INSTRUMENT FOR MEASURING MAGNETIC ATTRACTIONS, &C. 247 



polarity, by attracting the north pole of the needle. With 

 this instrument, however, pieces of iron or steel, after having 

 their state as to magnetism determined by it, and being then 

 submitted to any operation capable of affecting their polarity, 

 •can be brought a second time to the same precise position, 

 can be presented at the same angle to the needle, and can have 

 any change in their polarity determined in the most satisfac- 

 tory manner. 



As a substitute for the dipping needle, this instrument be- 

 ing much less expensive, and much more portable, may be 

 considered, perhaps, of some importance. The situation in 

 which bar-iron, void of permanent polarity, loses its magnetism 

 of position, is the plane of the magnetic equator. The obliqui- 

 ty of this plane, measured from the horizontal, is, in London, 

 19° 26', that is, in the line of its axis coinciding with the mag- 

 netic meridian. It dips towards the south, and is horizontal 

 in the east and west. The obliquity of the magnetic equator 

 being the complement of the dip, and my magnetic apparatus 

 enabling us to discover that obliquity, by pointing out the 

 angle of no-attraction in iron, we are furnished with a simple 

 process for determining the dip of the needle *. 



By means of this instrument, several curious results, in the 

 phenomena of magnetism, have been obtained. The experi- 

 ments were chiefly made on small bars of iron and steel, laid 

 in the groove of the moveable plate. As the instrument, at 

 the same time, was placed in a north and south position, ac- 

 cording to the compass, the bar, on being elevated or depres- 

 sed, traversed in the magnetic meridian. The moveable limb 



was 



* The remarks subjoined to No. 2. of the succeeding 'propositions point out 

 the method of finding the dip, and show what precautions are necessary to be 

 used to insure accuracy in the result. 



