EVERT AGE HAS ITS OWN DISCOVERIES. 171 



its own axis, was led to it by a curious experiment of 

 Savary's intended to point out the action of angular cur- 

 rents.' l The great discovery of Ampere, of the mutual 

 attractions and repulsions of electric currents, led to his 

 further discovery of the mathematical law of their action 

 according to distance. The discovery of a principle 

 generally follows upon that of some of the facts which it 

 co-ordinates, and a knowledge of this principle often leads 

 to the discovery of new facts which flow from it. The 

 discovery by Gralvani in the year 1791, that the limbs of a 

 frog were convulsed when he established a communication 

 between the nerves and muscles by means of metals, 

 coupled with the fact that Du Verney in 1700 knew 

 that the limbs of a frog were convulsed by the action of 

 electricity, 2 conduced in 1793 to the discovery by Volta of 

 voltaic electricity, which in its turn led to the discovery of 

 many new phenomena resulting from the action of that force 

 on different substances. An accumulation of lesser truths 

 ripens the conditions of discovery of a greater one. The 

 discovery of the great law of gravitation was brought about 

 in this way. The truths evolved by the labours of Wren, 

 Hooke, Halley, Kepler, and Huyghens, ripened the condi- 

 tions of Newton's great discovery. 3 Every age of mankind 

 has had its own particular great discoveries, which were 

 the necessary result of the natural extension of knowledge 

 to a particular state at the time ; and those discoveries 

 could not l^ave been made at a much earlier period, nor 

 have been much longer delayed, because, like distant lights 

 advancing towards us, they would have become more easily 



1 See De la Hive's Treatise on Electricity, English edition, vol. i. 

 p. 260. 



2 See Ency. Met. vol. iv. p. 220. 



Whewell, History of the Inductive Sciences, 3rd edit. vol. ii. 

 p. 118. 



