50 COMPRESSION OF THE EARTH. SECT. VI 



nutation of the earth's axis and the precession of the 

 equinoxes (N. 143) are occasioned by the action of the 

 sun and moon on the protuberant matter at the earth's 

 equator. And although these inequalities do not give 

 the absolute value of the terrestrial compression, they 

 show that the fraction expressing it is comprised be- 

 tween the limits T ^- and ^| . 



It might be e'xpected that the same compression 

 should result from each, if the different methods of ob- 

 servation could be made without error. This, however, 

 is not the case ; for, after allowance has been made for 

 every cause of error, such discrepancies are found, both 

 in the degrees of the meridian and in the length of the 

 pendulum, as show that the figure of the earth is very 

 complicated. But they are so small, when compared 

 with the general results, that they may be disregarded. 

 The compression deduced from the mean of the whole 

 appears not to differ much from * T ; that given by the 

 lunar theory has the advantage of being independent of 

 the irregularities of the earth's surface and of local at- 

 tractions. The regularity with which the observed 

 variation in the length of the pendulum follows the law 

 of the square of the sine of the latitude, proves the 

 strata to be elliptical, and symmetrically disposed round 

 the center of gravity of the earth, which affords a strong 

 presumption in favor of its original fluidity. It is re- 

 markable how little influence the sea has on the varia- 

 tion of the lengths of the arcs of the meridian, or on 

 gravitation ; neither does it much affect the lunar ine- 

 qualities, from its density being only about a fifth of the 

 mean density of the earth. For, if the earth were to 

 become a fluid, after being stripped of the ocean, it 

 would assume the form of an ellipsoid of revolution 

 whose compression is ^| ? . 7 , which differs very little 

 from that determined by observation, and proves, not 

 only that the density of the ocean is inconsiderable, but 

 that its mean depth is very small. There may be pro- 

 found cavities in the bottom of the sea, but its mean 

 depth probably does not much exceed the mean height 

 of the continents and islands above its level. On this 

 account, immense tracts of land may be deserted or 

 overwhelmed by the ocean, as appears really to have 



