32 FRAGMENTS OF SCIENCE 



the next question. The first step of the philosopher in 

 this direction was to prove, mathematically, that if this 

 law of attraction be the true one; if the earth be con- 

 stituted of particles which obey this law; then the action 

 of a sphere equal to the earth in size on a body outside 

 of it is the same as that which would be exerted if the 

 whole mass of the sphere were contracted to a point at 

 its centre. Practically speaking, then, the centre of the 

 earth is the point from which distances must be measured 

 to bodies attracted by the earth. 



From experiments executed before his time, Newton 

 knew the amount of the earth's attraction at the earth's 

 surface, or at a distance of 4,000 miles from its centre. 

 His object now was to measure the attraction at a greater 

 distance, and thus to determine the law of its diminution. 

 But how was he to find a body at a sufficient distance ? 

 He had no balloon? and even if he had, he knew that 

 any height to which he could attain would be too small 

 to enable him to solve his problem. What did he do? 

 He fixed his thoughts upon the moon a body 240,000 

 miles, or sixty times the earth's radius, from the earth's 

 centre. He virtually weighed the moon, and found that 

 weight to be TsWth of what it would be at the earth's 

 surface. This is exactly what his theory required. I 

 will not dwell here upon the pause of Newton after his 

 first calculations, or speak of his self-denial in withhold- 

 ing them because they did not quite agree with the ob- 

 servations then at his command. Newton's action in this 

 matter is the normal action of the scientific mind. If it 

 were otherwise if scientific men were not accustomed to 

 demand verification if they were satisfied with the im- 

 perfect while the perfect is attainable, their science, 



