and a Method of difcovering the Longitude. 38t 
mean dip with the marked end north, exceeding the mean 
dip with the marked end fouth by 8° 8’), then are the mean | 
dips from the beginning to be diminifhed by a or 244 X% 
cofine of the dips. If, again, we fuppofe no error on receiv- 
ing it, nor from the above accident, the mean dips to this 
day will be leffened by at or 122 X cofine of the dip.” 
What error or difference arofe between the extreme of the 
obferyed dips with the marked end north, and thofe with the 
marked end fouth, are no where fet down; they muft, how- 
ever, undoubtedly have been greater, and in all likelihood 
inuch greater than 8° 8’, feeing the mean refult of the whole, 
fmall and great, gave 8° 8’, The obfervations made by Lord 
Mulgrave are of the like nature and uncertainty. 
The whole of thefe inftruments were therefore totally in- 
adequate to the correct and ufeful purpofes of navigation, or 
indeed to any corre€tly ufeful purpofe whatever: they were 
however, as before obferved, the very beft that could be ob- 
tained; and in the making of which no expence whatever 
on the part of the Board of Longitude, nor pains on the part 
of thofe who made them, were wanting to render them as 
perfec as poffible, and fych as fhould anfwer the purpofes 
for which they were defigned; nor do I entertain any doubt 
they were as much fo as the nature of their conftruction could 
poffibly admit of. : 
My purfuits in hfe having been confiderably conneéted 
with objects of this kind, my own experience, corrobo- 
rated by the above and like teftimonies, together with the 
continual neceffity, which arofe from time to time, of in- 
creafing the magnetic orbit fo as to correfpond to the increafe 
of the variation (on the fuppofition that the true dip and va- 
riation might be accounted for and prediéted,) by means of 
moveable magnetic poles *, led me into endeavours to invef- 
tigate 
* William Whifton, M. A. in the 8th page of an hiftorical preface to 
2 p»mphict of his, entitled, « The Longitude and Latitude found by the 
Iuciinatory or Dipping Needle,” gives us the brief hhiftory of*all'theories 
then publifhed, in chefe words, {peaking of the magnetic poles and theories: 
« However, this -notion in general of moveable magnetic poles has al- 
Ways 
