124 
MAGNETISM. 
the bar was fir ft made very hot, even though allowed to 
acquire its molt magnetical ftate before being quenched, 
than if it had been heated only to this latter degree. Nay, 
he always found it ftronger if quenched while red hot. 
He alfo found that, when he heated a fmall (feel bar red 
hot, and quenched it while lying between two magnets, 
it acquired a much ftronger magnetic power than it would 
acquire in any other way. 
There are various methods of communicating a per¬ 
manent magnetifm to ferruginous bodies, by means of a 
bar rendered magnetic, by pofition, of, which the moft 
fimple is that defcribed Mr. Marcel, nephew to Mr. Leeu¬ 
wenhoek, whofe experiments were made in the year 1726. 
Being employed in making fome obfervations on the mag¬ 
netic power which he found in great pieces of iron, he 
took a large vice weighing 90 pounds, in which he fixed 
a large anvil weighing ialbs. The fteel to which he wifhed 
to give the magnetic power was laid upon the anvil in a 
north and fouth polition, which happened to be the dia¬ 
gonal of the fquare furface of the latter. He then took a 
four-cornered piece of iron an inch thick every way, 33 
inches long, weighing about 81 bs. having one end rounded 
and brightly polifhed, the other being tapered. Holding 
then the fteel faft upon the anvil with the one hand, he 
took the iron bar in the other, and, holding it perpendi¬ 
cularly, he rubbed the fteel hard, with the rounded part 
towards him, from north to fouth, always carrying the bar 
far enough round about to begin at the north. Having 
thus given 10 or 12 ftrokes, the fteel was turned upfide 
down, and rubbed as much on the other fide. Proceeding 
in this manner till it had been rubbed 400 times, the fteel 
was as ftrongly magnetic as if it had been touched by a 
powerful loadftone. The place where he began to rub 
was always the north pole. In thel'e experiments it fome- 
times happened that the virtue was imparted by a few 
ftrokes ; nay, by a lingle ftroke a fmall needle was made 
to receiver very coniiderable power. Thus he imparted 
to two compafs-needles Inch a degree of magnetic power, 
that one lifted three-fourths, and another a whole ounce 
of iron ; and, although thefe needles were anointed with 
linfeed oil to keep them from rulling, and a hard coat was 
thus formed upon them, they neverthelefs retained their 
power.' Thus alfo a knife was made fo ftrongly magneti¬ 
cal, that it would take up an ounce and three-fourths of 
iron. Four fmall pieces of fteel, each an inch long, and 
one twelfth of an inch broad, as thin as the fpring of a 
watch, were thus impregnated with the magnetic power, 
and then joined into a fmall artificial magnet; which at 
its.fi[ft formation took up eight times its own weight of 
iron; and, after being fix years kept in the moft careiefs 
manner, was found to have gained rather than loft any 
thing of its power. In the courfe of his experiments, 
Mr. Marcel found, that the end at which he began to rub 
was always the north pole, whatever polition the fteel was 
laid in. On rubbing a piece of fteel from one end to the 
middle, and then from the other end to the middle, it ac¬ 
quired two north poles, one at each end, the middle being 
a fouth pole. Beginning to rub from the middle towards 
each end, lie found a north pole in the middle, and a fouth 
pole at each extremity. 
6. Natural magnets may be imitated in the following 
manner : Take fome martial sethiops, or, which is more 
eafily procured, reduce into very fine powder the fcales of 
iron which fall from red-hot iron when hammered, and 
which are found abundantly in fmiths’ fliops. Mix this 
powder with drying linfeed oil, fo as to form it.into a 
very ftiff pafte, and thape it in a mould fo as to give it any 
form you require, whether of a terrella, a human head, or 
any other. This done, place it in a warm place for fome 
weeks, and it will dry fo as to become very hard. Then 
render it magnetic, by the proper application of powerful 
magnets, and it will acquire a coniiderable power. 
The late Dr. Gowin Knight proftefled a furprifing fkill 
in magnetifm, being able to communicate an extraordi¬ 
nary degree of attractive or repullive power, and to alter 
or reverie the poles at pleafure; but, as he refufed to dif- 
cover his methods upon any terms whatever (even, as he 
faid, though he fhould receive in return as many guineas 
as he could carry), thefe curious and valuable fecrets have 
died with him. In the 69th volume of the Philofophical 
Tranfaftions, however, Mr. Benjamin Wilfon hasgiven a 
procefs, which at lead diicovers one of the leading prin¬ 
ciples of Dr. Knight’s art, and may perhaps be a means 
of difcovering the whole to thofe who fhall be lefs re- 
ferved. The doftor’s procefs, according to Mr. Wilfon, 
was as follows : Having provided himfelf with a great 
quantity of clean iron-filings, he put them into a lar^e 
tub, that was more than one-third filled with clean water ■ 
he then, with great labour, worked the tub to and fro for 
many hours together, that the friftion between the grains 
of iron by this treatment might break off fuch fmaller 
parts as would remain fufpended in the water for a time. 
The obtaining of thefe very fmall particles in fufficient 
quantity feemed to him to be one of the principal defi- 
derata in the experiment. The water being by this treat¬ 
ment rendered very muddy, he poured the fame into a 
clean iron veffel, leaving the filings behind; and, when 
the water had (food long enough to be clear, he poured 
it out carefully, without difturbing fuch of the fediment 
as (till remained ; which now appeared reduced almoft to 
an impalpable powder. This powder was afterwards re¬ 
moved into another veffel in order to dry it; but, as he 
had not obtained a proper quantity thereof by this one 
ftep, he was obliged to repeat the procefs many times. 
Having at laft procured enough of this very fine powder, 
the next thing was to make a pafte of it, and that with 
fome vehicle which would contain a confiderable quan¬ 
tity of the inflammable matter ; for this purpofe he had 
recourfe to linfeed oil in preference to all other fluids. 
With thefe two ingredients only he made a ftiff pafte, 
and took particular care to knead it well before he mould¬ 
ed it into convenient (hapes. Sometimes, while the pafte 
continued in its foft ftate, he would put the impreftion of 
a feal upon the feveral pieces; one of which is in the 
Britiih Mufeum. This pafte was then put upon wood, 
and fometimes on tiles, in order to bake or dry it before 
a moderate fire, at about the diftance of afoot. He found 
that a moderate fire was moft proper, becanfe a greater de¬ 
gree of heat made the compolition frequently crack in many 
places. The time required for the baking or drying of 
this pafte was generally about five or fix hours, before it 
attained a lutficient degree of hardnefs. When that was 
done, and the feveral baked pieces were become cold, he 
gave them their magnetic power in any direction he 
pleafed, by placing them between the extreme ends of his 
large magazine of artificial magnets for a few feconds or 
more as he taw occafion. By this method the power 
they acquired was fuch, that when one of thele pieces 
were held between two of his belt ten-guinea bars, with 
its poles purpofely inverted, it immediately of itielf 
turned about to recover its natural direction, which the 
force of thofe very powerful bars was not fufficient to 
counteract. 
Circumjlances which tend to impair or dcjlroy the Magnetic Power . 
The magnetic power in all its modifications, whether 
of attraction, repulfion, or polarity, is in general tem¬ 
porary and perifhing. The belt magnets, whether na¬ 
tural or artificial, unlefs carefully preferved, with atten¬ 
tion to certain circumftances that will prefently appear, 
are obferved to have their magnetic power diminifhed. 
Natural magnets, and artificial magnets made of fteel 
tempered as hard as poflible, retain their power moft ob- 
ftinately, and feldom entirely lole it except under cir¬ 
cumftances which we know to be unfavourable to its du¬ 
rability. Magnets of fteel of a fpring temper, are much 
fooner weakened, lofe more of their force merely by keep- 
jpg, and finally retain little or none of it. . Soft fteel and 
iron 
