218 ON determinations of the absolute value, secular change, 
mum, in the same hemisphere, was determined a few years earlier (1828 and 1829), 
by the expedition of MM. Hansteen, Erman and Due ; and we may gather the result 
from the following passage in M. Erman’s ^ Reise um die Erde,’ which I quote from 
Mr. Cooley’s translation, vol. ii. p. 365 : — “The magnetical results of the last jour- 
ney were now examined more narrowly, and it was clear that we had in fact crossed 
the meridian of the Siberian magnetic pole between Irkutsk and Yakutsk. The mag- 
netic attraction of the earth was decidedly greater between Kirensk and Beresovoi 
Ostrov than at any other point which we had visited in the same parallel of latitude 
to the east or west. The pole sought for had there exhibited its greatest force, and 
extended its influence furthest to the south ; and consequently we must have been 
there on the same meridian with it. This probably took place at Parshinsk in 
longitude 111° 27' E.*.” Omitting the consideration of the small amount of secular 
change which may have taken place between the expedition of MM. Hansteen, 
Erman and Due and that of Captain Lefroy, we have here an interval of (270° 
— 1 1 1°~)159° as the approxiinate difference of longitude (on the side of Behring’s 
Straits and the adjacent continents) between the two northern centres. This differ- 
ence is diminishing by the effect of secular change, and the epoch, when the centres 
were 180° apart, must therefore have taken place a few years antecedently to either 
of the determinations above referred to ; probably about the close of the last century, 
when, as we learn by Professor Loomis’s discussion of the observations of magnetic 
inclination in the northern parts of the United States-f-, the inclination which had 
previously diminished in that quarter began to increase. 
The change in the geographical position of both the points of maximum in the 
northern hemisphere has been from west to east since the earliest period at which 
inferences of this nature could be drawn from the phenomena; the diminution of the 
meridional distance between them on the one side, and its increase on the other, being 
occasioned by the more rapid movement of translation of the minor maximum. It 
has been conjectured that the motion of the principal maximum might cease to be 
progressive in the easterly direction when the two centres or maxima should be 180° 
* It is obvious that M. Erman uses the term “ magnetic pole ” to designate the central point of a loop of 
the isodynamic lemniscates, or the point of greatest intensity of the force. This also is the sense in which it is 
employed by M. Kupffee, when he says “ il y a un pole magnetique dans la Siberie.” The term “ pole” cannot 
however he understood to have the same signification in those writings which assert a supposed connection 
between “ two magnetic poles ” and “ two poles of cold ” in the northern hemisphere ; for, in North America 
at least, the point of maximum intensity of the force is certainly very far distant from that of the lowest annual 
temperature : the “ magnetic poles ” in this case may possibly be intended to refer to the centres of the loops of 
th.Q isoclinal instead of those of i\ie iso dynamic (so-called) lemniscates. But either of these significations diflPers 
materially from M. Gauss’s definition of a magnetic pole, i. e. “ where the horizontal terrestrial force is zero.” 
I have subjoined this note in illustration of some remarks which I ventured to make in a former communication 
on the inconvenience of the employment of a term which appears to be used in different meanings by diflPerent 
writers. 
t In SiUiman’s Journal. 
