INSTRUMENT FOR MEASURING MAGNETIC ATTRACTIONS, &C. 255 



and I7 inches distant, produced a deflection of 27^°. The same 

 heated to redness, and then presented in the same position as be- 

 fore, produced a deflection of 60°. On repeating the experiment 

 with the bar a httle nearer to the compass, the mean deflection of 

 both ends of the bar was 15° when cold ; but when heated red-hot, 

 the mean repulsion was 77°. 



Remark. — On trying the efiect of heat on a magnet [bar No. 3.] 

 its power was found to be permanently weakened by heating to 

 300° or 400° ; but, from several experiments, it appeared that the 

 magnet atttacted and repelled as much when hot as cold. 



9. A bar-magnet, if hammered when in a vertical position, 

 or in the position of the magnetic axis, has its power increa- 

 sed if the south pole be upward, and loses some of its magne- 

 tism if the northend be upward. 



Exp. — The compass being placed at the distance of 4^ inches from 

 the south end of the maguetic bar No. 3., while the bar was laid 

 on the moveable plate of the instrimient, at an angle of 21-i-°, the 

 needle was drawn from its meridian 22°. The bar being then 

 struck ten smart blows on the end vdth a hammer, south pole up- 

 ward, its attraction was found to be increased to 26° ; ten more 

 blows increased it to 28° ; twenty more to 31° ; after which 

 there was little change produced, though forty more blows were 

 struck. The north end of the magnet was now foimd to repel the 

 needle 17^°. On striking this end, while held in the hand, north 

 pole up, after ten blows of the hammer, it only repelled 14°; and 

 the south end, which had not been struck, attracted the needle 3° 

 less than before. 



10. A bar of soft steel, without magnetic virtue, has its 

 magnetism of position fixed in it, by hammering it when in a 

 vertical position; and loses its magnetism by being struck 

 when in the plane of the magnetic equator. 



Bepiark. 



