CH. XXX, BRADLEY AND DELISLE. 265 



CHAPTER XXX. 



SCIENCE OF THE EIGHTEENTH CENTURY (CONTINUED). 



Bradley and Delisle, Astronomers — Aberration of the Fixed Stars — 

 Nutation of the Axis of the Earth — Delisle's Method of Measuring 

 the Transit of Venus — Lagrange and Laplace — Libration of the 

 Moon accounted for by Lagrange — Laplace works out the Long 

 Inequality of Jupiter and Saturn — Lagrange proves the Stability 

 of the Orbits of the Planets — Sir William Herschel constructs his 

 own Telescopes — Discovery of a New Planet — Discovery of Binary 

 Stars — Herschel studies Star-clusters and Nebuloe — Theory of 

 Nebulae being matter out of which Stars are made — The Motion of 

 our Solar System through Space— Weight of the Earth determined 

 by the Schehallien Experiment — Summary of the Science of the 

 Eighteenth Century. 



Astronomical Labours of Bradley and Delisle. — And 



now, as we approach the end of the eighteenth century, 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 men- 

 tioned were Bradley, the Astronomer- Royal (born 1692, died 

 1762), 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 us. 

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