SEQUEL TO COPERNICUS. 431 



We must not forget, in reading such passages, 

 that they were written under a belief that force 

 was requisite to keep up, as well as to change the 

 motion of each planet ; and that a body, moving in 

 a circle, would stop when the force of the central 

 point ceased, instead of moving off in a tangent to 

 the circle, as we now know it would do. The force 

 which Kepler supposes is a tangential force, in the 

 direction of the body's motion, and nearly perpen- 

 dicular to the radius ; the force which modern phi- 

 losophy has established, is in the direction of the 

 radius, and nearly perpendicular to the body's path. 

 Kepler was right no further than in his suspicion of 

 a connexion between the cause of motion and the 

 distance from the center; not only was his know- 

 ledge imperfect in all particulars, but his most 

 general conception of the mode of action of a cause 

 of motion was erroneous. 



With these general convictions and these phy- 

 sical notions in his mind, Kepler endeavoured to 

 detect numerical and geometrical relations among 

 the parts of the solar system. After extraordinary 

 labour, perseverance, and ingenuity, he was emi- 

 nently successful in discovering such relations ; but 

 the glory and merit of interpreting them according 

 to their physical meaning, was reserved for his 

 greater successor, Newton. 



