MAGNETISM. 



quire a sensible and permanent magne- 

 tism. This is generally the case with 

 the stationary iron bars belonging to a 

 building, and even with pokers and other 

 fire-irons which have long been kept in 

 an upright position. This circumstance 

 will also readily account for the perma- 

 nent magnetism of that class of iron ores 

 to which the loadstone belongs. Indeed 

 it is perhaps not going too far to assert 

 with Professor ^Robison that all the 

 magnetism which we observe, whether 

 in nature or art, is either the immediate 

 or the remote effect of the magnetism of 

 the earth. 



(109.) All the phenomena which we 

 have already described as the conse- 

 quences of induced magnetism proceed- 

 ing from ordinary magnets, are exem- 

 plified also in the case of that derived 

 from the magnetism of the earth. It is 

 most readily induced, but soonest lost, 

 in the softest kinds of iron and steel ; it 

 is slowly acquired, but more permanently 

 retained in hard-tempered steel. Per- 

 cussion promotes the change, of what- 

 ever kind it may be, which the position 

 of the bar relatively to the earth has a 

 tendency to produce. Hence we see the 

 reason why the steel bar described in 6 

 became permanently magnetical by 

 being struck, while in a vertical position, 

 with a hammer. Mr. Scoresby found 

 that even a bar of soft iron, held in any 

 position, except in the plane of the mag- 

 netic equator, may be rendered magne- 

 tical by a blow with a hammer or other 

 hard substance ; and both ends seem to 

 acquire, by this treatment, an equal de- 

 gree of magnetism. 



On the other hand, an iron bar, pos- 

 sessing permanent polarity, when placed 

 any where in a direction at right angles 

 to the magnetic position, and struck 

 several times, has its magnetism always 

 much weakened, and may even be de- 

 prived of the whole of its magnetism by 

 a single blow. This affords, indeed, an 

 excellent method of depriving iron of its 

 magnetism. Rough treatment of any 

 kind, such as filing or scouring the 

 surface of iron, and more especially 

 bending or twisting it, when in the mag- 

 netical position, tends to impart to it the 

 magnetism corresponding with that po- 

 sition ; or to destroy its previous mag- 

 netism, if it be subjected to the same 

 treatment in a position at right angles 

 to this. 



Iron heated to redness, and quenched 

 in water, in a vertical position, was found 

 by Mr. Scoresby to become magnetic j 



the upper end acquiring the southern, and 

 the lower end the northern polarity. Hot 

 iron, according to the same experimen- 

 talist, receives more magnetism of posi- 

 tion than the same when cold. An iron 

 bar is rendered magnetical by passing 

 an electrical discharge through its axis, 

 provided it be in a position favourable to 

 induction by the earth : and the polarity 

 it acquires corresponds with the effects 

 of this induction. Electricity appears 

 to act, in this instance, merely by its 

 mechanical agency, and independently 

 of a peculiar influence of another kind 

 which it possesses, and which will be 

 the subject of future inquiry. 



(110.) Let us now examine how far 

 Dr. Gilbert's hypothesis corresponds 

 with the actual phenomena of the varia- 

 tions of magnetic position in different 

 parts of the globe. For this purpose, it 

 will be necessary to revert to what was 

 explained in a former chapter regarding 

 the positions which a small needle as- 

 sumes when under the influence of a 

 strong magnet in its vicinity, and va- 

 riously situated with respect to it. These 

 positions, we have seen, are tangents 

 to a magnetic curve passing through the 

 two poles of the great magnet, arid 

 through the centre of the needle. The 

 direction of the tangent, which is the 

 same as that of the dipping-needle, 

 together with that of a vertical line, 

 or one perpendicular to the horizon, 

 will determine the plane of the magnetic 

 meridian, for it is the plane which in- 

 cludes both these lines. The compass- 

 needle, which turns in an horizontal plane 

 only, will arrive at its position of equi- 

 librium when it is situated in the plane 

 of the magnetic meridian, because it 

 then makes the nearest approach of 

 which it is susceptible, to the position of 

 the dipping-needle, which is that towards 

 which the magnetic influence of the 

 earth tends constantly to bring it. 



(111.) In those parts of the globe 

 where the dip is very small, the hori- 

 zontal needle is capable of taking a posi- 

 tion very nearly approaching to that of 

 the dipping-needle : hence the terrestrial 

 magnetism is exerted in bringing it to 

 this position with very little loss of its 

 force. This happens in the equatorial 

 regions of the earth. In high latitudes, 

 on the contrary, where the dip is great, 

 the forces which actuate the horizontal 

 needle, act more obliquely, and there- 

 fore to great disadvantage : hence the 

 compass-needle is more feebly impelled ; 

 the point of rest is less decidedly marked, 



