150 DISCOURSE ON THE STUDY 



by the motions they produce. Thus, the force ol 

 magnetism becomes known by the deviation pro- 

 duced by iron in a compass needle, or by a needle 

 leaping up to a magnet held over it, as certainly as 

 by that adhesion to it, when in contact and at rest, 

 which requires force to break the connection ; and 

 thus the currents produced in the surface of a quan- 

 tity of quicksilver, electrified under a conducting 

 fluid, have pointed out the existence and direction 

 of forces of enormous intensity developed by the 

 electric circuit, of which we should not otherwise 

 have had the least suspicion.* 



(144.) But when the cause of a phenomenon nei- 

 ther presents itself obviously on the consideration of 

 the phenomenon itself, nor is as it were forced on 

 our attention by a case of strong analogy, such as 

 above described, we have then no resource but in 

 a deliberate assemblage of all the parallel instances 

 we can muster ; that is, to the formation of a class 

 of facts, having the phenomenon in question for a 

 head of classification ; and to a search among the 

 individuals of this class for some other common 

 points of agreement, among which the cause will 

 of necessity be found. But if more than one cause 

 should appear, we must then endeavour to find, or, if 

 we cannot find, to produce, new facts, in which each of 

 these in succession shall be wanting, while yet they 

 agree in the general point in question. Here we 

 find the use of what Bacon terms " crucial instances" 

 which are phenomena brought forward to decide 

 between two causes, each having the same analogies 

 in its favour. And here, too, we perceive the utility 



* Phil. Trans. 1824. 



