Chap. VIL] GRAVITATION AND INERTION. 81 



The exact relative amount of the revolving force 

 in each orbit depends on the thickness of the surface 

 whose gravitation causes the revolving action, and of 

 this we have no definite knowledge. But, by taking 

 the amount by which the force of the nearest exceeds 

 that of the remotest part (or any other corresponding 

 points in the opposite hemispheres) an approximation 

 can be obtained, which is sufficiently accurate for our 

 present purpose.^ 



The apparent motion of the spots upon the sun 

 has long been supposed to indicate a motion of 

 rotation in the direction above suggested. And the 

 following tables show it to be a mere matter of fact 

 that in the different orbits of the planets the fractions 

 by which the force of the gravitation of the nearest 

 part of the sun exceeds that of the remotest are such 

 that their square roots represent the actual relative 

 velocities with which the planets move in their orbits 

 round the sun. 



Takmg the sun's diameter as 888,000 miles, the 

 relative distances from the nearest and remotest 

 parts of the sun respectively are approximately 

 as in the first column of the following table, in 

 which that diameter is taken as the unit of measure- 

 ment : — 



1 See Table of Forces, Proposition XIX., Chapter XXI. 



G 



