[56 SEVENTEENTH CENTURY. rx. ill. 



CHAPTER XIX. 



SCIENCE OF THE SEVENTEENTH CENTURY (CONTINUED). 



Transits of Mercury and Venus — Kepler foretells their occurrence — 

 1 63 1, Gassendi observes a Transit of Mercury — 1639, Horrocks 

 foretells and observes a Transit of Venus — 1676, Halley sees a 

 Transit of Mercury, and it suggests to him a method for Measuring 

 the Distance of the Sun — 1691-1716, Halley describes this method 

 to the Royal Society — Explanation of Halley's method. 



First transits ever observed of Mercury and Venus, 1631- 

 1639. — We must now pause for a moment before passing on 

 to Newton's discoveries in Optics, in order to mention a re- 

 markable astronomical suggestion made about this time by 

 the astronomer Halley (born 1656, died 1742), who was the 

 friend and disciple of Newton. 



You cannot fail to have heard and read something about 

 the expeditions sent last December, 1874, into all parts of 

 the world to observe the Transit (or Passage) of Venus 

 across the sun. The object of these observations was to 

 measure the sun's distance from the earth ; and Halley was 

 the first to propose this method of measurement, in 1691, 

 and to show how it might be accomplished. 



You know that the two planets Mercury and Venus are 

 nearer to the sun than our earth is, and are therefore con- 

 stantly passing between us and it. But usually they pass 

 either below or above the sun, and it is only rarely that 

 they cross over the bright disc, so as to be seen through 



