o6 
Edmund Halley. 
'February, 
Celebes ; in the remainder of the South Pacific Ocean, \ n 
South America, and the greater part of the South Atlantic 
Ocean, it is governed by a magnetic pole ib. from the 
South Pole in a meridian 20° west of the Straits of Ma- 
^ “ On this hypothesis Halley explains the variation ob- 
served in different places, and among others cites the two 
following instances: — On the coast of America, about Vir- 
ginia, New England, and Newfoundland, the variation was 
found to be west, being above 20° in Newfoundland, 30 m 
Hudson Strait, and 57° in Baffin’s Bay. 0ntte «‘“ 
Brazil, on the contrary, it was found to be east, being 12 a 
Cape Frio, and increasing to 2o£° at the Rio de la Plata, 
thence decreasing towards the Straits of Magellan. Thus, 
almost in the same geographical meridian, we find the 
needle at one place pointing nearly 30° west, at another 
20b 0 east : this is explained by the north end of the need 
in Hudson Strait being chiefly attrafted by the North 
American magnetic pole, whilst at the mouth of the Rio de 
la Plata the south end is attradted by the south magnetic 
pole situated west of the Straits of Magellan. _ Sailing 
north-west from St. Helena to the equator, the variation is 
always in the same diredtion, and slightly east. Here the 
South American is the chief governing pole, but its power 
is opposed by the attradtion of the North American and 
Asian South Poles, the balance as you recede from the latter 
being maintained by approach to the former.” 
“ During the next nine years Halley continued his investi- 
gation of the causes of magnetic variation, and in 1692 he 
made another communication to the Royal Society, m whic 
he endeavoured to meet two difficulties he had always felt 
in his former explanation ; one, that no magnet he had ever 
seen or heard of had more than two °PP<f h.e Poles ; the 
other, that these poles were not— at least all of them fixed 
in the earth, but slowly changed their positions, the lo- 
lowing observations are cited by Halley in proof of the 
motion of the magnetic system At London, in 1580, the 
variation was 11° 15' E., in 1622 it was 6 E„ in 1634 it 
was a° s' E., and in 1657 there was no variation ; whilst in 
1672 it was 2° 30' W., and in 1692, 6" W. At Pans the vari- 
ation was &° or 9 0 E. in 1550, 3 0 E. in 1640, o in 1666, and 
2 ° ao' W. in 1681. At Cape Comorin it was 14 20 W.in 
1620, 8° 48' W. in 1680, and 7° 30' W. in 1688. Halley 
considered the external parts of our earth as a shell, sepa- 
rated by a fluid medium from a nucleus or inner globe, which 
had its centre of gravity fixed and immovable in the common 
