.162 Transactions of the American Institute. 



current around these cores, the disc was magnetized and its poles 

 coincided with the points of contact of the cores. Thus this little 

 disc of two and a quarter inches in diameter gives us an approximate 

 representation of a section of the earth's magnestism, passing through 

 the two magnetic poles. 



We will now experiment with this disc as we did with the bar 

 magnet, and in the same manner examine its magnetic curves ; for 

 if the earth be a magnet, they must give us an idea of the lines in 

 which it also exerts on the needle its directing influence, and greatly 

 aid in giving clear conceptions of the magnetic actions which have 

 been observed by navigators and explorers. I now place the disc 

 upon the horizontal condenser of this lantern, and down npon it I 

 lower this plate on whose upper surface iron filings have been uni- 

 formly sifted. I now gently tap the plate. How beautifully these 

 lines form around the disc ! Here are its poles, for you observe the 

 lines all leading to these points within the dark circle of the magnet, 

 and directly over these points the lines tend directly to the center of 

 the disc. Here, however, at positions on the circumference midway 

 between the poles, you observe that the course of the lines for a short 

 distance is parallel to the surface of the magnet, and places so situate 

 determine points of its magnetic equator. 



With this tiny magnetic needle, suspended by a delicate silk fiber, 

 I will explore the magnetic condition of this bright space covered 

 with lines, which you observe surrounding the black circle of the 

 magnet. Wherever I place it you see that it always stands with its 

 length in the direction of a curve, and now that I move its point of 

 suspension over one of these lines you see it swinging round with the 

 bend of the curve, just as a car curves on a railroad track ; and you 

 will remark that this takes place when it is moved over any line, 

 whether near to or far from the disc. Now follow me as I move the 

 needle around the circumference of the magnet, while at the same 

 time I describe what takes place to a needle freely moving on its 

 center when placed over the earth at points corresponding in latitude 

 to those marked on this disc. The needle is now over the magnetic 

 pole of the disc, for you see it points toward its center ; thus stood 

 the needle of Sir James Ross on the west coast of Boothia. I 

 now progress southward, and observe how the end near the disc is 

 gradually moving away from it ; thus acts the needle on the earth 

 whenxarried south from its position of vertically. Now the little 

 needle is at a point of the disc marked with the latitude of New 

 York, :and you see it making an angle of about seventy-three degrees 



