vii LAW AND PRINCIPLE 173 



This is not the place to show that the law of universal 

 gravitation provides a complete explanation of Kepler's 

 three laws of planetary motion, or to discuss its profound 

 significance. A law of Nature is tested by its ability 

 to meet all the cases to which it may be applied ; and 

 in not a single instance has the law of gravitation been 

 found wanting in this respect. It explains the fall of 

 bodies on the earth, and the motions of the planets and 

 their satellites ; it enables the paths of comets to be 

 calculated, and the disturbances to which they may be 

 subjected in passing near more massive bodies ; it is 

 used to determine the masses of bodies in our solar 

 system, and of stars revolving round one another, and 

 is applied to calculate the tide-raising effects of the sun 

 and moon ; while by it tables of the moon's movements 

 are calculated to a high degree of perfection for use in 

 determining longitude at sea ; and it does all this without 

 any need of amendment. 



It has been truly said that " The tendency of the 

 human mind is to exaggerate the possibilities of the 

 unknown." Wherever there is ignorance, Nature is 

 dreaded as much as a child dreads darkness. The 

 appearance of a comet in the sky caused whole nations 

 in earlier days to tremble with fear because nothing 

 was known of the nature and movements of these bodies. 

 From the time that Newton showed that comets travel 

 round the sun in definite paths under the control of 

 gravitational attraction, the feeling of awe and anxiety 

 formerly produced by such celestial visitors has been 

 diminishing. They are now looked upon as interesting 

 spectacles instead of being regarded as heralds of 

 disaster. 



Edmund Halley was inspired by Newton's work to 

 calculate the paths of a number of comets. He found 



