ON GEOGRAPHT. S69 



were conducted. The first of these, which can be considered as accurate, was 

 executed by Picart in France, towards the end of the seventeenth century. 



But the spherical form is only an approximation to the truths it was cal- 

 culated by Newton, and ascertained experimentally by the French Acade- 

 micians, sent to the equator and to the polar circle, that, in order to represent 

 the earth, the sphere must be flattened at the poles, and prominent at the 

 equator. We may therefore consider the earth as an oblate elliptic spheroid; 

 the curvature being greater, and consequently every degree shorter, at the 

 ecjuator, than nearer the poles. If the density of the earth were uniform 

 throughout, its ellipticity, or the difterence of the length of its diameters, 

 would be ^li of the whole; on the other hand, if it consisted of matter of 

 inconsiderable density, attracted by an infinite force in the centre, the el- 

 lipticity would be only ^^5 ^^^^ whatever may be the internal structure of 

 the earth, its form must be between these limits, since its internal parts 

 must necessarily be denser than those parts which are nearer the surface. If 

 indeed the earth consisted of water or ice, equally compressible with common 

 water or ice, and following the same laws of compression with elastic fluids, 

 its density would be several thousand times greater at the centre than at the 

 surface; and even steel would be compressed into one fourth of its bulk, and 

 stone into one eighth, if it were continued to the earth's centre; so that there 

 can be no doubt but that the central parts of the earth must be much more 

 dense than the superficial. Whatever this difterence may be, it has been de- 

 monstrated by Clairaut, that the fractious expressing the ellipticity and the 

 ap|)arent diniinntion of gravity at the equator must always make together -j-fg-, 

 and it has been found, by the most accurate observatioiis on the lengths of 

 ])endulums in difterent latitudes, that the force of gravity is less powerful by 

 vf^ at the equator than at the pole, whence the ellipticity is found to be -j-^-g- 

 of the equatorial diameter, the form being the same as would be produced, if 

 about three eighths of the whole force of gravity were directed towards a 

 central particle,' the density of the rest of the earth being uniform. 



This method of determining the general form of the earth is much lesa 

 liable to error and irregularity, than the measurement of the lengths of de- 

 grees in various parts, since the accidental variations of curvature produced 

 by local diftcrences of density, and even by superficial elevations, may oftea 



