SECT. VI. DEPTH OF THE SEA. 51 



ance 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 gjg ; that given by the lunar theory has the advantage of 

 being independent of the irregularities of the earth's surface and 

 of local attractions. 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 centre of gravity of the 

 earth, which affords a strong presumption in favour of its 

 original fluidity. It is remarkable how little influence the sea 

 has on the variation of the lengths of the arcs of the meridian, 

 or on gravitation ; neither does it much affect the lunar inequal- 

 ities, from its density being only about a fifth of the mean 

 density of the earth. For, if the earth were to become fluid, 

 after being stripped of the ocean, it would assume the form of an 

 ellipsoid of revolution whose compression is 3^.3, 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 are profound 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 been the 

 case, without any great change in the form of the terrestrial 

 spheroid. The variation in the length of the pendulum was first 

 remarked by Richter in 1672, while observing transits of the 

 fixed stars across the meridian at Cayenne, about five degrees 

 north of the equator. He found that his clock lost at the rate of 

 2 m 28 s daily, which induced him to determine the length of a 

 pendulum beating seconds in that latitude ; and, repeating the 

 experiments on his return to Europe, he found the seconds' pen- 

 dulum at Paris to be more than the twelfth of an inch longer than 

 that at Cayenne. The form and size of the earth being deter- 

 mined, a standard of measure is furnished with which the dimen- 

 sions of the solar system may be compared. 



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