and a Method of difcovering the Longitude: 385 
gitude ought to be counted; I have always ufed it as fuch, 
and obtained the moft fatisfactory refults. 
Proxpiem the Second, being the Pracriceé. 
To determine the Longitude univerfally, 
Here are always given the fhip’s polar diflance, the mag- 
netic polar diftance, and the magnetic co-latitude, being 
_ three fides 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 eaft or nak 
(as the cafe may be) from the meridian aforefaid. 
N.B. The magnetic co-latitude is the diflance of the fhip 
from the neareft magnetic pole; the fhip’s polar diftance is 
the diftance of the hip from the pole of the earth next ad- 
joining the faid magnetic pole ; ; the polar diftance is the dif- 
tance between either magnetic pole and the pole of the earth 
neareft tHereto: moreover, the longitude thus obtained can 
never exceed 180. It may, braves, be eafily reduced as to 
that eftimated from any affumed firft meridian. Likewife the 
point M, thus affumed 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 ftands at 
right angles to the plane of the horizon, without regarding 
whether the magnetic poles themfelves are under, at, or aboye 
the furface of the earth. 
The method of obtaining and eftablifhing (from obfervation 
alone) certain data for inveftigating or corroborating perfect 
tables of magnetic latitude and co-latitude, appears (from the 
preceding theory) obvious, being, by carefully making different 
dips of the magnetic needle along the meridian aforefaid, and 
carefully obferving the refpeétive latitudes at which thofe dips 
are taken, and for greater certainty (if thought needful) con- 
tinuing tliem, as far as poffible, into both hemi/pheres; though 
it will alfo be perceived they may be inveftigated by obferying 
different dips, along any other great circle, pafling through 
the magnetic poles, or having determined the precife latitude 
and longitude of the magnetic poles. Correét tables may alfo 
be deduced from correét dips at various places; however, the 
VoL. VY. 2D firkt 
