CH. xxxi. BRADLEY AND DE LISLE. 275 



CHAPTER XXXI. 



SCIENCE OF THE EIGHTEENTH CENTURY (CONTINUED). 



Bradley Delisle Lagrange Laplace Sir William Herschel 

 Binary Stars Star-Clusters and Nebulse Motion of our Solar 

 System through Space Maskelyne Schehallion Experiment 

 Summary of the Science of the Eighteenth Century. 



Astronomical Labours of Bradley and Delisle. And 



now we must take up once more the history of astronomy, 

 which we have neglected since Newton died in 1727. 

 Since that time many good astronomers had been occupied 

 in making different observations, but the only two who 

 need be mentioned were Bradley, the Astronomer-Royal 

 (born 1692, died 1762), whose accurate observations of 

 the stars laid the foundation of all our knowledge of the 

 motion of our Solar System in space, and of the proper 

 motions of the stars, and Delisle (born 1688, died 1768). 

 Bradley explained two difficult astronomical problems ; the 

 first of these is called the aberration of the fixed stars, which 

 is an apparent movement of each fixed star in a small circle 

 in the heavens, but which is really the combined effect of 

 the yearly motion of our own earth, and of the time which 

 light occupies in coming down from the stars to u,9> His 

 second discovery is that of the nutation, or slight ostullation, 

 of the earth's axis. But these observations are dinjcult to 

 understand, but it is necessary to bear in mind that he 

 made them, for they are very important in astronomy. 

 Delisle will interest you because he proposed a second 



