WEIGHING THE WORLDS 



precisely as if the entire mass of the earth were 

 located at its center. The fact that the gravitational 

 influence of a spherical body is thus exerted was 

 demonstrated by Newton himself. To balance the 

 pull of a mountain against the pull of the earth is not 

 unlike testing the strength of a bar of steel by seeing 

 how much it is bent by a push or pull of known 

 force. A fundamental difficulty with the method, 

 however, is that the exact mass of the mountain itself 

 can never be known with absolute accuracy, although 

 careful surveys may determine the precise bulk of 

 the mountain, and deep borings may give a fairly 

 accurate knowledge of the kind of rock that makes 

 up its structure. Nevertheless a certain measure of 

 success attended this method. 



TESTS WITH A PENDULUM 



The first important efforts to utilize it were made 

 by a French commission of which M. Bouguer was 

 the most prominent member, which, as long ago as 

 1740, made an official trip to South America the chief 

 purpose of which was the measuring of a degree of 

 latitude at the equator. The commission supplement- 

 ed its work, however, by making careful observations 

 of the swing of the pendulum at various high alti- 

 tudes, and in particular on a plateau of the Andes, 

 as compared with the swing of the pendulum at sea 

 level. It has been understood since the time of Gali- 

 leo that a pendulum of given length oscillates with 

 unvarying rapidity at the sea level. The rate of 

 oscillation changes, however, if the pendulum is car- 

 ried to a high altitude, being thus farther removed 



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