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LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
General Remarks . — If vve take a general view of the magnetic declination in the 
southern hemisphere, particularly in the best-known portion of it, comprised between 
the tropics and the Antarctic Circle, we find that the phenomena present the same 
obvious and decided features of a duplicate system as do those of the northern hemi- 
sphere. If, following any of the geographical parallels, we carry our attention round 
the hemisphere, we find it divided into four spaces, in which opposite characteristics 
in regard to the direction of the needle alternately present themselves. In two of 
the spaces the change in the pointing of the needle, as the space is traversed in the 
direction of the parallel, is continuous and progressive towards the west, and in the 
other two continuous and progressive towards the east. If, for example, commencing 
with the meridian of 30° E. or thereabouts, we trace the parallel of —45° round the 
hemisphere, always proceeding in' an easterly direction till we return to the meridian 
at which we began, we shall find that we first pass through a space in which the 
direction of the north end of the needle becomes progressively more and more easterly , 
either by the decrease of westerly or increase of easterly declination ; we next pass 
into a second space, on entering which the continuity is broken, the progressive 
movement of the north end of the needle towards the east is arrested, and its direc- 
tion becomes now more and more westerly as we advance ; thence we pass, success- 
ively, into a third space which has the same characteristic as the first, and into a 
fourth which has the same as the second. 
The spaces here spoken of must be distinguished from those which are charac- 
terized by the exclusive prevalence of either east or west declination : they have a more 
simple and pure magnetical relation, implying the predominance within each space 
of one or the other of the two systems of magnetic forces which govern the direction 
of the needle. It may happen, or it may not happen, that in one of these spaces the 
direction of the needle may coincide in some point or points with the geographical 
meridian ; when this occurs, the space will comprise both east and west declination ; 
when it does not happen, the declination throughout the space will be exclusively 
east or exclusively west as the instance may be : but in either case, the change in the 
direction of the needle is always continuous and uniform in character throughout 
the space. It is well known that if the magnetic declination be computed on the 
supposition of a single central magnetic axis, there will be found two, and only two 
such spaces in each hemisphere. The systematic discordance which the declinations 
in the northern hemisphere presented when compared with the declinations so com- 
puted, and their agreement with the phenomena deducible from a double system of 
forces, led Halley to embrace the latter hypothesis. The declinations in the southern 
hemisphere present an arrangement strictly analogous to that in the northern, and 
conduct to the same conclusion, be that conclusion what it may. 
If, with Halley, we view the declinations in the Southern Pacific as principally 
influenced by the weaker system of forces, or by that to which is also to be ascribed 
the high intensity of the magnetic force in the same quarter, we should be prepared 
