lit 
MAGNE'TIS M. 
%o. The attractive power ofloadftones, in their fimilar 
petition to, but different diftances from, magnetic needles, 
is in the fufquiduplicate proportion of the difiances of 
their fnrfaces from their needles reciprocally ; or as the 
mean proportionals between the fquares and the cubes of 
thofe diftances reciprocally ; or as the fquare-roots of the 
fifth powers of thofe distances reciprocally. Thus the 
magnetic power of attraction, at twice the diftance from 
the furface of the loadltone, is between a fifth and fixth 
part of that power at the firlt diftance ; at thrice the dif¬ 
tance, the power is between the fifteenth and fixteenth 
part 5 at four times the diftance, the power is thirty-two 
times as fmall; and at fix times the diltance, eighty-eight 
times as fmall. Where it is to be noted, that the dif- 
tances are not taken, as in the laws of gravity, from the 
centre ; but from the furface; all experience affuring us, 
that the magnetic power refides chiefly, if not wholly, in 
the furfaces of the loadflones and iron ; without any par¬ 
ticular relation to any centre at all. The proportion here 
laid down was determined by Mr. Whiflon, from a great 
number of experiments of Mr. Haufkbee, Dr. Brook Tay¬ 
lor, and himfelf. Sir Ifaac Newton fuppoles magnetic at¬ 
traction to decreafe nearly in the triplicate ratio of the 
diftance: Mr. Martin obl'erved, that the power of his 
loadflone decreafed in the fefquiduplicate ratio of the dif- 
tances inverfely. Dr. Helfliam found it to be as the 
fquares of the diftances inverfely, which ratio agrees with 
that of the ingenious Mr. Michell. Others, as Dr. Brook 
Taylor and Mr. Mufchenbroek, are of opinion, that this 
power follows no certain ratio at all; but that it is much 
quicker at greater diftances than at fmall ones, and that 
it is different in different ftones. 
n. An inclinatory or dipping needle, of fix inches ra¬ 
dius, and of a prifmatic or cylindric figure, when it of- 
cillates along the magnetic meridian, performs there every 
mean vibration in about 6 " or ^bo " 1 and every fmall of- 
cillation in about 55", or 330" ; and the fame kind of nee¬ 
dle, four feet long, makes everyanean olcillation in about 
24", and every fmall one in about 22". 
12. The entire power of magnetifm in this country, as 
it affefts needles a foot long, is to that of gravity nearly 
as 1 to 300 ; and, as it affeCts needles 4 feet long, as 1 to 
600. 
13. The quantity of magnetic power accelerating the 
fame dipping-needle, as it ofcillates in different vertical 
planes, is ever as the co-fines of the angles made by thofe 
planes, and the magnetic meridian, taken on the horizon. 
Thus, if we would eftimate the quantity of forces in the 
horizontal and vertical fituations of needles at London, 
we fliall find that the latter, in needles a foot long, is to 
the entire force along the magnetic meridian, as 96 to 
loo ; and in needles'four feet long, as 9667 to 10000 ; 
whereas in the former, the entire force in needles a foot 
long, is as 28 to 100; and in thofe four feet long, as 2569 
to 10000. Whence it follows, that the power by which ho¬ 
rizontal needles are governed in tliefe parts of the world 
is but one quarter of the power by which the dipping- 
needle is moved. Hence, alfo, lince the horizontal needle 
is moved only by a part of the power which moves the 
dipping-needle 3 and that it only points to a certain place 
in the horizon, becaufe that place is the neareft its origi¬ 
nal tendency of any its fituation will allow it to tend to; 
whenever the dipping-needle ftands exaftly perpendicular 
to the horizon, the horizontal needle will not refpeft one 
point of the compafs more than another, but will wheel 
about every way uncertainly. 
14.. The time of ofciliation and vibration, both in dip¬ 
ping and horizontal needles equally good, is as their length 
direftly ; and the aft ual velocity of their points along their 
arc*, is always equal. Hence magnetic needles ar e,cateris 
paribus , ftill better, the longer they are; and that in the 
fame proportion with their length. 
15. The earth on which we live is fuppofed to in¬ 
clude within it a vaft fpherical magnet, concentrical there¬ 
to, having its own poles, meridians, equator, and pa¬ 
rallels ; and all much of the fame general nature of thofa 
with fmall terrelLz, or fpherical loadftones, in the poffef- 
fion of the curious among us. 
16. The power of a good terrdla, or fpherical loadftone, 
as it affefls a needle a foot long, is equal to the magnetic 
power of that internal loadltone, about two and a half, or 
three, diameters of fuch loadltone. From which confide- 
ration the quantity of magnetic attraction at all diftances 
from the internal loadltone for needles a foot long, may 
be determined ; and from the fame consideration it appears, 
that the diameter of this internal loadltone is about 1150 
miles. To which we add, that, in regard fir Ifaac New ton 
has demonftrated, that the power of gravity diminilhes 
within the earth, and is lei's there than at its furface, nearly- 
in the proportion of its greater nearnels to the centre, the 
magnetic power, at 2900 miles diftance from us, and nearly 
1060 from the earth’s centre, which is of the power of 
gravity here, will be fomewhat greater titan the power of 
gravity there 5 which limit is worthy our attention, gra¬ 
vity being llronger than magnetifm on the one tide of it, 
and weaker on the other; we mean as it aft'efts needles 
of one foot diameter. At that limit, therefore, at leaft 
near the magnetic poles, iron a foot long ought to be twice 
as heavy, and fall twice as faff, as any other natural body, 
viz. by the union of thofe two equal powers,gravity anti 
magnetifm; and of confequence, above that limit, fuch 
an iron ought to be lets than twice as heavy, below it more 
than twice as heavy as any other natural body. 
17. The earth’s internal loadltone is not fixed to our up¬ 
per parts, but is moveable with refpeft to them, and actu¬ 
ally revolves on the earth’s axis, from eaft to weft, in a cer¬ 
tain long period of time ; as appears from the conftant va¬ 
riation of the horizontal needle, as well as the regular in- 
creafe of inclination of the dipping-needle. The only 
way to render this motion, i. e. the variation, pollible and 
intelligible (to ufe Dr. Halley’s words), is to iuppofe it to 
turn about the centre of a globe, having its centre of gra¬ 
vity fixed and immoveable in the fame common centre of 
the earth. This moveable internal furface muff likewife be 
loofe,and detached from theexternal part of theglobe, which 
may be reckoned the thell, and the other the nucleus or in¬ 
ner globe, included within it, with a fluid medium between. 
Now, from the variation’s moving weft ward, it is plain, that 
the forefaid nucleus has not preciiely attained the fame de¬ 
gree of velocity with the exterior parts in their diurnal 
revolution ; but fo nearly equals it, that in three hundred 
and fixty-five revolutions the difference is lcarcely fentible; 
and muff probably have arifen from hence, that the im- 
pulle, whereby the diurnal motion was impreffed on the 
earth, was given to the external parts, «id thence com¬ 
municated to the internal. 
18. The internal magnet has one central pole north¬ 
ward, in the nature ot the poles of our common load¬ 
ftones ; but its fouthern pole appears not to be central, 
but rather circular, and that at a great diltance from the 
fouthern pole of the earth, 
19. The northern magnetic pole is now fituate, fays 
Mr. YVhifton, about the latitude of 764; i. e. 135° from 
the north pole of the earth, and about 30 0 eaftward from 
the meridian of London. 
20. The fouthern magnetic circular pole has its centre, 
or centre pole, nearly in the parallel ot 60 degrees ; and, 
in a meridian pafling along the ealt coaft of Borneo, abcvut 
117 degrees eaftward of London. Its radius is alfo an arc 
of a great circle of about 44 degrees. 
21. The refpeftive motion of the internal magnet, or 
the velocity, v. g. of its north pole, appears to be 27 de¬ 
grees in 144 years, i. e. upwards of one degree in five 
years; lo that it makes an entire revolution in 1920 years. 
22. The variation of magnetic needles from the azimuth 
of the meridians of the internal magnet is derived from 
the difference of the ftrength of the leveral parts of the 
internal magnet’s furface; which as it is only to be 
known by experience, that variation cannot be determined 
beforehand, unlefs where there are good accounts hovj 
3 much,. 
