PRELUDE TO THE EPOCH OF GALILEO. 19 



erroneously assigns as a reason, the smallness of 

 the point of contact 3 . But the most common mis- 

 take of this period was, that of supposing that as 

 force is requisite to move a body, so a perpetual 

 supply of force is requisite to keep it in motion. 

 The whole of what Kepler called his "physical" 

 reasoning, depended upon this assumption. He 

 endeavoured to discover the forces by which the 

 motions of the planets about the sun might be pro- 

 duced ; but, in all cases, he considered the velocity 

 of the planet as produced by, and exhibiting the 

 effect of, a force which acted in the direction of the 

 motion. Kepler's essays, which are in this respect 

 so feeble and unmeaning, have sometimes been con- 

 sidered as disclosing some distant anticipation of 

 Newton's discovery of the existence and law of cen- 

 tral forces. There is, however, in reality, no other 

 connexion between these speculations than that 

 which arises from the use of the term force by 

 the two writers in two utterly different meanings. 

 Kepler's Forces were certain imaginary qualities 

 which appeared in the actual motion which the 

 bodies had; Newton's Forces were causes which 

 appeared by the change of motion : Kepler's Forces 

 urged the bodies forwards; Newton's deflected the 

 bodies from such a progress. If Kepler's Forces 



3 In speaking of the force which would draw a body up an 

 inclined plane he observes, that " per communem anirni senten- 

 tiam," when the plane becomes horizontal, the requisite force is 

 nothing. 



C2 



