270 A Short History of Astronomy [C'n. x. 



discoveries, other investigations were carried out by him 

 before or during the same time. 



The earliest important piece of work which he accom- 

 plished was in connection with Jupiter's satellites. His 

 uncle had devoted a good deal of attention to this subject, 

 and had drawn up some tables dealing with the motion of 

 the first satellite, which were based on those of Domenico 

 Cassini, but contained a good many improvements. Bradley 

 seems for some years to have made a practice of frequently 

 observing the eclipses of Jupiter's satellites, and of noting 

 discrepancies between the observations and the tables ; and 

 he was thus able to detect several hitherto unnoticed 

 peculiarities in the motions, and thereby to form improved 

 tables. The most interesting discovery was that of a 

 period of 437 days, after which the motions of the three 

 inner satellites recurred with the same irregularities. 

 Bradley, like Pound, made use of Roemer's suggestion 

 (chapter VIH., 162) that light occupied a finite time in 

 travelling from Jupiter to the earth, a theory which Cassini 

 and his school long rejected. Bradley's tables of Jupiter's 

 satellites were embodied in Halley's planetary and lunar 

 tables, printed in 1719, but not published till more than 

 30 years afterwards ( 204). Before that date the Swedish 

 astronomer Pehr Viihelm Wargentin (1717-1783) had in- 

 dependently discovered the period of 437 days, which he 

 utilised for the construction of an extremely accurate set 

 of tables for the satellites published in 1746. 



In this case as in that of nutation Bradley knew that his 

 mathematical powers were unequal to giving an explanation 

 on gravitational principles of the inequalities which observa- 

 tion had revealed to him, though he was well aware of the 

 importance of such an undertaking, and definitely expressed 

 the hope " that some geometer,* in imitation of the great 

 Newton, would apply himself to the investigation of these 

 irregularities, from the certain and demonstrative principles 

 of gravity." 



On the other hand, he made in 1726 an interesting 

 practical application of his superior knowledge of Jupiter's 



* The word "geometer" was formerly used, as "geometre" still 

 is in French, in the wider sense in which " mathematician " is now 

 customary. 



