Gt SEVENTH REPORT—1837. 
I proceed to notice a few of the most striking inferences which 
are deducible from the observations of intensity recorded in 
this report. 
1. The lines of equal intensity are not parallel with the 
lines of equal dip, and the difference is systematic. 
In 1805 M. Biot published an investigation of the laws which 
should govern the dip and the intensity, in the hypothesis of a 
magnet situated at the centre of the earth, having its poles in- 
finitely near to each other, and directed to opposite points on 
the surface of the globe. It is a well-known consequence of this 
hypothesis, that the lines of equal dip and equal intensity on the 
earth’s surface should everywhere be parallel to each other. 
It has always appeared to me that the distinguished author of 
this investigation has been taken much beyond his meaning, 
when he has been supposed to have propounded this hypothesis 
as a general representation of the facts of terrestrial magnetism 
then known, or of those which should be shown by more ex- 
tensive experience. He was doubtless fully aware that, many 
years antecedently, the phenomena of the variation had been 
shown by Dr. Halley to be wholly irreconcileable with the 
geometrical deductions from a single central magnetic axis; 
and that Euler, who may in some degree be regarded as an op- 
ponent of Halley upon the subject generally, fully acquiesced 
in this conclusion. Accordingly, M. Biot made no comparison 
of the hypothesis with the variation, considering no doubt that 
its inapplicability in that respect had been already shown. A 
few facts of the dip were the only observations with which he 
compared the formule of his hypothesis, and with some of 
these it appeared to accord tolerably; but still there were 
anomalies which drew from him the acknowledgement, that to 
represent even those few facts of the dip, it would be neces- 
sary to add to the influence of the primary axis the supposi- 
tion of subordinate centres. That he had no expectation of its 
proving applicable to the intensity, any more than to the varia- 
tion, is, I think, beyond a question, when we read the following 
sentence: ‘‘Quant A la declinaison et 4 l’intensité nous avouons 
franchement que nous ne savons absolument rien sur leurs 
lois ni sur leurs causes: et si quelque physicien est assez 
heureux pour les ramener 4 un principe unique, qui explique en 
méme temps les variations de l’inclinaison, ce sera sans doute 
une des plus belles découvertes que Yon ait jamais faites.”* 
* Journal de Physique, vol. lix. p. 450. The state in which the question 
was left by Halley and Euler was, I believe, as follows: Halley decided in 
