CH. XXX. THE DENSITY OF THE EARTH. 277 



may, I hope, lead you to long and seek for more solid 

 food. Before, however, we take leave of Sir W. Herschel, 

 we must not forget to mention the faithful assistant who 

 was so great a help to him in his labours. 



When George III. gave Herschel his home and pension, 

 the astronomer sent to Hanover for his sister Caroline, and 

 she lived with him and received a small salary as his assist- 

 ant. She shared his night-watches and mapped down the 

 stars, star-clusters, and nebulae, as he came across them with 

 his slowly-moving telescope ; she helped to draw up his 

 catalogues, to write his papers, and to make his calculations. 

 In a word, she fulfilled one of the highest duties of a woman, 

 in becoming the patient helpmate of a great and noble mind ; 

 and for this reason although she never sought fame for her- 

 self, the name of Caroline Herschel will always be associated 

 with the labours of our great astronomer. Sir William died 

 in 1822, in his eighty-fourth year, leaving behind him a son, 

 the late Sir John Herschel, who will be mentioned in the 

 next chapter. 



Determination of the Density of the Earth by the Sche- 

 hallien Experiment, 1774. — After speaking of the wonders 

 of the vast universe, and of suns so distant that we cannot 

 even guess at the space which lies between them and us, we 

 must now come back to our little planet and mention a re- 

 markable experiment which was made in 1774 by Maskelyne, 

 who was then Astronomer- Royal of England. This was the 

 finding out of the weight of the earth compared to its size, 

 or in other words, the density of the earth. 



If our globe were made of one material, it would be easy 

 to weigh a small piece and multiply that by the size, which 

 we know pretty accurately, and so to get at the weight of the 

 whole. But as the rocks of the earth's crust differ very much 



