272 A Short History of Astronomy [Cn. x. 



place of a star, as seen in the telescope and noted at . the 

 time, from being a satisfactory permanent record of its 

 position. There are various instrumental errors, and errors 

 due to refraction ; again, if a star's places at two different 

 times are to be compared, precession must be taken into 

 account ; and Bradley himself unravelled in aberration and 

 nutation two fresh sources of error. In order therefore 

 to put into a form satisfactory for permanent reference a 

 number of star observations, it is necessary to make cor- 

 rections which have the effect of allowing for these various 

 sources of error. This process of reduction, as it is techni- 

 cally called, involves a certain amount of rather tedious 

 calculation, and though in modern observatories the process 

 has been so far systematised that it can be carried out 

 almost according to fixed rules .by comparatively unskilled 

 assistants, in Bradley's time it required more judgment, 

 and it is doubtful if his assistants could have performed 

 the work satisfactorily, even if their time had not been fully 

 occupied with other duties. Bradley himself probably 

 found the necessary calculations tedious, and preferred 

 devoting his energies to work of a higher order. It is 

 true that Delambre, the famous French historian of 

 astronomy, assures his readers that he had never found 

 the reduction of an observation tedious if performed the 

 same day, but a glance at any of his books is enough to 

 shew his extraordinary fondness for long calculations of 

 a fairly elementary character, and assuredly Bradley is not 

 the only astronomer whose tastes have in this respect 

 differed fundamentally from Delambre's. Moreover reducing 

 an observation is generally found to be a duty that, like 

 answering letters, grows harder to perform the longer it 

 is neglected ; and it is not only less interesting but also 

 much more difficult for an astronomer to deal satisfactorily 

 with some one else's observations than with his own. It 

 is not therefore surprising that after Bradley's death a 

 long interval should have elapsed before an astronomer 

 appeared with both the skill and the patience necessary 

 for the complete reduction of Bradley's 60,000 observations. 

 A variety of circumstances combined to make Bradley's 

 Observations decidedly superior to those of his predecessors. 

 He evidently possessed in a marked degree the personal 



