266 EIGHTEENTH CENTURY. pt. hi. 



This question is very difficult, as is also his second discovery 

 of the nutation, or slight oscillation, of the earth's axis ; but 

 it is necessary to bear in mind that he made these obser- 

 vations, for they are very important in astronomy. 



Delisle will interest you because he proposed a second 

 method of measuring the transit of Venus, which is now 

 used at stations where Halley's rule (see p. 160) cannot be 

 applied. Delisle's method consists in marking the time at 

 which the transit is seen to begin in one part of the world, 

 and to end in another ; instead of measuring, as Halley did, 

 the duration or length of time occupied by the whole tran- 

 sit as seen at each place. It requires that the clocks of all 

 the different stations from which the transit is observed 

 should be set exactly to the same time, and then it answers 

 as well as Halley's. 



These discoveries are all that need be mentioned during 

 the first half of the eighteenth century, but during that time 

 there had been born within a few years of each other three 

 men, Lagrange, Laplace, and Herschel, who were to light up 

 the close of the century with the most brilliant discoveries. 

 The two first of these were Frenchmen, the last we may fairly 

 claim as an Englishman ; for though he was born at Hanover 

 in 1738, of German parents, still Sir William Herschel came 

 over to England at the age of twenty- one, and all his dis- 

 coveries were made here. It was our King George III. who 

 gave him the pension which enabled him to devote himself 

 to science; and his son Sir John Herschel was, like his 

 father, one of our greatest astronomers, and made England 

 his home and country. 



Lagrange and Laplace. — Louis de Lagrange was born 

 at Turin in 1736. His father, who had been Treasurer of 

 War, lost all his fortune when his son was quite a child, and 



