io The Realm of Nature CHAP. 



and this possible explanation is termed an hypothesis. Then 

 he reasons deductively from the assumed explanation, usually 

 employing mathematics for the purpose, and so arrives at a 

 number of additional facts which must exist if the hypo- 

 thesis be true. These predicted facts may not be familiar 

 or may not occur naturally at all. In the latter case it is 

 necessary to seek them by making experiments, and so 

 important is this aid in some cases that the expression Ex- 

 perimental Science is often used in the sense of physical 

 science. If the facts predicted to exist in certain circum- 

 stances by hypothesis are not found, and if others which the 

 hypothesis could not account for appear, the hypothesis 

 is proved to be erroneous, or, at least, incomplete. Renewed 

 inductive reasoning from the wider basis of ascertained facts 

 must then furnish material for a fresh effort of imagination 

 and a new hypothesis to be similarly tested, and, if neces- 

 sary, rejected in turn. Should the facts agree with those 

 deduced from the hypothesis there is a probability of its 

 being true, but a great many tests must be thought of, 

 applied, and found realised before the hypothesis is accepted 

 as a true and complete explanation. An explanation of 

 facts found, tested, and proved to be true and complete 

 in this way is called a theory, and when a theory is con- 

 firmed by a great number of observations it is accepted as 

 a Law of Nature. 



19. Proof of a Theory. The process of testing an 

 hypothesis requires great caution in order to prevent mis- 

 takes. A long time and the labour of many observers are 

 often necessary to perfect a theory or demolish an incorrect 

 hypothesis. When Newton imagined the hypothesis of 

 universal gravitation, according to which the force that 

 causes a stone to fall to the ground also controls the motion 

 of the Moon round the Earth and of the Earth round the 

 Sun, he deduced from the hypothesis that the Moon in its 

 orbit should fall toward the Earth 15 feet in a minute. 

 Careful observation of the Moon's motion showed that it 

 was only bent toward the Earth 1 3 feet in a minute, and 

 therefore Newton abandoned his hypothesis as untrue. 

 Thirteen years later a new measurement of the size of the 



