Scientific Lectures. 153 



iron or of steel is magnetized by induction, the end of the bar nearest 

 the inducing magnet is of the opposite polarity to that end of the 

 magnet toward which it points. In the experiment just made, the 

 end of the magnet was its north pole, and the red end of the bar 

 nearest it has south polarity given it by induction. 



With labor on my part, encouraged by patience on yours, I have 

 at last put you in full possession of those tests which can determine 

 for us whether any given bar has magnetic properties or is devoid of 

 them ; and I have also experimentally shown you those conditions 

 necessary for a distant magnet to produce either temporary or perma- 

 nent magnetism in bars of iron or steel. The process of the experi- 

 mental establishment of these elementary principles, preparative to 

 our actual work with the earth as a magnet, has been tedious to you 

 and to me; some, thoughtlessly, may say puerile ; yet as well say 

 " every one knows that," and erase from your geometry its axioms, as 

 omit from our demonstration those elementary facts and principles 

 which constitute its very foundation, for 



* * "To tell by which of nature's laws, 

 The stone called magnet by the Greeks — since first 

 'Mong the Magnesians found — can iron draw. 



* * * * 



Here many principles we must first lay down, 

 And slow approach by long preparative, 

 Rightly to solve the rare phenomenon." 



Lucretius, Book VI, Trans, of C. F. Johnson. 



" The earth itself is a great magnet ;" if so, it also will confer on 

 this iron bar temporary magnetism when its length points toward the 

 earth's magnetic pole ; and, if vibrated while in this position, it will 

 also receive permanent magnetic properties. But here difficulties at 

 once meet us. Are we not begging the question when we talk 

 of drawing a line toward the earth's magnetic pole 1 That the 

 earth has magnetic poles is just the problem to be solved. True, but 

 we can do this; we know from experiment that if the earth be a 

 magnet, then this point of concentration of the magnetic effect, called 

 the pole, is situate somewhere in the depth of the earth, and this 

 depth we can (as has, indeed, been done) approximately determine 

 by experiments on a magnetic sphere of steel. Then, we can further- 

 more assume that these polar points exist somewhere on or near the 

 earth's axis of rotation, for a freely suspende 1 magnetic needle places 

 itself, here in New York, approximately in a plane passing through 

 this axis. Also, as the spear-end of the magnetic needle points toward 

 the geographic north, then either that part of the earth is of magnetic 



