xxv SLO W PROGRESS OF KNO WLEDGE 299 



to understand their motions, to realise what their 

 morning and evening settings mean, what their 

 turnings when they move straight toward us, why 

 they are driven back from us. We have learned 

 but a few years ago whether Jupiter would rise or 

 set, or whether he would retrograde the term that 

 has been applied to his retirement from us. People 4 

 have been found bold enough to say to us : You are 

 mistaken in thinking that any star ever stops orwheels 

 in its course. The heavenly bodies may not stand 

 or turn away. All advance ; once the signal is 

 given they start on their race. Their career will 

 end only with their existence. This eternal creation 

 has motions that suffer no recall. Should they once 

 be arrested, they will encounter obstacles in front 

 which are meantime held in place by the ordered, 

 regular march of the universe. 



XXVI 



WHAT then is the reason, you may ask, for the 

 apparent retrogression of some heavenly bodies ? 

 The appearance of slowness in their motion is 

 caused by their encountering the sun, as well as by 

 the character of their paths and the position of their 

 orbits, which are at certain periods calculated to 

 deceive the eye. Ships in the same way moving 

 under full sail seem withal to be stationary. Men 

 will some day be able to demonstrate in what 

 regions comets have their paths, why their course is 

 so far removed from the other stars, what is their 

 size and constitution. Let us be satisfied with what 

 we have discovered, and leave a little truth for our 

 descendants to find out. 



