MAGNETISM. 



55 



single pole, are brought near to each 

 other, and their directions rendered pa- 

 rallel: they are, therefore, made to 

 conspire in various actions which 

 require the joint operation of both 

 poles, such as that of eliciting magne- 

 tism by the double touch. The same 

 advantages, indeed, are procured by this 

 construction as \ve have already seen 

 obtain in the case of horse-shoe mag- 

 nets when compared with straight mag- 

 netic bars. Thus we find that a load- 

 stone which, in the natural state, would 

 appear to be exceedingly feeble, will pos- 

 sess, when properly armed, very consi- 

 derable magnetic powers. 



(217.) The armature of a loadstone 

 not only contributes to exalt its magnetic 

 virtue, but also furnishes us with the 

 means of preserving it uninjured. Its 

 two poles, being now transferred to the 

 extremities of the armatures, or to each 

 foot of the armature, as it has been 

 called, on connecting these poles, by ap- 

 plying to them a bar of soft iron, A (Jig. 

 58), we may effectually prevent the dissi- 

 pation of their magnetism. This cross- 

 bar performs a similar function with 

 relation to these poles that the iron plates 

 do to the loadstone itself it acts as a 

 secondary armature ; and we find, after 

 applying this bar, that the apparatus 

 gradually acquires greater power up to 

 a certain limit. Such therefore is the 

 mode in which loadstones, when not in 

 use, should always be kept, with a view 

 to the preservation of their powers. 



(218.) Directions of a similar kind, 

 and derived from the same principles, 

 apply also to the preservation of artificial 

 magnets. Horse-shoe magnets should 

 have a short bar of soft iron (A,y?#. 59) 

 Fig. 59. adapted to connect the two 

 poles, and should never be 

 laid by without having such 

 a piece of iron adhering to 

 them. Bar magnets should 

 be kept in pairs, lying pa- 

 rallel to each other, with 

 their poles turned in con- 

 trary directions, and the dis- 

 similar poles on each side 

 connected by a bar of soft 

 iron ; so that the whole may 

 form a parallelogram as in 

 fig. 60. They should fit into 

 A a box when thus arranged, 



so as to guard against accidental con- 

 cussions, and to preserve them from the 

 dampness of the atmosphere. Magnets 

 should be polished, not indeed with a 

 view to the increase of their magnetism, 



Fig. 60. 



but because they are then less liable to 

 contract rust. It is convenient that those 

 ends which have the northern polarity 

 should be marked with a line all round, 

 in order to distinguish the respective 

 poles in each magnet. 



CHAPTER VI. 

 Magnetic Instruments. 



(219.) IN every branch of science, 

 the value of a correct theory is best 

 estimated by the extent and importance 

 of its practical applications. The nearer 

 its approach to perfection, the greater 

 the assistance we derive from it in con- 

 structing instruments for the accurate 

 measurement of spaces, of times, or of 

 forces, and for the accomplishment of 

 the objects which relate to that particu- 

 lar science. It is thus that in mag- 

 netism, if the theory, developed in the 

 preceding part of this treatise, be cor- 

 rect, it ought to furnish principles for 

 the construction and management of 

 magnetic instruments, such as the com- 

 pass, dipping-needle, &c. Our limits 

 permit us only to point out the leading 

 principles which particularly deserve 

 attention in the case of each kind of 

 instrument. 



1. Of the Compass. 



(220.) The term compass is a general 

 name for all instruments calculated to 

 indicate the position of the magnetic 

 meridian, or of objects with reference to 

 that meridian, whether adapted for being 

 used on land, and in the bottoms of mines, 

 where we have a stable support at com- 

 mand, or for observation at sea, where 

 the perpetual agitation of the surface 

 deprives us of that advantage. 



The first class, which merely show 

 us the direction of the magnetic meri- 

 dian, includes the Land Compass, the 

 Mariner's Compass, and the Variation 

 Compass; the second, or those which 

 mark the angular distances of objects 

 from this meridian, are called Azimuth 

 Compasses. 



(221.) Whatever modifications may 

 be rendered necessary by the particular 

 purpose of the compass, the essential 

 parts of which it consists are the same 



