124 GREAT MEN OF SCIENCE 



pumps of a coal-mine, and more such engines were 

 soon built for the English coal-mines. More trustworthy 

 and efficient operation, and more general applicability 

 through rotary motion, was first attained seventy years later 

 by Watt, who added the new idea of a separate condenser. 



JAMES BRADLEY 

 i6g2-iy62 



The introduction of the Kepler telescope for astronomical 

 measurement, of which Roemer was the pioneer, and a 

 corresponding improvement in the accompanying divided 

 circles, resulted in increasing refinement in the determina- 

 tion of the positions of the fixed stars. Particularly in view 

 of the annual motion of the earth, astronomers had sought 

 with growing hope of success since Copernicus' time for 

 an apparent displacement, annually repeated, of nearer fixed 

 stars as compared with more distant ones. This displace- 

 ment is called the 'parallax' of the star, and its amount gives 

 us immediately the distance of a star in terms of the diameter 

 of the earth's orbit. Signs were also found of such dis- 

 placement in the position of certain fixed stars, and Halley 

 in 1 71 8, was the first to determine the proper motion of 

 Aldebaran, Arcturus and Sirius; an important discovery in 

 itself. 



But these motions were not periodic. Bradley was the 

 first to use sufficiently refined methods and sufficient patience 

 to discover annual displacements, the existence of which had 

 also been asserted (for example by Hooke); he was the first, 

 however, to determine them beyond doubt, so far as the 

 available means at that time permitted, and he also inter- 

 preted them correctly. In the year 1778, he had been suc- 

 cessful in making sufficient measurements of a star. It 



