vi INQUIRY AND INTERPRETATION 153 



by mathematics before it was found actually to exist 

 in Nature. 



Scientific theory is established by cumulative 

 evidence of this kind. The Copernican theory is held 

 to be true, because it accounts completely for the 

 varying aspects of the heavens as seen from different 

 parts of the earth and at different times ; it describes 

 the movements of the celestial machinery, and when 

 the law of gravitation had proved to be the mainspring, 

 the two principles combined provided astronomers with 

 everything required for the construction of an accurate 

 time-table of the heavenly bodies in the past, present 

 or future. The powerful instrument which has thus 

 been placed in the hands of astronomers, and the use 

 made of it, has been described in impressive words. 



Supply any man with the fundamental data of astronomy, the 

 times at which stars and planets cross the meridian of a place, 

 and others matters of this kind. He is informed that each of 

 these bodies whose observations he is to use is attracted by all 

 the others with a force which varies as the inverse square of their 

 distance apart. From these data he is to weigh the bodies, 

 predict their motion in all future time, compute their orbits, 

 determine what changes of form and position these orbits will 

 undergo through thousands of ages, and make maps showing 

 exactly over what cities and towns on the surface of the earth 

 an eclipse of the sun will pass fifty years hence, or over what 

 regions it did pass thousands of years ago. A more hopeless 

 problem than this could not be presented to the ordinary human 

 intellect. The men who have done it are therefore in intellect 

 the select few of the human race. The astronomical ephemeris 

 is the last practical outcome of their productive genius. Prof. 

 Simon Newcomb. 



Observation, interpretation, prediction, realisation, 

 are successive stages of scientific theory. Astronomy, 

 being the oldest branch of science, can furnish the most 

 perfect links in this chain of reasoning, but remarkable 



