190 PROGRESS OF SCIENCE IN THE CENTURY. 



measurement and calculation; on the other hand, 

 he emphasised the idea of change or, one may almost 

 say, of evolution. The heavens no longer seemed 

 fixed and unchanging, when it was shown that new 

 systems were being formed and that others were dying 

 away. 



Herschel's work was continued at Konigsberg by 

 Bessel; at the Russian observatory of Pulkowa by 

 Struve, succeeded in 1858 by his son Otto; and by 

 many other illustrious workers. In Britain the 

 father found an intellectual heir in the son, John 

 F. W. Herschel (1792-1871), whose Cape observa- 

 tions (1834-38) did for the Southern heavens what 

 had been done for the Northern. Published in 1847, 

 they represent the state of sidereal astronomy at the 

 middle of the century. " Not only was acquaint- 

 ance with the individual members of the cosmos 

 vastly extended, but their mutual relations, the laws 

 governing their movements, their distances from the 

 earth, masses, and intrinsic lustre, had begun to be 

 successfully investigated." * 



Improvements in telescopes and other instruments 

 aided in the progress of the sidereal astronomy to 

 which Herschel had given so much impetus, and with' 

 improved mechanical means was associated a re- 

 formed method of observation. Friedrich Wilhelm 

 Bessel (1784-1846), who made himself famous at 

 the age of twenty by calculating an orbit for Halley's 

 comet, did a gigantic piece of work by instituting 

 '(1813, 1830) a uniform system of " reduction " (or, 

 correction of observations) which lengthened out the 

 period of exact astronomy by half a century. In 

 other words he made a uniform correction of Brad- 

 ley's Greenwich observations, making allowances for 

 * A. M. Clerke. History, 1885, p. 65. 



