from the centre than the polar. 



In the above diagram, the round figure G represents a sec- 

 tion of the globe through its axis, N the north and S the 

 south pole ; a b the equatorial protuberance on one side of the 

 globe, c d that on the other. If the earth were a perfect 

 sphere, the moon''s attraction would have no disturbing effect 

 upon it. And though it is an oblate spheroid, the result 

 would be the same if the moon's orbit were in the plane of the 

 equator, that is in the position G m, as in this case the force 

 of attraction on the one side of the earth would exactly 

 balance that on the other. But the plane of the moon's orbit 



