The Evolution of the Sciences 



material objects there exists a force of attrac- 

 tion proportional to their masses and acting in 

 inverse ratio to the square of their distances. 

 Had it been possible for any doubts to subsist 

 after Newton regarding this attraction, Caven- 

 dish would have dispelled them in 1798 by 

 measuring it directly by means of very sensitive 

 balances. The experiment has since been re- 

 peated many times, and we now know precisely 

 that two masses, each equal to one kilogramme, 

 and placed one metre apart, attract one another 

 with a force equal to 6.7 millionths of a milli- 

 gramme. Now, the attraction of the earth for 

 a kilogramme placed at its surface is precisely 

 equal to the weight of the kilogramme. This 

 enables the mass of the earth to be determined 

 by simple proportion; it is found to be 5875 

 million billion metrical tons. Dividing this 

 figure by the volume of the earth we obtain its 

 mean density, which the most recent determina- 

 tions fix at 5.56. The earth is on an average 

 five and a half times heavier than water. But 

 this is only the mean density, and it is very easy 

 to see that in the superficial layers of the globe 

 the weight of the heaviest materials is consider- 



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