242 SCIENCE AND METHOD. 



orbit. Accordingly for the planets, by virtue of 

 Kepler's law, the effect varies in the inverse ratio of 

 Ja^, and it is therefore imperceptible except in the 

 case of Mercury. 



It is equally imperceptible in the case of the Moon, 

 because, though « is large, a is extremely small. 

 In short, it is five times as small for Venus, and six 

 hundred times as small for the Moon, as it is for 

 Mercury. I would add that as regards Venus and 

 the Earth, the motion of the perihelion (for the same 

 angular velocity of this motion) would be much more 

 difficult to detect by astronomical observations, because 

 the excentricity of their orbits is much slighter than in 

 the case of Mercury. 



To sum up, the only appreciable effect upon astronoin- 

 ical observations would be a motion of Mercury's peri- 

 helion, in the same direction as that which has been 

 observed without being explained, but considerably 

 smaller. 



This cannot be regarded as an argument in favour 

 of the new Dynamics, since we still have to seek 

 another explanation of the greater part of the anomaly 

 connected with Mercury ; but still less can it be 

 regarded as an argument against it. 



III. 



Lesage's Theory. 



It would be well to set these considerations beside 

 a theory put forward long ago to explain universal 

 gravitation. Imagine the interplanetary spaces full of 

 very tiny corpuscles, travelling in all directions at very 



