Interior of the Globe. 33 



to drag after it, and hence undergoes but a very slight change 

 of position. 



The magnitude and density of the globe being known ap- 

 proximately, and also the magnitude and density of the ring 

 abed, the action of a body like the moon, whose mass and 

 distance are known, can be subjected to mathematical calcula- 

 tion. 



Mr Hopkins first investigates the phenomena of precession 

 and nutation on the hypothesis that the earth is of uniform 

 density throughout ; and in this case the conditions of solidity 

 and fluidity are considered. His chief object here seems to 

 have been to test the accuracy of his process ; and we need not, 

 therefore, stop to notice the results. He then passes to the 

 case in which the earth is assumed to be, what it really is, a 

 body whose density is variable, increasing with the depth, and 

 is modified by the conditions of internal fluidity and solidity. 



The mean density of the globe is about the same with that 

 of the heaviest iron ore, or 5<J. Keeping in mind that the 

 force of attraction is in proportion to the quantity of matter, 

 let us assume the ring abed to be of the same density with 

 the sphere G, or five and a half times the weight of water. In 

 this case its efi^ect, under the moon'*s attraction, in disturbing 

 the position of the earth's axis, would be in proportion to its 

 relative mass, and its distance from the earth's centre. But 

 if the density of the ring were only that of brick, or two times 

 the weight of water, its disturbing effect upon a sphere of the 

 superior density of iron ore would be comparatively trifling. 

 If, again, the ring had merely the density of pine wood, which 

 is but the eighth part of that of iron ore, its disturbing effect 

 would scarcely be appreciable by the nicest observation. 



But the density of the ring, and the average density of the 

 globe, are not the only elements involved in the problem. We 

 have further to inquire into the constitution and distribution 

 of the matter in the interior of the globe. We know that it 

 is solid at the surface. It is solid to the centre — that is to 

 say, is it composed of parts immoveable inter se ? We know 

 also from volcanoes, that there is fluid matter within it, that 

 is, matter whose parts are moveable, and obedient inter se to 

 the laws of gravitation, external attraction, and centrifugal 



VOL. XXXVII. NO. LXXIII. JULY 1844. C 



