CiTAP. XXI.] THE HEAVENS. 237 



PEOPOSITION XI. 



Theorem. — The retarding action indicated by the fore- 

 going Proposition is apparent as regards the axial 

 rotation of the sun, and also as regards the axial 

 rotations of the earth and of the planets Saturn 

 and Mars. 



As regards the sun ; because the velocity indi- 

 cated by the relative velocities of planetary motions, 

 which make the squares of their periods as the cubes of 

 their distances from the centre of the sun, would give 

 a planet at the sun's surface a period of "1169 of a 

 day instead of 28 days. 



As regards the earth ; because the velocity indi- 

 cated in the same manner by the motion of the moon 

 would give the earth's equator a period of 80 minutes 

 instead of 24 hours. 



As regards Saturn ; because its rings, extended 

 round the plane of its equator, and not checked by 

 matter extending to the polar regions, revolve in a 

 shorter period than the surface of Saturn below 

 them. 



As regards Mars ; because its moon is in fact 

 sufficiently near to it to be carried round in a shorter 

 period than that of the rotation of the surface of 

 Mars. 



