276 DISCOURSE ON THE STUDY 



periodic time. Relying on these grand discoveries, 

 we are enabled to look forward, from the point of 

 time which we now occupy, many thousands of years 

 into futurity, and predict the state of our system 

 without fear of material error, but such as may arise 

 from causes whose existence at present we have no 

 reason to suppose, or from interference which we 

 have no right to anticipate. 



(306.) A correct enumeration and description of 

 the fixed stars in catalogues, and an exact know- 

 ledge of their position, supply the only effectual 

 means we can have of ascertaining what changes 

 they are liable to, and what motions, too slow to de- 

 prive them of their usual epithet, fixed, yet sufficient 

 to produce a sensible change in the lapse of ages, 

 may exist among them. Previous to the invention 

 of the compass, they served as guides to the navi- 

 gator by night ; but for this purpose, a very mo- 

 derate knowledge of a few of the principal ones 

 sufficed. Hipparchus was the first astronomer, who, 

 excited by the appearance of a new star, conceived 

 the idea of forming a catalogue of the stars, with a 

 view to its use as an astronomical record, " by 

 which," says Pliny, "posterity will be able to dis- 

 cover, not only whether they are born and die, but 

 also whether they change their places, and whether 

 they increase or decrease." His catalogue, containing 

 more than 1000 stars, was constructed about 128 

 years before Christ. It was in the course of the la- 

 borious discussion of his own and former observations 

 of them, undertaken with a view to the formation of 

 this catalogue, that he first recognised the fact of that 

 slow, general advance of all the stars eastward, when 



