l893-] COOKE — THE FIGURE OF THE FARTH. 213 



THE FIGURE OF THE EARTH. 



(Abstract.) ' 



The figure of the earth is due to the constant action of forces 

 which determine the pressure of the particles constituting the globe. 

 It is the same that it would be if the earth were composed entirely of 

 water. Such a globe of water affected only by its own gravitation 

 would assume the form of a perfect sphere. Its rotation would bring 

 into operation another force called centrifugal force. Both these 

 forces act upon each particle separately — gravitation drawing it 

 towards the centre of the globe and the other force urging it away 

 from its own centre of gyration in the direction of the radius. At 

 the equator the centre of gyration is the centre of the earth and the 

 operation of the centrifugal force is there in direct opposition to the 

 earth-pull. In every change of latitude along any meridian there is 

 a change in the direction in which the force of gravitation, or earth- 

 pull, acts upon the particles; but the direction in sj-tace in which the 

 centrifugal force urges them does not change, for the centres of 

 gyration change along the axis with every change of latitude. 



The intensity of the earth-pull, affecting particles of the same 

 mass, varies slightly by reason of the ellipticity of the earth from the 

 equator along any meridian to the pole ; but the intensity of the 

 centrifugal force varies as the cosine from maximum, at the equator, 

 which is about one two hundred eighty-nmih of the earth-pull, and 

 vanishes at the pole. Inasmuch as the earth-pull changes from 

 opposition at the equator for every change of latitude, the weaker 

 force (the centrifugal) is resolved — one component acting vertically, 

 or in direct opposition to the earth-pull, and the other at right 

 angles to it and towards the equator. 



It was claimed that the earth-pull upon a particle at the 6oth 

 parallel of latitude minus the vertical component of the centrifugal 

 force affecting the same particle, plus its horizontal component, 

 constitutes a combination of forces greater than any affecting particles 

 upon any other parallel of latitude. The force so combined with 

 that of the earth-pull is .366 of the whole centrifugal force of a 

 particle at the equator ; at the 30th parallel the vertical component 

 of the centrifugal force of a particle is equal to the horizontal 

 component and one counteracts the effect of the other in its pressure. 

 At the 70th degree the difference between the horizontal component 

 and the vertical, in favor of the horizontal, is equal to .3 of the 

 centrifugal force of a particle at the equator. The equilibrium is 



