LIEUT.-GENERAL SABINE ON TERRESTRIAL MAGNETISM. 
383 
pressions appertaining to this branch of the physical sciences. Thus: — I have used 
the terms magnetic 'pole and poles in their original and natural sense, as designating 
those points on the surface of the globe where the intensity of the magnetic force is a 
maximum in comparison with adjacent localities — in other words, the centres of the 
lemniscates into which, at our present epoch in the magnetic cycle, the isodynamic lines 
of highest intensity resolve themselves. 
In one of the earliest systematic treatises on the phenomena of terrestrial magnetism 
as derived from and resting on observation, (that of Halley in the Philosophical Trans- 
actions for 1683,) it is unhesitatingly affirmed, on the evidence of well-assured facts, 
that “ the globe of the earth may be regarded as one great magnet having four magne- 
tical poles, ox points of attraction , two of them near each pole of the equator; and that 
in those parts of the globe which lie near any of those magnetical poles, the needle is 
chiefly governed thereby, the nearest pole being always predominant over the more 
remote.” It is in this, its original signification, bearing date so early as 1683, that I 
have employed the term “ magnetic pole.” The magnetic surveys which have been so 
carefully and systematically made in the course of the present century over almost 
all the accessible parts of the globe, may now be considered to have fully confirmed 
the truth of Halley’s remarkable anticipation, and may be claimed in full justification 
by those who, following in the steps of our sagacious and illustrious countryman, employ 
the terms which he was amongst the first to use (and the first to assign to them their 
true significancy), in the same sense in which he himself employed them. The Halleian 
hypothesis, and the incontrovertible facts on which it rested, were for a time obscured 
by the prevalence of a uniaxial hypothesis, in which the points of maximum force were 
assumed to be identical with the points where the inclination is 90° ; around which, as 
their centre, the isodynamic and isoclinal lines were supposed to be arranged symmetri- 
cally in parallel lines ; hence the term magnetic pole came, in the uniaxal hypothesis, 
to be attached to the points where the inclination is 90°, that element being the easier 
of determination, and consequently the one most popularly regarded. But the accept- 
ance of the uniaxial hypothesis has gradually yielded to the progressive advance of 
observational knowledge ; and one of its ablest expositors (Biot), as far back as the com- 
mencement of the present century, was constrained to admit that, even in the case of 
apparently the most simple of the three elements (viz. the Inclination), a single magnetic 
axis would afford no sufficient explanation of the phenomena, unless it were supple- 
mented by the supposition of subordinate centres ; whilst as regarded the known facts 
of the Declination and Intensity, they must be held to be wholly inexplicable on the 
supposition of a single magnetic axis (Beport of the British Association, 1837, p. 64). 
Preserving, therefore, the sense in which Halley employed the term magnetic pole, 
and in accordance with the evidence, now fully established by observation, of the exist- 
ence of four such points on the surface of the globe, the magnetic eguator is most pro- 
perly defined as the line connecting those points in each geographical meridian where 
the intensity of the earth’s magnetism is less than in any other point situated in the 
