160 Transactions of the American Institute. 



Let me try if I can give you some insight into their significance. 

 The plate sprinkled with iron dust was placed upon the magnet and 

 nothing remarkable appeared. Why ? Simply because all pheno- 

 mena, without exception, are either motions or the results of motion ; 

 and no motions of the iron particles could take place until the plate 

 was vibrated; then they sprang into the air, and the magnet, from its 

 proximity, rendered every iron grain magnetic by induction, while 

 the same directing principle joined their north and south poles. 

 Thus, while in the air, or when gliding over the plate, they were 

 deflected into those curious curves which is the figure of their dance. 

 These lines thus show the directions in which act the combined mag- 

 netic radiations from the two poles. They are the lines of magnetic 

 force. They indicate far more than this ; but for the present what 

 I have shown you will suffice. Now, if these curves are actually 

 formed of iron filings whose greatest lengths lie in these lines, and 

 whose contiguous ends are north and south poles, and if these lines 

 truly mark out the resultants of the combined actions of the two 

 poles, then this tiny magnetic needle which I have by a silk fiber 

 suspended from this thin wire, will, if brought anywhere in this field 

 of curves, always place its length in the direction of a line ; more- 

 aver, when the center of the needle is moved along one of these lines, 

 the length of the needle will always come into the line. I now 

 bring all this to the test of experiment. You see all happens just as 

 I predicted. 



To lead the mind from the small to the great, from our puny 

 experiments to the action of the great earth itself, I have magnetized 

 this disc of steel ; therefore, this metallic circle has magnetic poles, 

 like the bar we have just experimented with. Hence, if " the earth 

 is a great magnet," this disc gives an ideal section of its magnetism, 

 and should produce on a small scale what the earth does on a grand 

 one. Yoyagers and philosophers have stated that the earth has two 

 magnetic poles or points in its mass toward which the magnetic 

 line converge.* When the little exploring needle was carried 

 around one end of the bar magnet, you will remember that it 

 always kept pointing toward one point, which we called the 

 magnetic pole of the bar. So, in the case of this disc, will the explor- 

 ing needle, when brought near the region of its poles, always point 

 in lines which converge to these inner centers of force ; hence it 



* Gilbert, in his De Magnete, Lib. i, ch. 2, sayB : " This is that Robert Norman (a good seamen 

 and an ingenious artificer) who first discovered the dip of magnetic iron." Norman made this very 

 important observation in 1576. 



