612 SPECIAL METHODS OF DISCOVERY. 



that a quantity cannot pass from one amount to another 

 without passing through? all intermediate degrees of mag- 

 nitude according to the intermediate conditions.' ' New- 

 ton used the law of continuity to suggest, but not to 

 prove, the doctrine of universal gravitation. Let, he said, 

 a terrestrial body be carried as high as the moon, will it 

 not still fall to the earth ? and does not the moon fall by 

 the same force ? ' c Every philosopher has the power of 

 applying this law, in proportion as he has the faculty of 

 apprehending the ideas which he employs in his induction, 

 with the same clearness and steadiness which belong to 

 the fundamental ideas of quantity, space, and number.' 1 

 6 The method of gradation consists in taking intermediate 

 stages of a property in question, so as to ascertain by 

 experiment, whether, in the transition from one class to 

 another, we have to leap over a manifest gap, or to follow 

 a continuous road.' ' Faraday made a gradation of electric 

 conductors to non-conductors, and thus discovered that 

 there was no real division between the two classes of 

 bodies. He also showed that voltaic and frictional elec- 

 tricity were only different grades of the same force.' 2 



1 See page 562. 



2 "Whewell, Philosophy of the Inductive Sciences, vol. ii. pp. 559-563. 



