304 



Analyses of Books. 



[Oct. 



$ 



T-V-. 



north pole ; and henrc is explained why on approaching the lower end of an iron 

 bar to the north end of a needle, it will repel it; while, if we approach the upper 

 end of the bar to the same end of the needle, it will attract it. 



The following experiment, given by M. Biol, in his " Precis Elenientaire de 

 Physique," torn. ii. p. 7, will at once illustrate ivhat is above stated. 



" Suspend, a magnetized needle at its centre, by 

 an assemblage of untwisted silk, and equilibrate it by a 

 counterweight at the south end, so that it may maintain 

 itself in the horizontal plane, as well as in that of the 

 magnetic meridian. Now take a bar of soft iron, A B, 

 about a metre and a half in length, and two centimetres 

 square : then, inclining this bar very nearly in the plane 

 of the magnetic dip, bring its lower end. A, near the 

 north end of the needle, and the latter will be repelled : 

 approach, on the contrary, the upper end, B, (as in the 

 bottom figure) by gradually lowering the bar parallel to 

 itself, and the same end will be attracted. We see then 

 that in this stale of inclination the bar is found suddenly 

 animated with the magnetic influence of the terrestrial 

 globe, as it would have been presented to any other mag- 

 net; ils inferior or lower half possessing a magnetism 

 contrary to that which predominates in our hemisphere, 

 viz. the auitral magnetism ; and the upper half requiring 

 the opposite species, or the boreal. 



" The two ends of the bar are, therefore, in the same 

 state as the needle, a b, itself, and this is the reason that 

 on bringing the end A of the bar to the extremity a of the 

 needle, we observe repulsion, while on approaching the 

 other end of the bar to the same end a of the needle, we 

 find attraction. 



" In order to show that these phenomena depend on a 

 sudden animation impressed u])on the bar in virtue of its 

 position, we have only to invert it end for end, its in- 

 clination remaining the same, when each of its extremities will produce again the 

 same |)heni)mena as we have described a!)ove ; and consequently, these phenumena 

 will be directly the reverse of those which the same ends produced in the first in- 

 stance. The magnetic poles of the bar have been therefore suddenly changed by 

 this inversion ; but in order that this effect may be inslantaneuus, it is requisite to em- 

 ploy a bar of soft iron, and not one of hard iron or sleel." 



141. I have made several experiments, with a view of either confirming or 

 confuting this doctrine, and have met with some results which I find very difiScult 

 to reconcile with it : I shall, however, only stale the following: As we are parti- 

 cularly requested in the preceding directions to employ a bar of soft iron, on 

 account of the facilities with which it changes its poles, I was naturally curious to 

 ascertain what would be the result if we made use of a bar of hard steel; as this is 

 known to receive its magnetic properties very slowly, and to retain it obstinately 

 when once acquired. I, theiefore, procured a steel bar of the above description 

 three feet long, <'in inch broad, and half an inch thick, and had it rendered, accord- 

 ing to the expression of the workmen, " as hard as fire and water could make it;" 

 and 1 must say that I was not at all surprised to find that it produced precisely the 

 same eflVcts as the softest iron, changing its poles with its position (to adopt the 

 language of our author) with equal facility. 



Now this being undeniably the case, why is the reader requested to use soft iron, 

 " and not hard iron or steel," unless it be to hide a blot in the theory, which I can- 

 not see any very easy way to remove. 



14-2. That this is an important objection to the hypothesis in question is obvious 

 from the follow ing remarks of Dr. Robison. (Supplement to Encyclopaedia Britan- 

 nica. Art. Magnetism, No. 46.) " 1 1 is of jreculiar importance to remember, that 

 the acquisition of magnetism is gradual, and that the gradation is the more percep- 

 tible in proportion as the steel is of a harder temper. When a magnet is brought 

 to one end of a bar of common iron, ils remote extremity, unless it be exceedingly 

 long, acquires its utmost magnetism immediately. But when the north end of a 

 magnet is applied to one end of a bar of hard steel, the part in contact immediately 

 becoincs a south pole, and tlie far end is not yet aflfected. We observe a north 



