D IPPI N G 
are ftationary ; and if the magnetic poles are (fationary, 
(as appears clearly evident front above,) the meridian 
thus difcovered is the Firjl (or only) Meridian from which 
the longitude ought to be counted ; I have always ufed 
it as fuch, and obtained the mod fatisfaffory refults.” 
Problem II. being the Practice. —To determine the 
Longitude* univer{ally .—Here are always given the fltip’s 
polar diftance, the magnetic polar diftance, and the mag¬ 
netic co-latitude, being three (ides of a fpherical triangle, 
to determine the angle at the pole of the earth, oppofite 
the magnetic co-latitude, being at all times and places 
the longitude of the (hip ea(f or weft (as the cafe may be) 
from the meridian aforefaid. The magnetic co-latitude 
means the diftance of the (hip from the neareft magnetic 
pole; the (hip’s polar diftance is the diftance of the (hip 
from the pole of the earth next adjoining the faid mag¬ 
netic pole ; the polar diftance is the diftance between 
either magnetic pole and the pole of the earth neareft 
thereto : moreover, the longitude thus obtained can never 
exceed 180. It may, however, be eafily reduced as to 
that eftimated from any aflumed firft meridian. Likewife 
the point M, thus a (Fumed as the magnetic pole, may, 
to prevent perplexity, be better underftood as being that 
point on the furface of the globe at which the dipping- 
needle (lands at right angles to the plane of the horizon, 
without regarding whether the magnetic poles themfelves 
are under, at, or above, the furface of the earth. 
“ The method of obtaining and eftablifhing (from ob- 
fervation alone) certain data for inveftigating or corro¬ 
borating perfedt tables of magnetic latitude and co-la. 
titude, appears (from the preceding theory) obvious, 
being, by carefully making different dips of the mag¬ 
netic needle along the meridian aforefaid, and carefully 
obferving the refpedtive latitudes at which thofe dips 
are taken, and for greater certainty (if thought needful) 
continuing them, as far as pofiible, into both herni- 
fpheres ; though it will alfo be perceived they may be 
inveftigated by obferving different dips, along any other 
great circle, palling through the magnetic poles, or hav¬ 
ing determined the precife latitude and longitude of the 
magnetic poles. Correct tables may alfo be deduced 
from correct dips at various places; however, the firft 
method is not only the mod eafy, but on many accounts 
the mod preferable ; and for this purpofe the corredt dips 
of the magnetic needle, taken at the following places, 
(regard alfo being had to determine their corredt latitudes 
and°longitudes, particularly the former, except near the; 
magnetic poles, where it is neceflary to corredtly deter¬ 
mine both,) will be found fufficient, namely at the Falk¬ 
land I (lands, Buenos Ayres, Oroonoque, Trinidad, Bnr- 
badoes. Saint Luke, Martinique, Guadaloupe, Antigua, 
Saint Martin’s, Bermudas, the Atlantic Ocean, in or near 
the fame longitude up to and near the Ifle of Sables, at 
the ifland of Cape Breton, the fouth (ide of the river 
Saint Lawrence in the longitude aforefaid ; then going 
round to the north-eaft (hore of the Labrador Coaft, and 
obferving thereon at Davis’s Inlet, and fo on up to the 
northward under the meridian aforefaid, up to and into 
Baffin’s Bay, and there carefully determining the latitude 
and longitude of the north magnetic pole ; or firft going 
into Baffin’s Bay, and afterwards proceeding to the l'outh- 
ward. Next, by carefully obferving the magnetic incli¬ 
nation along the oppofite meridian in Ruflia, China, the 
Chinefe Seas, Indian Ocean, &c. as at the mouth of the 
I.eno, at Olikfminftcoi, at Nutfchink or Albazin, at Pekin, 
Nankin, Nimpo, Formofa, the Philippine Kies, Gindano, 
Celebes, Flores, the north and fouth (ide of New Holland, 
and fo continuing to obferve along the Indian Ocean, in 
or near the meridian aforefaid, up towards the fouth pole 
of the earth, until the place of the fouth magnetic pole 
be carefully determined, i. e. whatever pole may be firft 
convenient to determine. The number of places herein 
fet forth are mentioned in order that- navigators and 
others, as opportunity may offer, may know where and 
-NEEDLE. 851 
how to make the advantageous obfervations herein al¬ 
luded to. 
“ It may alfo be proper to add, that hitherto the places 
of the magnetic poles have always been attempted to be 
found by means of tile magnetic variation; a method 
whereby the moll able mathematicians have bepn conti¬ 
nually led aftray ; for, as the magnetic variation is con¬ 
tinually changing, their conclufions were rendered a# 
uncertain as the variation itfelf, of which (it had fo hap¬ 
pened from the method they purfued) they knew little 
more than that it did change. Had tire prefent theory 
occurred to them, it would have been foon and effedtu- 
ally eftabliftied. It may be farther proper to remark, 
that the obfervations of the magnetic inclination herein 
(fated, and made at London, do of themfelves alone efta- 
blifh another important conclufion, likewife quite diffe¬ 
rent from the general opinion of the learned, which fup- 
pofes that the magnetic variation aftedts (z. c. increafes 
or diminifties) the magnetic inclination; for, from the 
time in which the magnetic inclination was firft obferved 
at London to the prefent time, the variation has ftiifted 
upwards of 36° without affedting the inclination ; and 
therefore, after knowing .the refult of fo very great a 
change in the magnetic variation without having aff'edfed 
the inclination, there can be no reafon to conclude that 
it can in any refpedt be aif'edfed thereby : a circumftance 
that renders the theory here pointed out (fill more valu¬ 
able. 
“ Thus, by new' and interefting application and tefti- 
mony of unerring fcience, pradfical obfervation, and fair 
conclufion, I have, I flatter myfelf, cleared the magnetic 
theory of the abftrufe and einbarrafling confiderations 
which have hitherto bewildered it, and thereby laid the 
foundation of a mod (imple, general, and ufeful, method 
of determining a (flip’s corredt place at fea, without any 
regard to the fun, moon, or (tars, to good or bad wea¬ 
ther, to the time of the day, the day itfelf, months, years, 
feafons, or centuries, except fo far as may be faid to re¬ 
gard the magnetic inclination, which mud always be 
known from obfervation; as alfo the latitude of the (hip, 
which mult likewife be known, either from obfervation, 
or dead reckoning. The great advantage of my method 
of difeovering the longitude by the dip is, that it is eafily 
underftood by the molt ordinary capacity ; that to others, 
in every inftance, it becomes an auxiliary or corrobora¬ 
ting proof, and, in the time of greateft neceflity and peril, 
a mod eftimable fubftitute; not requiting, in this cafe, 
any calculation whatever. Thus, from the preceding 
theory, the dip in all places remaining the fame, the 
navigator, being perhaps feveral days without feeing 
fun, moon, or (tars, fo as to make any life of either to¬ 
wards determining his longitude, has this benefit left 
him, that he can, at any moment of the day, in which 
the extremity of the horizon can be feen, determine his 
correct dip, and therefore his vicinity towards the land 
he wifhes to make or avoid; where othervvife he might 
either be caff away, or, under the fuppofition that he 
was near thereto, keep either lying-to, or draining his 
(hip in endeavouring to get no farther to leeward, when 
at the fame time he might be an hundred miles or more 
from (hore, and fo lofe the very wind he wanted ; after 
which, (fiould contrary winds a(fail him, the injury alfo 
becomes obvious. 
“ The principles of the whole of my nautical magnetic 
inftruments (except a variation frame for converting all 
(leering into variation compalfes) is the application of a 
quadrant, fextant, or double fextant, fitted for this pur¬ 
pofe, to the vertical magnetic axis of the horizontal 
needle for determining the variation, and to the lateral 
magnetic inclinatory axis of the dipping-needle, for de¬ 
termining the dip. Thefe needles are alfo differently 
fufpended from others : the dipping-needle, for example, 
is always at liberty to refort to, and remain at reft in, 
the common interfeff ion of the lines of tlve magnetic me¬ 
ridian 
