82 SEVENTH REPORT—1837. 
maximum 1°85, the proportion would be 2°5 to 1. It seems 
probable that this is rather under than over the difference 
existing in the present distribution of the intensity. If the 
centres change their relative places, by having unequal mo- 
tions, both the absolute and the relative values of the max- 
imum and minimum must be variable. 
This report has already occupied so large a portion of the 
annual volume, that I feel the propriety of not permitting the 
inferences of an individual judgment to trespass further on 
its pages. Ihave endeavoured, to the best of my power, to 
place the facts themselves before the reader in such a manner, 
that, on the one hand, he may have no difficulty in tracing every 
observation to its original source,—and on the other, that by the 
assemblage of the results in one view, he may be enabled with 
the greater facility to draw his own conclusions. 
Having in a former report described M. Hansteen’s theory 
of the magnetism of the earth, and given the formule for the 
variation, dip, and intensity deduced from his hypothesis of two 
excentric axes of unequal force, it may be expected that I should 
conclude this report by comparing some of the observed inten- 
sities with the results computed by the formula. I may there- 
fore add a few words to show that the proper time for a detailed 
comparison of this kind has not yet arrived, because observa- 
tion is stillinarrear of theory. Until observation has supplied 
the materials which theory has required for the correct assign- 
ment of the elements of calculation, such a comparison could 
not be otherwise than imperfect. 
The geographical positions of the magnetic poles in the 
Magnetismus der Erde were derived from observations made 
between 1787 and 1800, which were insufficient to furnish them 
in more than a very general manner. Since that period also, 
changes, of the nature anticipated by M. Hansteen, appear to 
have taken place in the positions of the poles; which conse- 
quently require to be assigned afresh (as well as more cor- 
rectly), in order that the results computed by the formula may 
represent observations of a more recent date. ‘The materials 
proper for this purpose are observations in the vicinity of the 
netic intensity, even with a good instrument, on account of the shortness of the 
period during which the needle will continue to vibrate, and the consequent 
necessity of commencing with a large are of vibration. With an inferior instru- 
ment the limits of error are of course much wider still. In high magnetic la- 
titudes the statical method deserves a decided preference over the method of . 
horizontal vibrations, inasmuch as a moderate error of the dip will scarcely have 
an appreciable effect on the intensity ; and over that by verticul vibrations, inas- 
much asit admits of much greater exactness. 
he 
