LIEUT.-COLONEL SABINE ON TERRESTRIAL MAGNETISM. 
117 
to expect that if the geographical limits of the adjacent spaces, having the charac- 
teristics referred to, were determined at different epochs, the alteration in the position 
of the spaces, if any, would show the existence of a secular change in the system 
itself ; that it would indicate the direction of such change ; and, if the intervals were 
sufficiently long in reference to the precision with which the determinations were 
made, the average rate of the movement of translation might also be inferred. 
In this view a knowledge of the geographical position of the limiting lines, or of 
lines drawn so as to separate one of these spaces from the next, may have a particular 
value. In the part of the Pacific Ocean which is now referred to, the separating lines, 
as for distinction they may be called, coincide nearly in direction with geographical 
meridians, and are therefore crossed nearly at right angles by vessels pursuing a 
course from east to west, or from west to east. Prior to our own times, the epoch of 
Captain Cook’s voyages is perhaps that in which the observations of the declination 
in the Southern Pacific may be regarded with the most confidence. The determina- 
tions of that period have been collected by M. Hansteen into a map, of which he 
assigns the year 1/70 as the mean epoch. It is one of those published in the Atlas 
of the Magnetism us der Erde, and comprehends the results obtained by Byron, 
Carteret, Wallis, Cook in three voyages, Ekeberg also in three voyages, and 
Abercrombie. If in this map we draw lines separating the spaces which have the 
opposite magnetic characteristics referred to, and compare them with the corre- 
sponding lines which we may draw in Erman’s map of the Declination in 1827-1830, 
published in the Magnetic Instructions of the Royal Society, we find an effect of 
secular change very distinctly shown in the altered position of the separating lines. 
These lines, A and B, are drawn in the accompanying Plate # , where the two epochs, 
1770, and 1827-1830, are brought into comparison. In the map of 1827-1830, the 
separating lines occupy a considerably more westerly position than in the earlier map, 
the difference amounting to about 10° of longitude. Hence we are led to the con- 
clusion, that the spaces in the Southern Pacific, distinguished by certain magnetic 
characteristics, undergo a movement of translation, of which the general direction is 
from east to west. This direction is the opposite to that in which the change is 
known to take place in the corresponding quarter in the northern hemisphere (viz. 
in the Siberian quarter), where the secular movement is from west to east. 
We are not without earlier, though possibly it may be supposed less precise, evi- 
dence of the effect of secular change in the Southern Pacific. From Halley’s chart 
of the variation lines for 1700, we are enabled to draw the separating line B for that 
epoch, when we find it to have been between the longitudes of 305° and 310°. In a 
still earlier map drawn by Hansteen for the year 1600 (Magnetismus der Erde, Atlas, 
No. 1), representing the observations of the very able and scientific navigators of that 
period, we find the position of the same line to have been about 333° of east longitude. 
In the observations of Captain Ross’s voyage, we have the most recent evidence of 
the progressive westerly movement of the magnetic phenomena in the Southern 
* Plate XII. 
