$$ 97, 98] The Cassel Observatory 129 



him to devote most of his energy to the duties of govern 

 ment, and his astronomical ardour abated. A few years 

 later, however (1575), as the result of a short visit from 

 the talented and enthusiastic young Danish astronomer 

 Tycho Brahe ( 99), he renewed his astronomical work, and 

 secured shortly afterwards the services of two extremely able 

 assistants, Christian Rothmann (in 1577) and J 'oost Biirgi 

 (m 1579). Rothmann, of whose life extremely little is 

 known, appears to have been a mathematician and theo- 

 retical astronomer of considerable ability, and was the 

 author of several improvements in methods of dealing 

 with various astronomical problems. He was at first a 

 Coppernican, but shewed his independence by calling 

 attention to the needless complication introduced by 

 Coppernicus in resolving the motion of the earth into 

 three motions when two sufficed (chapter iv., 79). His 

 faith in the system was, however, subsequently shaken by 

 the errors which observation revealed in the Prussian Tables. 

 Biirgi (1552-1632) was originally engaged by the Landgrave 

 as a clockmaker, but his remarkable mechanical talents 

 were soon turned to astronomical account, and it then 

 appeared that he also possessed unusual ability as a 

 mathematician.* 



98. The chief work of the Cassel Observatory was the 

 formation of a star catalogue. The positions of stars were 

 compared with that of the sun, Venus or Jupiter being 

 used as connecting links, and their positions relatively to 

 the equator and the first point of Aries (r) deduced; 

 allowance was regularly made for the errors due to the 

 refraction of light by the atmosphere, as well as for the 

 parallax of the sun, but the most notable new departure 

 was the use of a clock to record the time of observa- 

 tions and to measure the motion of the celestial sphere. 

 The construction of clocks of sufficient accuracy for the 

 purpose was rendered possible by the mechanical genius 

 of Biirgi, and in particular by his discovery that a clock 

 could be regulated by a pendulum, a discovery which he 



* There is little doubt that he invented what were substantially 

 legarithms independently of Napier, but, with characteristic inability 

 or unwillingness to proclaim his discoveries, allowed the invention 

 to die with him. 



9 



