95 
end of A, and its Unmarked end on the mark- 
ed end of 15 ; then apply the north end of 
the magnet F, and the south end ot 1), to the 
middle of the bar C, the opposite ends being 
elevated as in the figure; draw E and I) 
asunder along the bar C, one towards A, the 
other towards B, preserving the same ele- 
vation ; remove F and D a toot or two from 
the bar when they are off the ends, then 
'bring the north and south poles ot these 
magnets together, and apply them again to 
-the middle of the bar C as before : repeat the 
same process live or six times, then turn the 
bar, and touch the opposite surface in the 
same, manner, and afterwards the two re- 
maining surfaces; bv this means the bar will 
acquire a strong fixed magnetism. 
2. Place the two bars which are to be 
touched parallel to each other ; and then unite 
•the ends by two pieces of soft iron, called 
supporters, in order to preserve, during the 
•operation, the circulation of the magnetic 
matter ; the bars are to be placed so that the 
marked end 1) (fig. 6), may.be opposite the 
unmarked end 15; then place the two -at- 
tracting poles G and l on the middle of one 
•of the bars -to be tonclied, raising the ends, so 
that the bars may form an obtuse angle ot 400 
or 150 degrees ; the ends G and L ot the 
bars are to be separated two or three tenths 
of an inch from each other. Keeping the 
bars in this position, move them slowly oyer 
the bar A 15, from one end to the other, going 
from end to end about fifteen times. Hav- 
ing dofie this, change the poles of the bars 
(/.<-. the marked end of one is always to be 
against the unmarked end of the other), 
and repeat the same operation on the bar 
C D, and then on the opposite faces of the 
bars. The touch thus communicated may 
-be further increased, by rubbing the different 
.faces of the bars with sets of magnetic bars, 
-disposed as in fig. 7. 
In these operations all the pieces should 
be well polished, the sides and ends made 
quite flat, and the angles quite square. 
A magnet bent so that the two ends almost 
meet, is called a horse-shoe magnet, fig. 3. 
To vender it magnetic, place a pair of mag- 
netic bars against the ends of the horse-shoe, 
with the south end ot the bar against that 
<of the horse-shoe which is intended to be 
the north, and the north end of the bar to 
that which is to be the south ; the contact, 
or lifter of soft iron, to be placed at the other 
end of the bars. Also rub the surfaces ot 
the horse-shoe with a pair of bars placed in 
the form of a compass, or with another horse- 
shoe magnet, turning the poles properly to 
the poles of the horse-shoe magnet ; being 
careful that these bars never touch the ends 
f,f the straight bars. If the bars are sepa- 
rated suddenly from the horse-shoe magnet, 
its force will' be considerably diminished ; 
.to prevent this, slip on the lifter, or support, 
to the end of the horse-shoe magnet, but in 
such a manner, however, that it may not 
touch the bars ; the bars may then be taken 
away, and the support slid to its place. 
Magnetism is best communicated to com- 
pass-needles by the two folio wffig methods: 
Procure a pair of magnetic bars, not less 
■than six inches in length. Fasten the needle 
clown on a board, and with a magnet in each 
hand draw them from the centre upon the 
.needle outwards; then raise the bars to a 
considerable distance from the needle, and 
MAGNETISM. 
bring them perpendicularly down upon the 
centre, and draw them over again. r l his 
operation repeated about twenty times will 
magnetize the needle, and its ends will point 
to the poles contrary to those that touched 
them. 
Over one end of a combined horse-shoe 
magnet, of at least two in number, and six- 
inches in length, draw from its centre that 
half of the needle which is to have the con- 
trary .pole to the end of the magnet : raise 
the needle to a considerable distance, and 
draw it over the magnet again ; this repeated 
about twenty times at least, and the same for 
the other half, will sufficiently communicate 
the power. 
A set of bars arc exceedingly useful fol 
magnetizing other liars, or needles of com- 
passes, &c. their power may also be increas- 
ed when lost or impaired by mismanagement, 
■&ic. A set of such bars, viz. six bars and 
the two iron conductors, may be preserved 
in a box ; taking cate to place the north 
pole -of one contiguous to the south pole of 
the next, and that contiguous to the north 
pole of the third, &c. as shewn in fig. 8. 
After what has been said above, we need 
not describe how a knife, or any piece of 
steel, ex c . may be rendered magnetic, or in 
what manner a weak magnet may be render- 
ed more powerful. But it may perhaps be 
necessary to say something concerning the 
communication of magnetism to crooked 
bars like A 15 C, fig. 9- 
Place the crooked bar flat upon a table, 
and to its extremities apply the magnetic bars 
D F, EG; joining their extremities EG, 
with the conductor or piece of soft iron F G ; 
then to its middle apply the magnetic bars 
placed at an angle : or you may use two bars 
only, placed as shewn in tig. 9, and stroke the 
crooked bar with them from end to end, fol- 
lowing the direction of that bent bar ; so that 
on one side of it the magnetic bars may stand 
in the direction of the clotted representation 
LK. In this manner, when the piece of 
steel A B C has been rubbed a sufficient num- 
ber of times on one side, it must be turned 
with the other side upwards, &e. 
In communicating magnetism, it is best to 
use weak magnets first, and those that are 
stronger afterwards ; but you must be very 
careful not to use weak after strong magnets. 
A magnet loses nothing of its own power 
by communicating to other substances, but 
is rather improved. 
Every kind of violent percussion weakens 
the power of a magnet. A strong magnet 
has been entirely deprived of its virtue, by 
receiving several smart strokes of a hammer; 
indeed, whatever deranges or disturbs the 
internal pores of a magnet will injure its mag- 
netic force. 
Fill a small dry glass tube with iron filings, 
press them in rather close, and then touch 
the tube as if it was a steel bar, and the tube 
wiU attract a light needle; shake the tube, so 
that the situation of the filings may be dis- 
turbed, and the magnetic virtue wifi vanish. 
Magnets should never he left with two 
north or two south poles together ; for when 
they are thus placed, they diminish and de- 
stroy each other’s power. Magnetic bars 
should therefore be always left with the op- 
posite poles laid against each other, or by 
connecting their opposite poles by a bar of 
iron. The power of a magnet is increased 
by lotting a piece of iron remain attacked t<* 
one or both of its poles, A single magnet 
should therefore be always thus left. 
The difference of steel in receiving mag- 
netism is very great, as is easily proved by 
touching in the same manlier, and with the 
same bars, two pieces of steel ot equal size, 
but of different kinds. With some sorts of 
steel, a few strokes are sufficient to impart 
to them all the power they are capable of 
receiving ; other sorts require a longer ope- 
ration ; sometimes it is impossible to give 
them more than a small degree of magnet- 
ism. 
A piece of spring-tempered steel will not 
retain as much magnetism as hard steel ; solt 
steel still less, and iron retains scarcely any. 
Iron when oxy dated loses its magnetism. 
The construction and the use ot the prin- 
cipal magnetical instruments, &c. — The mag- 
netical instruments may be reduced to three 
principal heads; viz. 1st. the magnets or 
magnetic bars, which are necessary to mag- 
netize needles of compasses, or such pieces 
of steel, iron, &c. as may be necessary for 
divers experiments; and which have al- 
ready been sufficiently explained in the pre- 
ceding pages: 2dly, the compasses, such as 
are used in navigation, and for other pur- 
poses, which are only magnetic needles 
justly suspended in boxes, and which, ac- 
cording to the purposes for which they are 
particularly employed, hare several appen- 
dages, or differ in size, and in accuracy of 
divisions, &c. whence they derive the dif- 
ferent names of pocket compasses, steering 
compasses, variation compasses, and azimuth 
compasses: and 3dly, the dipping-needle. 
Tlic magnetic needles which are com- 
monly used at sea, are between four and six 
inches' long ; but those which are used for 
observing the daily variation, are made a 
little longer, and their extremities point the 
variation upon an arch or circle properly di- 
vided and affixed to the box. 
The best shape of a magnetic needle is 
represented in iigs. 10 and 1 1 ; the first of 
which shews the upper side, and the second 
shews a lateral view of the. needle, which is 
of steel, having a pretty large hole in the 
middle, to which a conical piece of agate is 
adapted by means of a brass piece O, into 
which the agate-cap (as it is called) is fastened. 
Then the apex of this hollow cap rests upon 
the point of a pin F, which is fixed in the 
centre of the box, and upon which t he nee- 
dle, being properly balanced, turns very 
nimbly. For common purposes, those nee- 
dles have a conical perforation made in the 
steel itself, or in a piece of brass which is 
fastened in the middle of the needle. 
A mariner’s compass, or compass generally 
used on board of ships, is represented in fig. 
12. The box, which contains the card or 
Hy with the needle, is made of a circular 
form, and either of wood, or brass, or cop- 
per. It is suspended within a square wooden 
box, by means of two concentric circles, 
called gimbalds, so fixed by cross axes n, a , 
a, a, to the two boxes (see the plan, fig. 13), 
that the inner one, or compass-box, shall re- 
tain a horizontal position in all motions of 
the ship, whilst the outer or square box is. 
fixed with respect to the ship. The compass 
box is covered with a pane of glass, in order 
that the motion of the card may not be dis- 
turbed by the wind. Y\ hat is called thq 
