SEQUEL TO COPERNICUS. 289 



Per damna per caedes, ab ipso 

 Sumit opes animumque ferro. 

 Untamed its pride, unchecked its course, 

 From foes and wounds it gathers force. 



We have spoken of the influence of the motion of the earth on the 

 motions of bodies at its surface ; but the notion of a physical connec- 

 tion among the parts of the universe was taken up by Kepler in an- 

 other point of view, which would probably have been considered as 

 highly fantastical, if the result had not been, that it led to by far the 

 most magnificent and most certain train of truths which the whole ex- 

 panse of human knowledge can show. I speak of the persuasion of 

 the existence of numerical and geometrical laws connecting the dis- 

 tances, times, and forces of the bodies which revolve about the central 

 sun. That steady and intense conviction of this governing principle, 

 which made its development and verification the leading employment 

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

 than as an example of profound 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, metaphysical." In the twentieth chapter of that work, he endeavors 

 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 centre of 

 all the orbits, namely, in the sun, which urges each body the more 

 vehemently in proportion as it is nearer ; but in more distant spaces 

 languishes in consequence of the remoteness and attenuation of its 

 virtue." 



We must not forget, in reading such passages, that they were writ- 

 ten 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 cir- 

 cle, 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 perpendicular to the radius; the 

 VOL. I. 19 



