OCTOBEB 25; 1907] 



SCIENCE 



533 



mass as the earth, made of a material as 

 nearly incompressible as granite, could not 

 exist; it would be gravitationally unstable. 

 The body would take up some such state of 

 aggTeg'ation as that illustrated in Fig. 4, 

 and its center of gravity would have an 

 eccentric position. 



Now how would an ocean rest on a 

 gravitating sphere of which the center of 

 gravity does not coincide with the center 

 of figure? Its surface would be a sphere 

 with its center at the center of gravity 

 (Fig. 5). The oceanic region would be on 

 one side of the sphere and the continental 

 region on the other side. It was pointed 

 out many years ago by Pratt that the exist- 

 ence of the Pacific Ocean shows that the 

 center of gravity of the earth does not 

 coincide with the center of figure. There 

 is no necessity to invoke some great catas- 

 trophe to account for the existence of the 

 Pacific Ocean, or to think of it as a kind 

 of pit or sear on the surface of the earth. 

 The Pacific Ocean resembles nothing so 

 much as a drop of water adhering to a 

 greasy shot. The force that keeps the 

 drop in position is surface tension. The 

 force that keeps the Pacific Ocean on one 

 side of the earth is gravity, directed more 

 towards the center of gravity than the 

 center of figure. An adequate cause for 

 the eccentric position of the center of 

 gravity is fovmd in the necessary state of 

 aggregation which the earth must have 

 had if at one time it was as compressible 

 as granite. The theory of gravitational in- 

 stability accounts for the existence of the 

 Pacific Ocean. 



But we can go much farther than this 

 in the direction of accounting for the con- 

 tinental and oceanic regions. We keep in 

 mind the eccentric position of the center 

 of gravity, and try to discover the effect 

 of rotation upon a planet of which the 

 center of gravity does not coincide with 



the center of figure. The shape of a ro- 

 tating planet must be nearly an oblate 

 spheroid ; but the figure of the ocean would, 

 owing to its greater mobility, be rather 

 more protuberant at the equator than the 

 figure of the planet on which it rests. The 

 primary effect of the rotation of the earth 

 upon the distribution of continent and 

 ocean is to draw the ocean towards the 

 equator, so as to tend to expose the arctic 

 and antarctic regions. "We have seen that 

 both arctic and antarctic are parts of the 

 continental region. But there is an im- 

 portant secondary effect. Under the in- 

 fluence of the rotation the parts of greater 

 density tend to recede further from the 

 axis than the parts of less density. If the 

 density is greater in one hemispheroid than 

 in the other, so that the position of the 

 center of gravity is eccentric, the effect 

 must be to produce a sort of furrowed sur- 

 face; and the amount of elevation and de- 

 pression so produced can be described by 

 an exact mathematical formula. It has 

 been proved that this formula is the sort 

 of expression which mathematicians name 

 a spherical harmonic of the third degree. 



The shape of the earth is also influenced 

 by another circumstance. We know that 

 at one time the moon was much nearer to 

 the earth than it is now, and that the two 

 bodies once rotated about their common 

 center of gravity almost as a single rigid 

 system. The month was nearly as short as 

 the day, and the moon was nearly fixed in 

 the sky. The earth must then have been 

 drawn out towards the moon, so that its 

 surface was more nearly an ellipsoid with 

 three unequal axes than it is now. The 

 primary effect of the ellipsoidal condition 

 upon the distribution of continent and 

 ocean would be to raise the surface above 

 the ocean near the opposite extremities of 

 the greatest diameter of the equator. But, 

 again, owing to the eccentric position of 



