ON THE MAGNETIC INTENSITY OF THE EARTH. 67 
1823 I had the good fortune to enjoy opportunities of observing 
the magnetic phenomena over a portion of the globe amounting 
to about one-eighth of its surface, or the quarter of an hemi- 
sphere. In comparing, on my return to England, the observa- 
tions of dip with M. Biot’s formula, the differences between cal- 
culation and experiment were seen to be not atsingle stations only, 
but systematic, extending over large spaces of the globe; the 
discrepancies were also so great as (in the words which I em- 
ployed in 1825) to make it “‘ certain that no two positions could 
** be assigned to the magnetic poles, which would enable a cal- 
** culation of the dip as a function of the magnetic polar distance, 
** in which differences from fact should not be found of 10° and 
“upwards.” Further, in comparing the observations of dip and 
intensity with the parallel course, which, according to the hy- 
pothesis, the lines of equal dip and equal intensity should 
preserve, their irreconcilability with this law was shown to be so 
great and so systematic as to be ‘ decisive’ against the sup- 
** posed relation of the force to the observed dip; and equally 
**so against any other relation whatsoever, in which the re- 
“spective phenomena might be supposed to vary in corre- 
** spondence with each other.” Another important difference 
was also pointed out. Inthe hypothesis the maxima of dip 
and intensity are coincident: with this the observations were 
at variance ; those of the intensity placing its maximum several 
degrees to the southward of the geographical position which 
the bhservations of dip indicated as that of the dip of 90 de- 
rees*, 
In 1830 M. Erman returned from a journey in which he had 
carried magnetic observations over a space on the globe still 
more extensive than mine, and (which should be specially no- 
ticed) so entirely distinct from mine, that we had not a single 
* The observations of intensity arranged around their own centre presented 
much less discordance with the laws of an uniaxal hypothesis than appeared 
in those of the dip when referred 'to the position of the pole as indicated by the 
dip of 90 degrees. By substituting in the formula of that hypothesis the “ iti- 
nerary distance from the maximum of intensity” for the “‘ magnetic polar di- 
stance,” and employing this formula as an empirical representation, it was 
found to correspond with the facts of the intensity within the district comprised 
by my observations, with no very material discrepancies. In that portion of 
the hemisphere in which the influence of the primary centre is predominant, 
ne variations of the intensity may be easily imagined not to differ greatly from 
the effect of a single axis; and such is apparently the fact. It happened that 
my observations, extensive as they were, fell within that limit; had they been 
pursued a few degrees further to the eastward, the influence of the Siberian 
centre would have become more sensible, and the uniaxal formula would have 
ceased to afford even an approximate representation of the facts. But this 
perhaps will be better understood when the sequel of the report has been read, 
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