382  «=Theory-of the Situation of the Magnetic Poles, 
tigate fome theory more conformable to the laws of Nature*; 
and which would reconcile the continual difagreement be- 
tween men of fcience on this fubject: and this defire alfo of 
neceffity led me into endeavours to form more perfect nau- 
tical inftruments, fo without more accurate obfervations no 
effential utility whatever could arife to navigation from any 
kind of theory however perfect. To detail the train of reafon- 
. ‘ways feemed fo probable, or rather neceffary, ever fince the variation has 
been itfelf found to vary, and this after a certain regular manner; allo, 
that Mr. Phillips before Mr. Bond afferted the fame, and ftated the revo- 
Tution to be in 370 years; and after all, our learned Dr. Halley, who has 
far outdone every body upon this fubjeét, has determined it to be fo, only 
he has thonght himfelf obliged to add the hypothefis of two other fixed 
poles; and from the joint efleéts of all four poles, and from thofe only, 
has he been able to bring this variation of the variation to fome kind of 
fy{tem agreeable to the obfervaticns. He has alfo been obliged to lengthen 
the period of the moveable poles’ revolution; and as Mr. Bond had en- 
Jarged Mr. Phillips’s number fiom 370 years to 60d, fo has Dr “Halley 
enlarged the fame farther, ftom 690 to 7e0 years;” and Mr. Whifton, in 
page 58 of the pamphlet itfelf, flates the revolution of Dr. Halley’s inner 
nucleus, or north magnetic pole, to be in 1920 years, which, he obferves, 
is much flower than that of Mr. Phillips, Mr. Bond, or Dr. Halley. 
Since Whifton’s time, thofe who attempted the magnetic theories are : 
the celebrated Euler, who, as ail others have hitherto done, fuppofes 
that the magnetic poles move; Mr. Lorimer, Mr. Churchman, Mr. 
Walker, &c.; but of all thefe, Mr. Churchman is the only perfon that 
determines their revolution, which he ftates to be, the northernmoft in 
1096 years, and the fouthernmoft in 2289 years. 
I fhall forward, for a future number of the Philofophical Magazine, a 
more full account of the theories that have hitherto prevailed, in which f 
Shall enter more fully, than my prefent mits will allow, into the truth of 
my new theory, and of the method by which I have been enabled to prove 
the fallacy of the former. In the mean time any perfon, even by roughly 
perufing the obfervation of latitudes, longitudes, and dips, made on the 
north-weft coaft of America, and comparing them with that at Londen, and 
alfo with thofe made off the eaft coaft of South America and in the Chi-+ 
nefe feas, will foon be enabled to afcertain,; fu fficiently near for the purpo!e 
of remaining fatisfied, that the fituatiow of the magnetic poles is as de- 
feribed: my determinations are, however, drawn from a multitude of cal- 
culations, and the refult compared with all the magnetic dips and varia- 
tions (taken in all the different places on fhore) I could procure. 
_ ™ Vaccount for the change of the magnetic variation by original mag’ 
nelic meridians and a magnetic annulus. Nz 
ing 
