xxiv.] EMPIRICAL KNOWLEDGE, EXPLANATION, &c. 535 



That light travels with a uniform high velocity was 

 proved by Roemer from observations of the eclipses of 

 Jupiter's satellites. Corrections, were thenceforward made 

 in all astronomical observations requiring it, for the 

 difference of absolute time at which an event happened, 

 and that at which it would be seen on the earth. But 

 no person happened to remark that the motion of light 

 compounded with that of the earth in its orbit would 

 occasion a small apparent displacement of the greater 

 part of the heavenly bodies. Fifty years elapsed before 

 Bradley empirically discovered this effect, called by him 

 aberration, when reducing his observations of the fixed 

 stars. 



When once the relation between an electric current and 

 a magnet had been detected by Oersted and Faraday, it 

 ought to have been possible for them to foresee the diverse 

 results which must ensue in different circumstances. If, 

 for instance, a plate of copper were placed beneath an 

 oscillating magnetic needle, it should have been seen that 

 the needle would induce currents in the copper, but as 

 this could not take place without a certain reaction against 

 the needle, it ought to have been seen that the needle 

 would come to rest more rapidly than in the absence of the 

 copper. This peculiar effect was accidentally discovered 

 by Gambey in 1824. Arago acutely inferred from 

 Gambey's experiment that if the copper were set in 

 rotation while the needle was stationary the motion 

 would gradually be communicated to the needle. The 

 phenomenon nevertheless puzzled the whole scientific 

 world, and it required the deductive genius of Faraday 

 to show that it was a result of the principles of electro- 

 magnetism. 1 



Many other curious facts might be mentioned which 

 when once noticed were explained as the effects of well- 

 known laws. It was accidentally discovered that the 

 navigation of canals of small depth could be facilitated 

 by increasing the speed of the boats, the resistance 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 predicted this 



1 Experimental Researches in Electricity, ist Series, pp. 24 44. 



