106 DISSERTATION SECOND. [faktii. 



the centrifugal force at the equator is the 289th part of 

 gravity, diminishing continually as the cosine of the lati- 

 tude, on going from thence toward the poles, where it 

 ceases altogether. From the combination of this force, 

 though small, with the force of gravity, it follows, that 

 the line in which bodies actually gravitate, or the plumb- 

 line, cannot tend exactly to the earth's centre, and that 

 a true horizontal line, such as is drawn by levelling, if 

 continued from either pole, in the plane of a meridian 

 all round the earth, would not be a circle but an ellipse, 

 having its greatest axis in the plane of the equator, and 

 its least in the direction of the axis of the earth's rota- 

 tion. Now, the surface of the ocean itself actually tra- 

 ces this level as it extends from the equator to either 

 pole. The terraqueous mass which we call the globe must 

 therefore be what geometers call an oblate spheroid, or 

 a solid generated by the revolution of the elliptic meridi- 

 an about its shorter axis. 



In order to determine the proportion of the axes of 

 this spheriod, a problem, it will readily be believed, of 

 no ordinary difficulty, Newton conceived, that if the wa- 

 ters at the pole and at the equator were to communicate 

 by a canal through the interior of the earth, one branch 

 reaching from the pole to the centre and the other at 

 right angles to it, from the centre to the circumference 

 of the equator, the water in this canal must be in equili- 

 bria, or the weight of fluid in the one branch just equal 

 to that in the other. Including, then, the consideration 

 of the centrifugal force which acted on one of the bran- 

 ches but not on the other, and considering, too, that the 

 figure of the mass being no longer a sphere, the attrac- 

 tion must not be supposed to be directed to the centre, 

 but must be considered as the result of the action of all 

 the particles of the spheriod on the fluid in the canals; 



