740 TERRESTRIAL MAGNETISM. 



One of the magnets, a small bar of steel, is suspended by the stirrup 

 and thread (fig. 373), and the north pole is marked after the bar 

 has come to rest in a definite position ; a second magnetic bar is 

 now suspended in the place of the first, and when it has come to rest, 

 the like and unlike poles of the two magnets are brought succes- 

 sively near to one another. 



FIG. 37-i (A an. proj., A and B real size). 



A needle which balances on a pivot may be made from a piece of 

 a thin steel knitting needle which is made red-hot but not heated 

 too much over a charcoal fire. It is then bent into the shape shown 

 in fig. 375, again made red-hot and thrown into water in order 

 to temper it. It is then magnetised by being rubbed with an 

 electro-magnet or a permanent magnet. A piece of glass tube is 



FIG. 375 (real size). 



drawn out to a point, being kept in the flame all the time, so that 

 it may be melted off and closed at a short distance from the 

 wider part of the tube. The closed conical end is cut off by a scratch 

 of the file and the pressure of the fingers, heated until sealing-wax 

 melts upon it, and when the sealing-wax is cool, the needle is heated 

 in the middle and pressed hot upon the glass cap. The point of a 

 darning-needle, of which the eye-end is fixed into a small piece of 

 wood, serves as pivot. If the needle is not well balanced when 

 placed upon the pivot, the end which dips down is ground upon the 

 grindstone until the needle is made to swing in a horizontal position. 



The fact that a freely suspended magnetised needle 

 ultimately sets in a definite position more or less north 

 and south, can only be explained by supposing the 

 earth itself to be a magnet. The magnetic poles of the 

 earth are near the terrestrial poles, but do not quite 



