294 HISTORY OF PHYSICAL ASTRONOMY. 



diminution of the periodic time in the successive 

 revolutions ; from which he inferred the existence 

 of a resisting medium. Uranus still deviates from 

 his tabular place, and the cause remains yet to be 

 discovered. 



Thus it is impossible that an assertion, false to 

 any amount which the existing state of observation 

 can easily detect, should have any abiding preva- 

 lence in astronomy. Such errours may long keep 

 their ground in any science which is contained 

 mainly in didactic works, and studied in the closet, 

 but not acted upon elsewhere ; which is reasoned 

 upon much, but brought to the test of experiment 

 rarely or never. Here, on the contrary, an errour, 

 if it arise, makes its way into the tables, into the 

 ephemeris, into the observer's nightly list, or his 

 sheet of reductions; the evidence of sense flies in 

 its face in a thousand observatories; the discre- 

 pancy is traced to its source, and soon disappears 

 for ever. 



In this favoured branch of knowledge, the most 

 recondite and delicate discoveries can no more 

 suffer doubt or contradiction, than the most pal- 

 pable facts of sense which the face of nature offers 

 to our notice. The last great discovery in astro- 

 nomy, the motion of the stars arising from aber- 

 ration, is as obvious to the vast population of 

 astronomical observers in all parts of the world, as 

 the motion of the stars about the pole is to the 

 casual night-wanderer. And this immunity from 



