246 DESCRIPTION OF A MAGNETIMETER, BEING A NEW 



means of this plate of lead, which has a screw d at each cor- 

 ner, the whole apparatus is readily put into a horizontal po- 

 sition. As the instrument is put together by screws, it can 

 be easily taken to pieces, so as to become exceedingly por- 

 table. 



Hitherto this instrument has been principally used for esti- 

 mating the magnetism of position in iron, — for ascertaining 

 the existence of permanent polarity in iron or steel, — for mea- 

 suring and comparing magnetic attraction or repulsion, as pro- 

 duced in iron by certain means independent of the magnet, — 

 and for determining the magnetic dip. 



From ignorance of, or inattention to, the magnetism of po- 

 sition, various erroneous conclusions have been drawn and 

 propagated respecting the phenomena of the magnet. For in- 

 stance : In several popular works on this subject, that I have 

 consulted, it is stated, that " if an iron-bar have gained a ver- 

 ticity by being heated red-hot and cooled again, north and 

 south, and then hammered at the two ends, its virtue will be 

 destroyed by two or three smart blows on the middle *." 

 Now there is a particular case in which this may be true j but 

 most generally the statement is inaccurate : for by hammering 

 a bar of iron at the ends, or in the middle, magnetism may be 

 destroyed, produced, or inverted, according to the position in 

 which the bar is held. The effect of position is indeed such, that 

 a bar that has been presented to a magnetic needle, and found 

 free from magnetism, when presented a second time, a quar- 

 ter of an inch nearer to the needle, or at an angle a degree or 

 two nearer the horizontal, may appear to have gained south 



polarity, 



* See Hutton's Math. & Phil. Did. &c. article Magnet' 



