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 s 

 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 effect 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 llesearches in Electricity, ist Series, pp. 24-44. 

 Paragraphs 81-139. 



1 Jamin, Cours cle Physique, torn. iii. p. 297. 



111 Airy, On Tides and Waves/ Encyclopaedia Metropolitana, p. 348 *. 



