32 On the Constitution and Structure of the 



perfect sphere. The difference is very small ; but when we 

 also recollect that the moon's attraction at d counteracts her 

 attraction at a, and that it is only the difference between the 

 one attraction and the other, depending on the inequality ol 

 the distance, which disturbs the earth's position — and further, 

 that the mass or weight of the disturbing agent, the moon 

 (which is the measure of her power), is only the 68th part of 

 that of the earth — when all these circumstances are considered, 

 it might be inferred that the effect of causes so very minute 

 would be inappreciable. Such an inference, however, would 

 be erroneous. In truth, the effect was discovered first, and 

 led to the knowledge of the cause. Dr Bradley detected a 

 change in the latitude of the stars, which, after increasing for 

 nine years, diminished for the next nine, and amounted in all 

 to eighteen seconds. He observed that its period coincided 

 exactly with that of the revolution of the moon's nodes, and 

 was thus led to the discovery of the cause. 



Cavendish's celebrated experiments with lead balls have 

 been lately repeated at the expense of Government ; and the 

 conclusion drawn from them is, that the mean density or 

 w^eight of the earth is rather more than 5 4 times the weight 

 of an equal bulk of water.* Now, the rocks at the surface are 

 only about 2| times the weight of water, and to make up the 

 mean density or weight of the whole to 51^, it follows that the 

 interior must be as much above that as the surface is below 

 it. We thus arrive at the conclusion, that the density in- 

 creases with the depth beneath the surface. 



Astronomers simplify the problem by considering the pro- 

 tuberance, a b c df eiS a, ring detached from the spherical mass. 

 The action of the moon in shifting the axis of such a ring, 

 revolving in free space, would be very great ; but it is reduced 

 to the very minute quantity I have mentioned, because the 

 sphere, to which the ring is attached, has no tendency to 

 change its position, and resists the change in the ring by its 

 inertia. The ring, in short, has an incomparably larger mass 



* The proportion, as given in the last volume of the ''Penny Cyclopsedia," 

 is 5G6 to 100. This workj though bearing a humble title, is invaluable 

 for the accuracy and great amount; as well as the accessible form, of 

 the knowledge it communicates. 



