]25 
MAGNETISM. 
jroti feldom retain magnetic power when removed from 
the magnet where they required it, unlefs their metallic 
(late undergoes fonie change. 
The following circumftances have been obferved to be 
moft powerful fn dimimthing or deftroying the power of 
magnets. 
1. Nothing has fo much effe& in impairing the power 
of a magnet as keeping it in an improper pofition, that is, 
too far front the magnetic line. If the axis of the mag¬ 
net be placed in a dire&ion that is at right angles with 
the magnetic meridian, that is, in this latitude nearly eaft- 
north-eaft and weft-north-weft, it will fooneft lofe its mag¬ 
netic power ; and, if it be placed in the magnetic line, but 
in a contrary pofition, or with the north pole where the 
fouth pole fliould be, if permitted to vibrate freely, it will 
gradually become weaker every day ; and, unlels it be a 
natural magnet, or an artificial one made of very hard 
tempered Iteel, it will, in no very long time, entirely lofe 
its magnetic power. 
2. The diflipation of magnetic power is greatly pro¬ 
moted by heating the magnet. The heat of boiling water 
has a fenfible effeft in this way ; but, if the magnet be 
expofed to a red heat, its power is entirely deftroyed, as 
has been long known. Dr. Gilbert obferved that the 
power of magnets w'as deftroyed by a heat that was not 
fufficient to make the metal vifible in the dark ; and Mr. 
Canton found that the heat of boiling water weakened 
the power of a magnet, but that the greateft part of this 
was recovered as the magnet cooled. If the heat be ap¬ 
plied when the magnet lies in a pofition moft favourable 
to the diftipation of magnetifm, the power is fooneft de¬ 
ftroyed ; hence, the belt way to deprive iron or Iteel of 
accidental magnetifm is, to heat it red-hot, and allow it 
to cool while lying in a direction perpendicular to the 
magnetic line. M. Coulomb has afcertained that at aoo 
degrees of heat, two fifths of the magnetifm of a magnet 
is diftipated, and that at 500 degrees the whole is loft. 
3. It is very extraordinary that the power of a magnet 
is impaired by rough vjage. Dr. Gilbert obferved that a 
magnet which he had powerfully impregnated was greatly 
weakened by a fingle fall on the floor; and fince his time 
it has been obferved, that when a magnet falls on a ltone, 
or receives any concuffion that makes it ring, it is injured 
much more than by being beaten with any thing foft and 
yielding. When a natural magnet is ground with coarle 
powders, in order to bring it to any required form, it is 
confiderably weakened. This (hows the propriety of al¬ 
tering the natural form of loadltones as little as poflible 5 
and, where this is necefiary, of doing it as expeditioufty 
as may be, by cutting them brifkly in the thin difks of a 
lapidary’s wheel. 
4.. Magnets placed near each other, with their Jimilar 
poles oppojite, always weaken each other ; and, when a pow¬ 
erful magnet is placed near a weaker, with their fimilar 
poles oppofed, the polarity of the weaker is frequently 
reverfed, that is, if the pole were north it becomes fouth, 
and vice verjd. When the weaker magnet is a natural 
load (tone, or has been made of hard tempered fteel, its 
original polarity is reftored when the improper pofition 
is changed ; but, if it has been made of (pring-tempered 
fteel, the alteration is generally permanent, and often as 
complete as while the magnets were in the neighbour¬ 
hood of each other. 
MAGNETICAL EXPERIMENTS. 
The properties of the magnet have been employed to 
perform feveral tricks, which excited a ccnfiderable degree 
of aftoniffiment in thofe who firft beheld them. No means 
indeed more fecret, and at the fame time more proper forac- 
tion, could be employed than magnetifm; fince its influence 
is (topped by no body with which we are acquainted, except 
iron. This idea was firft conceived by the celebrated Co- 
mus, who varied in a fingular manner the different tricks 
performed by this agent, fo that alt Paris flocked with the 
utmoft eagernefs to the places where they were exhibited. 
VOL. XIV. No. 961. 
He was admired by the ignorant, who confidered him as a 
force re r ; while the learned endeavoured to difcover the ar¬ 
tifice, which however was a profound fecret fo long as no 
one fufpefled magnetifm to be the principal caufe of it. 
1. The Magic Telefcope .—Thole who exhibit thefe tricks 
often employ a pretended magic telefcope, by means of 
which they can fee, it is faid, through opaque bodies. It 
is nothing more than an inftrument in the form of a tele¬ 
fcope, at the bottom of which, that is, towards the objeft- 
glafs, there is a magnetic needle, which affumes its proper 
direftion when the telefcope is placed upon the fide which 
that objeCt-glafs forms. To conftrucf this telefcope, 
provide a turned tube of ivory, wider towards the end 
where the objeft-glafs is placed ; but the ivory mud be of 
fufficient thinnefs to admit the light through to the infide. 
The narrow end is furnifhed with an eye-glafs, which 
ferves to (how more diftindlly the infide of it. The other 
end alfo is furnilhed with a glafs, which has the appear¬ 
ance only of an objeft-glafs, the pofterior furface of it be¬ 
ing opaque, fo as to ferve for the bafe or bottom of a fort 
of compafs or magnetic needle, w-hich turns on a pivot 
fixed in its centre. When the telefcope refts on the end 
containing the objeft-glafs, this needle aflumes a horizon¬ 
tal pofition, and points towards the north, or towards a 
magnetic needle in the neighbourhood. It is necefiary 
alfo to have a real telefcope, fimilar in appearance to the- 
other, in order that it may be ftiown inftead of it, which 
may be done by dexteroufly fubftituting the one for the 
other. When you wilh to employ the pretended magic 
telefcope, place it with the objeft-glafs downwards upon 
any thing you intend to examine ; and, if there be a mag¬ 
net or piece of magnetifed iron below it, the needle will 
turn to that fide. 
2. The communicative Piece of Money .—Take a crown or dol¬ 
lar, and drill a hole in the fide of it, in which place a piece 
of wire, or a large needle well poliffied, and ftrongly touched 
with a magnet. Then clofe the hole with a fmall piece of 
pewter, that it may not be perceived. Now, the needle in 
the magic telefcope before defcribed, when it is brought 
near to this piece of money, will fix itfelf in a dire&ioa 
correfponding to the wire or needle in that piece. De¬ 
fire any perfon to lend you a crown-piece or dollar, which 
you dexteroufly change for one that you have prepared as 
above. Then give the latter piece to another perfon, and 
leave him at liberty either to put it privately in a fnuff- 
box, or not; he is then to place the box on a table, and 
you are to tell him by means of your glafs, whether the 
crown is or is not in the box. Then, bringing your tele¬ 
fcope clofe to the box, you will know by the motion of 
the needle, whether it be there or not; for, as the needle 
in the telefcope will always keep to the north of itfelf, if 
you do not perceive it has any motion, you conclude the 
crown is not in the box. It may happen, however, that 
the wire in the crown maybe placed to the north, in which 
cafe you will be deceived. Therefore, to be fure of fuc- 
cefs, when you find the needle in the perfpeitive remain 
ftationary, you may on fonie pretence defire the perfon to 
move the box into another pofition, by which you will 
certainly know whether the crown-piece be there or not. 
You mull remember that the needle in the perfpeClive mud 
here be very fenfible, as the wire in the crown cannot pof- 
fibly have any great attractive force. 
3. Severalfigures being given which a perfon has arranged clofe 
to each other in a box, to tell through the lid or cover what number 
they form .—If you are defirous of employing the ten ci¬ 
phers, take ten fmall fquares, of an inch and a half on 
each fide, and on the upper face of each make a groove; 
but let thefe grooves be in different directions ; that is to 
fay, the firft intended for the number 1 mult proceed di- 
reftly from the top to the bottom; the fecond mud devi¬ 
ate to the right, fo as to form an angle equal to a tenth 
part of the circumference; the third an angle of two tenths ; 
and fo of the reft ; which will give ten different pofitions. 
Then introduce into thefe grooves fmall bars of fteel, well 
magnetifed, taking care to turn their north poles to the 
K. k proper 
