430 HISTORY OF FORMAL ASTRONOMY. 



speak of the persuasion of the existence of numeri- 

 cal and geometrical laws connecting the distances, 

 times, and forces of the bodies which revolve about 

 the central sun That steady and intense conviction 

 of this governing principle, which made its deve- 

 lopement and verification the leading employment 

 of Kepler's most active and busy life, cannot be 

 considered otherwise than as an example of pro- 

 found sagacity. That it was connected, though 

 dimly and obscurely, with the notion of a central 

 agency or influence of some sort, emanating from 

 the sun, cannot be doubted. Kepler, in his first 

 essay of this kind, the Mysterium Cosmographicum, 

 says, " The motion of the earth, which Copernicus 

 had proved by mathematical reasons, I wanted to 

 prove by physical, or, if you prefer it, metaphy- 

 sical." In the twentieth chapter of that work, he 

 endeavours to make out some relation between the 

 distances of the planets from the sun and their 

 velocities. The inveterate yet vague notions of 

 forces which preside in this attempt, may be judged 

 of by such passages as the following : " We must 

 suppose one of two things : either that the moving 

 spirits, in proportion as they are more removed 

 from the sun, are more feeble ; or that there is one 

 moving spirit in the center of all the orbits, namely, 

 in the sun, which urges each body the more vehe- 

 mently in proportion as it is nearer; but in more 

 distant spaces languishes in consequence of the 

 remoteness and attenuation of its virtue." 



