MAGNETISM. 



53 



with them two pair of "the hard bars, 

 placed between their iron armatures, at 

 the distance of about half an inch from 

 one another. The soft bars may now 

 be laid aside, and two of the hard bars, 

 placed between their iron armatures, 

 may be magnetized by means of the 

 other four, which should be held apart 

 at the lower end, at an interval of about 

 one fifth of an inch ; to which distance 

 they are to be separated after they are 

 set on the parallel bar, and brought to- 

 gether again after they are taken off. 

 The same process as that above described 

 is now to be continued until each pair 

 has been touched two or three times 

 over. 



The whole of this process may be 

 gone through in less than half an 

 hour; and each of the larger bars, if they 

 had been previously well hardened, may 

 be made to lift twenty-eight troy ounces, 

 or even more. Bars thus impregnated 

 will give to a hard bar of the same size 

 its full virtue in less than two minutes ; 

 and may, therefore, answer almost every 

 purpose in natural philosophy much 

 better than the natural loadstone, which 

 has seldom sufficient power to impreg- 

 nate hard bars. 



12. Horse-shoe Magnets. 



(209.) Magnets in the form of a 

 straight bar are less convenient when 

 the action of both the poles is wanted, 

 as happens in various experiments, es- 

 pecially such as concern the raising of 

 weights by the force of magnetic attrac- 

 tion. In order to bring the two poles 

 near each other, artificial magnets are 

 often made in the shape of a horse-shoe, 

 (fig. 56,) or sometimes a more semicircu- 



Fig. 56. 



Fig. 57. 



lar form is given to them (fig. 57). These 

 horse-shoe magnets, as they are called, 

 may be rendered magnetic by the same 

 process as a straight bar ; the magnets 

 by which they are rubbed being, of 



course, made to follow the curvature of 

 the bar. The method of^Epinus is best 

 suited for the communication of mag- 

 netism to bars of this shape. 



(210.) Horse-shoe magnets, that have 

 their poles brought very near to each 

 other, are exceedingly convenient as 

 substitutes for the compound magnets 

 employed in the process of magnetizing 

 by the double touch. They fulfil, in- 

 deed, all the purposes of compound 

 magnets in this operation ; and if placed 

 at once on the middle of the needle to 

 be magnetized, with the poles turned in. 

 a direction the reverse of that of the 

 poles intended to be given to the needle, 

 and then moved backwards and forwards 

 along the surface of the needle, taking 

 care to pass over each half of it an equal 

 number of times, and repeating the same 

 operation on the other side, the needle is 

 speedily and effectually rendered mag- 

 netic. The readiness with which this 

 may be put in practice, and the absence 

 of all previous preparation, are strong 

 recommendations in favour of this form 

 of magnet. 



(211.) Powerful magnetic batteries 

 are sometimes constructed by uniting a 

 number of horse-shoe magnets, laying 

 them one over the other with all their 

 poles similarly disposed, and fastening 

 them firmly together in a leathern or 

 copper case. 



' $ 13. Preservation of Magnets. 



(212.) From what has been already 

 said respecting those circumstances 

 which tend to produce or to destroy 

 magnetism, we may easily devise rules 

 for the preservation of magnets, for, 

 unless kept with care, and with the ob- 

 servance of certain precautions, they 

 soon lose their power. 



(21 3.) If a single magnet be kept in 

 an improper position,that is, one differing 

 much from that which it would assume in 

 consequence of the action of terrestrial 

 magnetism, in process of time it becomes 

 gradually weaker ; and this deterioration 

 is most accelerated when its poles have 

 a position the reverse of the natural one. 

 Under these circumstances, indeed, un- 

 less the magnet be made of the'hardest 

 steel, it will in no long time lose the 

 whole of its magnetic power. Two 

 magnets may also very much weaken 

 each other if they be kept, even for a 

 short time, with their similar poles 

 fronting each other. The polarity of the 

 weaker magnet, especially, is rapidly 

 impaired, and sometimes is found to be 



