MIRACLES OF SCIENCE 



then noting the difference of weight due to the inclu- 

 sion or exclusion of one small boy. Indeed, it is 

 further stated that the test would determine whether 

 or not the boy wore boots! It is interesting to ob- 

 serve that the results obtained by Professor Poynt- 

 ing are in close agreement with those secured by the 

 Cavendish method. He makes the average mass of 

 the earth 5.493; as against the 5.527 of Professor 

 Boys. The most recent investigations, then, show 

 the sagacity of Newton's original guess, that the 

 mass of the earth would be found to be between five 

 and six times that of a similar globe of water. 



WEIGHING THE MOON 



In finding the mass of the moon or of any other 

 sidereal body, the great law of universal gravitation 

 is invoked as in weighing the earth. We have, in- 

 deed, no other means of making such a test, and 

 before Newton's exposition of the law of gravitation 

 no one could have had more than the vaguest notion 

 of the mass of any planetary or stellar body. Granted 

 a knowledge of the law of gravitation, together with 

 a recognition of the principle of inertia, it becomes 

 possible to calculate the mass of many of the astro- 

 nomical bodies with comparative ease; provided the 

 size of the orbit in which the bodies move can be 

 determined. 



The law of gravitation expresses the fact that the 

 attraction between any two masses of matter varies 

 inversely as the square of their mutual distance and 

 directly as the product of their masses. The law of 

 inertia states the fundamental principle that any 



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