i The Study of Nature u 



Earth, and consequently of the distance of the Moon, gave 

 him more accurate data, and applying these to his hypothesis 

 and to the observations, he found that the discrepancy 

 vanished. This assured him of the truth of his hypothesis, 

 which has ever since taken rank as a theory and a law of 

 Nature. 



20. Test of a Law of Nature. A law of Nature has 

 no exceptions ( 14) ; the only test by which a theory can be 

 accepted as of this rank is the successful prediction of future 

 effects. The theory of gravitation enables astronomers to 

 calculate the relative position of the Sun, Moonj planets, and 

 stars as seen from all parts of the Earth's surface. This is 

 regularly done by a government office in London, and the 

 positions for stated times each day are published three years 

 in advance in the Nautical Almanac. From the tables of 

 this work the captains of ocean-going vessels are able to 

 work out their exact place on the ocean by observations 

 of the positions of the heavenly bodies ( 92). The 

 smallest deviation from truth in the expression of the law of 

 gravitation would throw the results into confusion and lead 

 to almost certain shipwreck. No such confusion has ever 

 occurred, and every successful sea-voyage is one proof more 

 that the law of gravitation was fully understood in the past, 

 and holds in the present. The appointments made for 

 the appearance of the Sun, Moon, and planets amongst 

 special groups of stars at definite times, in the Nautical 

 Almanac are analogous to the appointments_for the arrival 

 of trains at stations made in official railway time-tables. 

 Observation of the fulfilment of time-table predictions very 

 soon demonstrates that the hypothesis in* accordance with 

 which they are framed is not exact, and cannot be depended 

 upon for timing watches or determining our position on the 

 Earth. 



21. Magnitude of Nature. The Scientific Method is 

 applicable to the acquisition of knowledge of any kind, but 

 it has been most used in the study of Nature. It is neces- 

 sary that each scientific investigator should confine himself 

 to one department of Nature in which he finds the facts and 

 tries to reason out the theories connecting them. Thus we 



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