CuAi>. XXI.] THE HEAVENS. 247 



of the sun. For by treating the attempted velocity as 

 infinite, the table would give the maximum velocities 

 of lagging, which would make the relative periods of 

 revolution exactly the measure of the relative velo- 

 cities of lagging. 



No actual proof that this latter is not the measure 

 of the relative lagging velocity is yet given, except the 

 fact that it seems scarcely reasonable to suppose that 

 it can be so ; and it appears most probable that the 

 revolving force acts from what at any given instant 

 may be the centre of gravity of the nearer hemisphere 

 of the sun. And as this would give a still greater 

 velocity of lagging than that indicated in the table, 

 and therefore be intermediate between the maximum^ 

 obtained by treating the periods of revolution as 

 exactly the measure of the relative lagging, and the 

 minimum, obtained as shown by the table ; and as 

 those extremes do not themselves differ sufiiciently 

 to affect the purpose of the present arguments, the 

 simple ratio indicated by the periods of revolution 

 may (until greater precision is required for questions 

 of detail) be treated as the measure of the lagging 

 velocities. 



Therefore, as the apparent velocity of Mercury's 

 orbital motion is 14732" per day and that of Neptune 

 21^" ; the former revolves in 88 days and the latter 

 in 60126 days ; and the ratio of orbital motion and 

 lagging is approximately as given in the following 

 table : — 



