118 
LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
Pacific. The separating lines A and B, deducible from the observations in 1842, 
are seen in the Plate to be in both cases considerably to the west of those derived 
from the observations of 1827-1830. 
The whole body of evidence therefore, from the earliest observations to the latest, 
is consistent in showing a progressive movement to the westward of the spaces in the 
Southern Pacific, characterized by certain magnetic peculiarities, which in Halley’s 
view indicated the proximity and predominance of the weaker system of forces. It 
is worthy of notice that the rate of progression, deduced from the changes of position 
shown at the several epochs, differs much less from a uniform rate than might have 
been anticipated from the nature of the evidence we possess, even supposing the 
actual rate to have been uniform in nature ; whilst the magnitude of the whole 
change which appears to have taken place since the phenomenon became the subject 
of observation, in round numbers 50° of longitude in two centuries and a half, can 
scarcely fail to fix the attention. These are facts which, when the true physical causes 
of the magnetism of the globe shall occupy the earnest attention of philosophers, will 
probably attain an importance which at present perhaps we scarcely sufficiently esti- 
mate. But an endeavour to place distinctly before our minds facts of which the 
explanation must be deemed an essential condition of a satisfactory solution of this 
great problem, may not be without its use even at the present time. It may be also 
useful to call the attention of navigators to the value which may hereafter attach to 
determinations which may be made with instruments which are on board every ship, 
and in constant employ for the ordinary purposes of navigation. The position of the 
lines separating the spaces which have been the subject of discussion, has the advan- 
tage of being even more easily determined by observations on board ship than that 
of any particular declination line ; in crossing them, the declination, if previously 
decreasing, will then begin to increase, and if previously increasing will begin to 
decrease ; the determination is therefore independent of compass error, which is 
a much more prevalent source of error than is generally supposed ; and if the ship’s 
course be steady for some days together, which in the latitudes in question is very 
frequently the case, it is also in a great measure independent of the disturbance 
occasioned by the ship’s iron. A very cursory inspection of the general table of the 
declinations observed by the Erebus and Terror suffices to show that they must have 
crossed the separating line (A) about the 15th of March 1842, when their latitude 
was about —59° and longitude 221°; and the line (B) about the 27th or 28th of 
the same month in latitude about —59°, and longitude 275°*. 
Should the circumstance occur that one of the separating lines in the course of its 
progressive change of place should pass over a magnetic observatory, the epoch of 
its passage would be precisely determined. There is some reason for believing that 
* The line A passes through the culminating points of the southerly inflexion of the declination lines, of 
which the present position is shown in the Declination Map at the close of this paper to be about 220° east 
longitude. The line B passes through the culminating points of the northerly inflexion of the declination lines 
about the longitude of 276°. 
