128 Remarks on Professor Hansteen’s 
iem has the peculiar advantage, that every approximation to an 
accurate solution is immediately available in practice. Under 
this point of view, the work of Mr Hansteen, just quoted, may 
be regarded as a considerable acquisition. It was undertaken^ 
we are told, in consequence of a prize-question proposed in 
1811, by the Royal Society of Denmark, in these terms : Is 
the supposition of one magnetical axis sufficient to account for 
the magnetical phenomena of the earth, or are two necessary 
And the ideas which the author explains in a memoir, crowned 
on that occasion, form the ground-work of the present treatise. 
His attention had been excited to the inquiry some years pre- 
viously, by the inspection of a terrestrial globe, marked with 
some delineations pertaining to the subject. In particular, he 
observed an elliptic figure drawn round the south pole, and de- 
signated regio polaris magnetlca ; one of the foci being called 
regiofbrtiorj the other regio dehilior. As the figure profes- 
sed to be constructed by Mr Wilcke, from data furnished by 
the observations of Cook and Fourneaux, this statement led Mr 
Hansteen to compare the representation with the facts ; and the 
Comparison being favourable to Wilcke‘’s accuracy, he was next 
induced to study the theory of Halley, which he had till then 
considered quite extravagant. The result of his inquiries was, 
that Halley’s theory, in so far as regards the existence of four 
poles or of two magnetic axes, is perfectly correct ; being diffe- 
rent only in the position assigned to these poles, and erroneous 
in supposing them to arise from an interior moveable globe or 
terrella. The theory of Halley certainly gains a powerful ac- 
cession of strength from such an auxiliary ; and though even 
under its present shape it should prove in magnetism but like 
the cycles and epicycles of Ptolemy in astronomy, still it is of 
no small use to possess an hypothesis, which shall connect 
by any plausible principle, the phenomena of so compli- 
cated and vast a department of knowledge. But whatever be- 
come of his theory, Mr Hansteen’s book has a more undoubted 
species of merit. It contains an abundant store of facts relat- 
ing to the variation, dip, and intensity, — many of them collected 
from the most obscure and widely separated sources. The task 
of investigating the elementary laws of magnetic force is at 
least undertaken. Mr Hansteen displays considerable skill in 
