52 The Realm of Nature CHAP. 



extent having been found by experiment, was a measure of 

 the attraction between the small and large spheres. The 

 weight of the small balls is the measure of the attraction of 

 the Earth upon them, and as the distance of the small balls 

 from the centre of the Earth is known, the mass of the 

 Earth can be calculated from the known mass of the large 

 leaden spheres. Mr. Vernon Boys has recently succeeded 

 in making a very fine elastic thread of quartz which acts 

 as an extremely sensitive spring, and can be used to measure 

 the force of attraction between bodies as small as ordinary 

 bullets. 1 As the result of several independent methods, 

 the mass of the Earth has been found to be the same as if 

 it were a globe of homogeneous substance 5 J times as dense 

 as water ; the mean density of the Earth is thus said to be 



5-5- 



86. The Earth in Motion. On a clear morning the 

 bright disc of the Sun appears somewhere on the eastern 

 horizon, rises slowly and wheels round the sky, then, as 

 slowly sinking, it disappears somewhere on the western 

 horizon. When the Sun is visible its light fills the whole sky, 

 which appears as a bright blue dome unless clouds interrupt 

 our view of it. Sometimes a glimpse may be had of the 

 Moon, as a ghostly white broken disc like a little fleecy cloud ; 

 very rarely, indeed, the bright light of a planet is visible, or 

 the weird form of a comet. At night the curtain of the 

 Sun's excessive light is dropped, and we see that the whole 

 sky is really gemmed over with bright points or stars, as if 

 a dome or hollow sphere of black paper pricked with in- 

 numerable holes had been wheeled between us and the Sun. 

 This star -dome appears to revolve round the Earth, the 

 various marks on it preserving an unaltered arrangement. 

 The stars have been grouped into fanciful constellations, 

 which are easily recognised and serve as a rough-and-ready 

 way of naming any definite part of the sky. By a curious 

 mixture of guessing and of reasoning on the observations 

 which they made, Copernicus and Galileo and their followers 

 came to the conclusion that the regular changes in the 

 appearance of the sky from hour to hour and month to 

 month could only be accounted for by the Earth having at 



