Royal Society. 25 
l^he Caufe of the change of the Variation of the Magnetical 
Needle, with an Hypothefis of the Stru6lure of the internal 
part's of the Earth, hy Mr. Edm. Halley, Phil. Tranf. 
K° 195. p. 553. 
MR. Halley^ in his theory of the variation of the magnetical 
needle, came at length to this general concluiionj "Jiz. 
that the globe of the earth might be fuppofed to be one great 
magnet, with four magnetical poles or points of artra61:ion, two 
of them near each pole of the Equator j and that in thofe parts 
of the world, which lie near any of thole magnetical poles, the. 
needle is chiefly governed thereby, the neareft pole being always 
predominant over the more remote 5 and he there endeavoured to 
Hate and limit the prefent pofition of thole poles on the furface 
of our globe- yet he found two difficulties not ealily to be fur- 
mounted • the one was, that no magnet, he had ever feen or heard 
of, had more than two oppofite poles, whereas the earth had vi- 
fiblyfour, and perhaps more 5 and lecondly, it was plain, that 
thele poles were not, at leaft all of them, fixed in the earth, but 
fliifted from place to place, as appeared by the great changes in 
the needle's direction within this lall century of years 5 not only 
at London^ where this great difcovery was firft made, but almoft 
all over the globe of the earth ; whereas it is not known, or ob- 
served, that the poles of a load-ftone ever fliifted their place in 
the Hone, nor, conliderin^ the compa£tneis of its lubftance, can it 
eafily be fuppolcd : Thele difficulties made the author quite de- 
fpair of ever being able to account for this phsenomenon, when in 
an accidental converfation he lighted on the following hypothefis : 
It is fufficiendy known and allowed, that the variation of the 
needle changes, but that this change is gradual and univerlal will 
appear by the following examples^ at London in 1580, the vari- 
ation was obierved by Mr. 'Burro-'xs to be 11° 15' to the eaft j in 
i(5'2 2, the lame was found by Mr. Gunter to be only 6° to the 
eall^ in 1^34, Mr. G(?///^r^;;i obferved it 4° 5' to the eaft; in 
1(557, Mr. 'Borid obierved that there was no variation 2t London^ 
in 1(572, Mr. Halley himfelf obferved it 2° 30' to the weft; fa 
that in 112 years the dire6lion of the needle has changed no lels 
than 1 7 degrees : At ^Paris^ Oronttus FiUieus^ about the year 
1550, reckoned about 8 or 9° eaft variation; in ie>4P, it was 
found 3° to the eaft; in \666^ there was no variation there, and 
i6%i it was found to be 2° 30' to the weft : At Cape d' AgulhaSy 
the moft Ibutherly promontory of Africa^ about the year id'cc, 
the needle pointed due north and lb uth v;ithout variation, whcnc? 
Vor. Ilf, D the 
