2 



ELECTRO-MAGNETISM. 



this day, remained inexplicable, although 

 we have acquired the knowledge that 

 electricity, under certain modifications, 

 will produce every effect of magnetism. 

 Electricity, we know, may be transferred 

 from one body to another, but mag- 

 netism can be excited by induction only, 

 and is incapable of any similar kind of 

 transference. Still, however, there ex- 

 isted many positive facts, which, inde- 

 pendently of all analogy, demonstrated 

 that the magnetic needle was occasion- 

 ally influenced in its movements by the 

 action of electricity ; and that, in certain 

 cases, the magnetic properties could be 

 excited by electric explosions. The ap- 

 pearance of the aurora borealis, which 

 has all the characters of an electric phe- 

 nomenon, has been very frequently ob- 

 served to be accompanied by a disturb- 

 ance in the position of the compass ; 

 and a delicately suspended magnetic 

 needle has generally exhibited, on these 

 occasions, very frequent oscillations. 

 Lightning, which is still more decidedly 

 electric, has been known, in numberless 

 instances, to destroy, and sometimes to 

 reverse the polarity of the compass- 

 needle ; and many disastrous accidents 

 happening to ships, in consequence of 

 their mistaking their course, may very 

 probably have been owing to this cause. 

 In confirmation of this, we meet with a 

 narrative recorded in one of the early 

 volumes of the Philosophical Transac- 

 tions*, in which the ship Alexander, 

 being one hundred leagues from Cape 

 Cod, in latitude 48, encountered a 

 violent thunder-storm ; the mast was 

 struck by lightning, which also reversed 

 the poles of all the compasses in the 

 ship, a change which was not discovered 

 till the ensuing night, when the stars 

 appeared, and it was found that they 

 had been steering in the opposite course 

 to that which they intended. It is also 

 stated, that in one of the compasses, the 

 end which had before pointed to the 

 north now pointed to the west. An- 

 other instance is recorded in the same 

 workf, where a stroke of lightning 

 passed through a box containing a 

 great number of knives and forks, melt- 

 ing some, and scattering the rest about 

 the room. It was found that all those 

 which were not melted had been ren- 

 dered strongly magnetic, so as to take 

 up large nails, and other pieces of iron, 

 placed near them. 



(5.) Experiments were tried with the 



* Vol. xiv. p. 520. 



t Vol. xxxix. p. 74. 



electrical battery, in imitation of these 

 effects, and in order to ascertain the 

 circumstances on which they depended. 

 But although steel bars were easily ren- 

 dered magnetic by passing strong elec- 

 tric shocks through them, yet the results 

 were by no means uniform, and no 

 general law could be traced as govern- 

 ing the production and distribution of 

 the polarity thus induced. A large pro- 

 portion of the effects appeared to be re- 

 ferable to the concussion which the par- 

 ticles of the bar received in consequence 

 of the violence with which the accumu- 

 lated torrent of electricity rushed through 

 them, thereby giving efficacy to the in- 

 ductive influence of the earth. This 

 influence, it is well known, depends al- 

 together, as was explained in the Treatise 

 on Magnetism ( 109), on the position 

 of the bar with relation to the direction 

 of the dipping needle, which is the same 

 as that of the action of terrestrial mag- 

 netism. The experiments of Mr. Scores - 

 by*, made with a view of determining 

 the amount of this influence when aided 

 by electric concussion, fully confirmed 

 the principle upon which that mode of 

 explaining the phenomenon rests, by 

 showing that the action of a powerful 

 electric shock is, in a great measure, 

 similar to that of a blow from a ham- 

 mer, or to the forcible twisting of the 

 iron, or any other kind of mechanical 

 violence. 



(6.) There still, however, remained 

 many anomalous appearances, to the 

 explanation of which this principle did 

 not furnish the most slender clue. These 

 anomalies presented themselves more 

 especially when the electric discharge 

 was made to pass transversely, or in 

 oblique directions, through the bar ; for 

 it was found, in those cases, impossible 

 to predict what direction the induced 

 poles would assume, or even whether 

 any distinct polarity would be commu- 

 nicated to the bar when so treated. 



Nothing illustrates more forcibly the 

 proneness of the human mind to draw 

 general conclusions from insufficient 

 data, than the various opinions so con- 

 fidently maintained by different experi- 

 mentalists on this subject. D'Alibard 

 thought he had demonstrated, by his 

 experiments, that the electric discharge 

 imparts a northern polarity to that point 

 of a steel bar at which it enters, and a 

 southern polarity to that at which it 



* Transactions of the Royal Society of Edin- 

 burgh, vol. ix. 



