" Next in order I will proceed to discuss by what law of Nature it 

 comes to pass that iron can be attracted by that stone which the Greeks 

 call the Magnet from the name of its native place, because it has its origin 

 within the bounds of the country of the Magnesians. This stone is more 

 wondered at because it often produces a chain of [iron] rings hanging 

 down from it. Thus you may see five and more suspended in succession 

 and tossing about in the light airs, one always hanging from the other 

 and attached to its lower side, and each in turn one from the other ex- 

 periencing the binding power of the stone : with such a continued cur- 

 rent its force flies through all. 



" In things of this kind, many things must be established before you 

 can assign the true law of the thing in question, and it must be ap- 

 proached by a very circuitous road ; wherefore all the more I call for an 

 attentive ear and mind." LUCRETIUS, DC Rerum Natura, Lib. VI., 

 Munro's Translation, p. 317. 



This lecture is a plain statement of the elementary facts of magnet- 

 ism, of one magnetic theory, and of the methods to be pursued in master- 

 ing both. It has already circulated among the teachers mentioned on its 

 title-page, and I had some doubts as to the propriety of its insertion 

 here. But, on reading it, it seemed so likely to be helpful, that my 

 scruples disappeared. J. T. 



