242 



THE OCEAX. 



[Book X. 



PEOPOSITION XVII. 



Data. — The sun's revolving force endeavours to 

 revolve the planets m equal periods. 



Definition. — The superimposed objects n and 

 M (Fig. 21), resting at a on the earth's surface, 

 EAW, are revolved about the earth's centre, c, in 

 equal periods ; the more distant, N, movmg in a 

 greater orbit, and with a greater velocity of motion, 

 than M. 



Theorem. — Those attempted velocities are decreased 

 as the square roots of the sun's revolving force in the 

 respective orbits decrease. 



