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feiTuginous) possessing two dissimilar poles, that have the property 

 of attracting or repelling other magnets. Dissimilar poles attract each 

 other ; similar poles repel each other. 



Magnets may be divided into two classes — permanent magnets and 

 inductive magnets : the former being generally composed of hard steel 

 that has been magnetized, or the natural magnet, or loadstone. Induc- 

 tive magnets are generally composed of soft iron, or cast iron, and are 

 only magnets whilst under the influence of a permanent magnet, and 

 the poles are determined by the relative position with regard to such 

 permanent magnet. 



This earth is to be regarded as a permanent magnet, influencing 

 every ferruginous body which is not under the superior influence of 

 some other permanent magnet ; consequently, all soft or cast iron on 

 the surface of the earth at all times exercises a magnetic influence. 



The term " permanent " must be understood in a restricted sense, 

 both as it is applied to this earth or hard steel artificial magnets. It 

 lias been shown that not only do the magnetic poles of the earth vary in 

 an age, but also, to a certain extent, there is a diurnal change in the 

 intensity, dip, and variation of the compass. Also, steel artificial 

 magnets, or needles, are subject to change. Since the difference 

 between the nature of the body capable of becoming a permanent magnet 

 depends upon its hardness, it is evident that no ferruginous body is 

 either sufficiently soft to be a perfect inductive magnet, or sufficiently 

 hard to be a permanent magnet ; and this remark particularly applies 

 to malleable iron, such as the angle iron and the plate iron of which 

 iron ships are built, which are hardened to a considerable extent 

 by being pressed between rollers. Such iron partakes of the nature 

 both of permanent and inductive magnets. 



If the magnetism of a ship that produces deviation were altogether 

 permanent, its influence would not vary in different hemispheres ; on 

 the other hand, if all the magnetism were inductive, it would be 

 reversed in passing to an opposite hemisphere. 



In J 820, Mr. Barlow introduced a method of correcting the compass 

 by means of soft-iron plates, and for some years his method was consi- 

 dered perfect ; but since that period great changes have taken place 

 both in the construction of the ship and in the nature of the voyages ; 

 both calculated in a much greater degree to test the accuracy of the 

 compass. In those days the helmsman was not allowed to wear shoes 

 with iron naUs in them ; but now, even in the construction of wooden 

 ships, wrought iron has been introduced to a very great extent, such as 

 diagonal riders, knees, boat's davits, tillers, &c., whilst in steamers more 

 considerable portions of iron are employed ; and, lastly, many ships are 

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