MAGNETISM. 
114 
magnets have in different circunjftances; for two magnets 
that are placed with their attracting poles towards each 
other, will have their power, by that means, increafed ; 
and, on the contrary, if their repelling poles be placed to¬ 
wards each other, their power will thereby be diminifhed ; 
and this increafe or diminution of power will be in a 
greater or lefs degree, according as the magnets are nearer 
to, or farther from, each other; whence, in all the experi¬ 
ments made on this fnbjefl, the attraction and repulfion 
come perpetually nearer to an equality, the greater the 
diftance of the two magnets is with which the experi¬ 
ments are made, and vice verfd. And fo great is the ef¬ 
fect of magnets on each other, that, when the repellent 
poles of a large magnet and a fmall one are brought into 
contaCf, the fmall one (hall fometimes have its repeilency 
changed into attraction. Mr. Michell alfo infers from 
other experiments, that the attraction and repulfion of 
magnets decreafe as the fquares of the diltances from the 
refpeCtive poles increafe. The differences of opinion in 
this refpect are af’cribed by him to the want of making 
proper allowances for that property of magnets, in confe- 
quence of which they attraCf or repel equally at equal 
diftances, together with the increafe and diminution of 
power in the magnets with which the experiments were 
tried. 
Sir Ifaac Newton fuppofes magnetic attraClioa to de¬ 
creafe nearly in the triplicate ratio of the diftance. Dr. 
Hellliam found it to be as the fquares of the diftances 
inverfely, which ratio agrees with that of the ingenious 
Mr. Michell. Mufchenbroeck thought he obferved, that 
its attractive force decreafed in the quadruple ratio, or 
as the 4th powers of the diftances. Thus, if at a line 
diftance a particle of iron is attracted with a force equal 
to 1, at 2 lines diftance that force will be 16 times lefs; 
at three lines, 81 times lefs; at 4 lines, 256 times lefs; 
and fo of the reft. This aCtion perhaps decreafes ftill 
more rapidly; for, in a fliip of war laden with large iron 
cannon, it is not obferved that they have a fenfible ac¬ 
tion on the compafs. In our opinion, however, it would 
be prudent to remove them to as great a diftance as pof- 
fible. 
A magnet attraCfs another magnet with lefs force than 
a piece of iron : this has been confirmed by many other 
experiments. But the attraction between two magnets 
begins from a greater diftance than between the magnet 
and iron. The attraction between a given magnet and a 
piece of iron is lubjeCt to a variation arifing from the 
weight and fhape of the iron ; there being a limit, in the 
weight and fhape of the iron, in which the magnet will at¬ 
tract: it more forcibly than either a greater or a fmaller 
one ; but this moft advantageous weight and extenfion of 
the piece of iron can only be determined by aCfoal expe¬ 
riment, it being various according to the various nature, 
ftrength, and fhape, of the magnet, as well as of the iron. 
Magnetic attraction takes place between the magnet and 
fitch ferruginous bodies as were not magnetic before, or 
between the contrary poles of two magnets ; but, when 
two magnets are placed with their poles of the fame name 
toward each other, then, inftead of attracting, they repel 
each other. However, it often happens, that though the 
north pole of one magnet be placed near the north pole 
of another magnet, or the fouth pole of the one be placed 
near the fouth pole of the other, yet they attract each 
other; and fometimes they fliow no attraction nor repui- 
iion. In order to reconcile this apparent contradiction, it 
is neceffary to mention firft another phenomenon, which 
takes place whenever a piece of ferruginous fubftance is 
brought near a magnet; and which indeed is the founda¬ 
tion of, and ferves to explain, a great many other appear¬ 
ances, ctherwife unintelligible, in the fcience of magne- 
tifm. The phenomenon, in fliort, is this: When a piece 
of iron, or any other fubftance that contains iron, is 
brought within a certain diftance of a magnet, it becomes 
itfelf a magnet, having the poles, the attractive power, 
and in Ihort every property, of a real magnet. That part 
of it which is neareft to the magnet acquires a contrary 
polarity; thus, if an oblong piece of iron, A B, be brought 
within a proper diftance of a magnet, fo that the extre¬ 
mity A of the iron may be oppolite the north pole of the 
magnet, then this fame extremity A will become a fouth. 
pole, and the other extremity B will become a north pole, 
The magnetifm acquired by being placed within the in¬ 
fluence or the fph'ere of activity of a magnet, in foft iron 
lafts only whilft the iron continues in that fituation, and, 
when removed from the vicinity of the magnet, its roa^- 
netifni vanifhes immediately; but with hard iron, and es¬ 
pecially with fteel, the cafe is quite different; for the 
harder the iron or the fteel is, the more permanent is the 
magnetifm which it acquires from the influence of a mag¬ 
net ; but it will be in the fame proportion difficult to ren¬ 
der it magnetic. If, for inftance, a foft piece of iron and 
a piece of hard fteel, both of the fame fhape and fize, be 
brought within the influence of a magnet at the fame dif¬ 
tance, it will be found that the iron will appear much more 
magnetic than the fteel; but, if the magnet be removed, 
the foft iron will inftantly lofe its magnetifm, whereas the 
hard fteel will preferve it for a long time. 
From thefe obfervations two confequences are evidently 
deduced, viz. firft, that there is no magnetic attraction 
but between the contrary poles of two magnets ; for the 
iron, or other ferruginous body, that is prefented to the 
magnet, muft become itfelf a magnet before it be at¬ 
tracted ; and fecondly, it appears why a magnet muft at¬ 
tract a piece of foft iron more forcibly than hard iron, 
and much more than hard fteel, viz. becaufe the hard 
iron, and more efpecially the hard fteel, does not become 
fo ftrongly magnetical as foft iron, when prefented to a 
magnet. 
The law of repulfion, being always exerted between 
magnetic poles of the fame name, nearly as ftrong as the 
attraction between thofe of different name, remains cer¬ 
tain and immutable; but it often happens, that one of the 
magnets, being more powerful than the other, will change 
the pole of that other magnet, in the fame manner as it 
gives magnetifm to any other piece of iron which is ex- 
pofed to its influence, and then an attraction will take 
place apparently between magnetic poles of the fame 
names ; though in faCt it is an attraction between poles 
of different names, becaufe one of them has been aCtua’lly 
changed. Thus, fuppofe that a powerful magnet be 
placed with its north pole very near the north pole of a 
weak magnet; it will be found, that inftead of repelling 
they will attraCf each other, becaufe that part of the weak 
magnet, which before was a north pole, has been changed 
into a fouth pole by the aCfion of the Itrong magnet. 
As thofe bodies which are poffeffed of any magnetifm 
cannot be very readily affeCfed by the influence of another 
magnet, for the very fame caufe which renders them ca¬ 
pable of retaining any magnetifm at all, namely, thehard- 
nefs ; and as the power of a magnet diminilhes in propor¬ 
tion to the diftances from its furface; it follows, that, 
when the north or fouth pole of a weak magnet is from a 
confiderable diftance gradually brought near the like pole 
of a powerful magnet, the pole of the weak magnet can¬ 
not be changed very eafily 5 hence, beyond a certain dif¬ 
tance, viz. before the faid pole be changed, the two mag¬ 
nets muft exert a repulfion againft each other; but, when 
the fmall magnet has been brought fo near the powerful 
one as that its pole may begin to be changed, then nei¬ 
ther an attraction nor a repulfion will take place; and, 
when the two magnets are approached nearer than that 
limit, then, the pole of the weak one being changed, an 
attraction will enfue. 
After thefe obfervations, the ingenious reader may ea¬ 
fily imagine that the decreafe of repulfion between homo-, 
geneous magnetic poles muft be at leaft as much, if nos 
more, irregular than the decreafe of the attraction at dif¬ 
ferent diftances. It is likevvife evident, that many objeCts 
mult be had in view, in attempting to inveftigate the law 
of that decreafe. 
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