170 THE PRINCIPLES OF SCIENCE. 



in 1824. Arago acutely inferred from Gambey's experi- 

 ment that if the copper were set in rotation while the 

 needle was stationary the motion would gradually be 

 communicated to the needle. The phenomenon never- 

 theless puzzled the whole scientific world, and it required 

 the deductive genius of Faraday to show that it was a 

 necessary result of the principles of electro-magnetism k . 

 By an act of deductive reasoning Faraday anticipated 

 that a piece of copper rotating between the poles of a 

 powerful magnet must experience a kind of resistance 

 which will soon bring it to rest, and this eifect he proved 

 to exist in a decisive experiment 1 . 



Many other curious facts might be mentioned which 

 when once noticed were explained as the effects of well- 

 known natural laws. It was accidentally discovered that 

 the navigation of canals of small depth could be greatly 

 facilitated by increasing the speed of the boats, the resist- 

 ance being actually reduced by this increase of speed, 

 which enables the boat to ride as it were upon its own 

 forced wave. Now mathematical theory might have pre- 

 dicted this result had the right application of the formulae 

 occurred to any one m . Giffard's injector for supplying 

 steam boilers with water by the force of their own steam, 

 was, I believe, accidentally discovered, but no new prin- 

 ciples of mechanics are involved in it, so that it might 

 have been theoretically invented. The same may be said 

 of the curious experiment in which a stream of air or 

 steam issuing from a pipe is made to hold a free disc 

 upon the end of the pipe and thus apparently obstruct 

 its own free outlet. The possession then of a true theory 

 does not by any means imply the foreseeing of all the 



k 'Experimental Researches in Electricity,' ist Series, pp. 24-44. 

 Paragraphs 81-139. 



1 Jamin, ' Cours de Physique,' torn. iii. p. 297. 



in Airy, ' On Tides and Waves,' Encyclopaedia Metropolitana, p. 348 *. 



