CH. xxxi. THE DENSITY OF THE EARTH. 287 



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 calcula- 

 tions. 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 herself, 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 

 Schehallion 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 remarkable 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 com- 

 pared 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 in weight, and we do not know what the middle of 

 the globe is made of, this plan is not possible. We know, 

 however, that every atom of matter has the power of attrac- 

 tion, so that if we could find out how much attraction our 

 earth possesses, by comparing it with the attraction of some 

 other body which we can weigh, then we could arrive at the 

 weight of the earth. 



Now Newton, in his ' Principia,' had pointed out that a 

 plumb-line, that is, a piece of string with a weight of lead at 



