SoO 
DIPPING-NEEDLE. 
pends on the pofition of the lines of equal dip, in thefe 
parts of tlie world, which it is found do lie about four¬ 
teen or fifteen degrees from the parallels. Hence he 
argues, that as we can have needles of five, fix, feven, 
eight, or more, feet long, which will move with ftrength 
i'ufficient for cxadt obfervation ; and fince microfcopes 
may be applied for viewing the fmalleft divilions of de¬ 
grees on the limb of the inftrument, it is evident that the 
longitude at land may thus be found to lefs than four 
miles. And as there have been many obfervations made 
at lea with the fame inftrument by Noel, Feuille, See. 
which have determined the dip ufually within a degree, 
fometimes within one half or one third of a degree, and 
this with fmall needles, of five or fix, or at the molt nine, 
inches long; it is inferred, that the longitude maybe 
found, even at lea, to lefs than half a quarter of a degree. 
7 'ofind the' Longitude or Latitude by the Dipping-Needle .—• 
If the line's of equal dip, below the horizon, be drawn 
on maps, or fea-charts, from good obfervations, it will 
be eafy, from the longitude known, to find the latitude; 
and from the latitude known, to find the longitude either 
at fea or land. Suppofe, for example, a perfon travel¬ 
ling or failing along the meridian of London, fiiould find 
that the angle of dip, with a needle of one foot, was 75 0 ; 
the chart will flievv that this meridian, and the line of 
dip, meet in the latitude of 53 0 n' ; which is therefore 
the latitude fought. Or, if lie be travelling or failing- 
along the parallel of London, i. c. in 51 0 31' north lati¬ 
tude, and find the angle of dip 74 0 ; then this parallel, 
and the line of this dip, will meet on the map on i° 46' of 
eafl longitude from London ; which therefore is the longi¬ 
tude fought. See the article Compass, vol. iv. p.892. 
Since the above-mentioned experiments, an improve¬ 
ment has been made in the dipping-needle compafs, by 
P. R. Nugent, efq.’ This we fhall explain in his own 
words: “ My purfuits in life having been conliderably 
connected with objects of this kind, and my own expe¬ 
rience having convinced me of the errors and miftakes 
arifing from part theory with the dipping-needle, led me 
into endeavours to invelligate fome theory more con¬ 
formable to the laws of nature, and which would recon¬ 
cile the continual difagreement between men of fcience 
on this fubjeCt: and this defire alfo led me into endea¬ 
vours to form nautical inffruments more appropriate to 
this theory. I account for the change of the magnetic 
variation by original magnetic meridians and a magnetic 
annulus. Upon this principle the following problems are 
founded, viz. 
Method of di/covering the Longitude, the Magnetic Inclination 
and Latitude of the Ship being given. 
Problem I. being the Theory. — To determine the La¬ 
titude and Longitude of the Magnetic Poles, and, in confcquence, 
for the purpofe of difeovering the Longitude, to afeertain a Firjl 
Meridian. —Let N, in the annexed diagram, reprefent the 
north pole of the earth, M the magnetic north pole, S 
Smeerenberg harbour in Spitlbergen, and L London : 
alfo, let arches of great circles be drawn from London 
to the north magnetic pole, to the north pole of the 
earth, and to Smeerenburg, and alfo from Smeerenburg 
to the north magnetic pole. There will thus become 
formed three fpherical triangles, which are refolved as 
follows: id, In the triangle N LS are given the fides N L 
. and N S, being the co-latitudes of London and Smeeren¬ 
berg, and the included angle LN S being their difference 
of longitude, to determine the angle of pofition NLS 
and the third fide LS. 2d, In the triangle LSM are 
given the three fides LS (juft found) LM and SM 
(LM and SM being the magnetic polar diftances corre- 
fpondiiig to thecorreCf dips at London and Smeerenberg), 
to determine the angle MLS, from which fubtraCling 
the angle NLS lately found, there remains the angle of 
pofition NLM. Laftly, In the triangle M LN are now 
given the fides LM and LN, and the included angle 
r 
MLN, to find the angle LNM, the longitude of the 
north magnetic pole (and the meridian of the earth pafliug 
the magnetic pole from the pole of the earth. 
“ The place of the magnetic poles being thus obtain¬ 
ed, the queftion next divides itfelf into two points of 
view, namely, Whether the magnetic poles or points 
thus determined are ftationary or otherwife i the folutioa 
of which depends on obfervation and philofophic infer¬ 
ence. From obfervation it is remarked, in page 121 of 
the late Lord Mulgrave’s Account of his Voyage towards 
the North Pole, that * There is no reafon at prefent t@ 
fuppofe that the dip is liable to any variation in the fame 
place at different periods of time. It having been ob- 
ferved at London by Norman, who firft difeovered it in 
1592, to be 71 0 50', and by Mr. Naime, in 1772, about 
72 0 : the difference between thefe obfervations, taken at 
fuch diffant periods of time, is fmaller than that found 
between feveral of Mr. Nairne’s obfervations compared 
with each other; and therefore we have no reafon to 
conclude that the dip has altered fince Norman’s time. 
The care with which his inftrument was conftru&ed, and 
his obfervations made, leave 11s no room to doubt of their 
accuracy.’ Again, Mr. Cavallo, in his Treatife on Mag- 
netifm, p.65, remarks, (fpeaking of the dip,) ‘ Its alte¬ 
ration in the fame place at different times is very fmall ; 
thus in London, about the year 1576, the north pole of 
the dipping-needle flood 71 -50 below the horizon, and 
in the year 1775 it flood at 72-03: the alteration of the 
inclination in fo many years amounting to lefs than a 
quarter of a degree, which may be attributed to the error 
of the inffruments, fince, as will appear in the fequel, 
thofe inffruments are far from having attained to any de¬ 
gree of perfection even in the prefent age.’ Now, from 
philofophic inference it will naturally follow, that if rhe 
inclination of the magnetic needle, at different periods of 
time, at any one place, remain conftant, the magnetic poles 
are 
<5, 
