ON THE MAGNETIC INTENSITY OF THE EARTH. 65) 
_ The light in which I have thus considered M. Biot’s essay is 
the same, I think, in which it was regarded at the time, by his 
distinguished coadjutors in this and so many other branches of 
science. MM. Gay Lussac and Humboldt, in closing the ac- 
count of their magnetic observations on the continent of Europe — 
in 1805 and 1806, remark as follows: ‘* Les inclinaisons corre- 
_ spondantes données par la théorie d’aprés M. Biot, sont toutes 
beaucoup plus grandes, car les plus petites différences vont a 
_prés de 4°. En supposant la position de l’équateur magnétique; 
Tigoreusement déterminée, il en resulteroit qu’en Europe, il y 
a une inflexion considérable des paralléles magnétiques vers 
Yéquateur, occasionnée par l’influence de quelque centre parti- 
eulier. Mais pour tirer aucune conclusion a cet égard, il est 
prudent d’attendre que des observations exactes et plus nom- 
breuses fournissent des bases solides, sur lesquelles on puisse 
- élever une théorie rigoreuse qui les embrasse toutes*.” It is 
here fully recognised that M. Biot’s was not “ cette théorie 
rigoreuse” which, resting on the solid basis of induction from 
a competent assemblage of facts, should have a proportionate 
claim to be regarded as a general representation of the pha- 
nomena. 
In showing the incompatibility with subsequent observations 
| of this ‘‘ abstraction mathématique,” as M. Biot himself de- 
_signated it, I do not therefore consider myself as opposing 
_ either his opinions or his expectations. 
_ It has sometimes appeared to me that the very simplicity of 
| the laws of this hypothesis has tended to counterbalance in 
_ some degree the advantage it produced, in recalling attention 
_ to a subject; the interest in which had been for some years 
_ suspended. Apart from the question of accordance or non- 
favour of four poles, as the best representation of the phenomena: Euler hesi~ 
tated to accede to this until it should be shown more decisively that the pha- 
mena might not be represented by a single excentric axis, having its semi- 
| axes of unequal length; claiming in such case the preference for the latter 
| supposition over that of four poles, as being more suitable for geometrical de- 
ductions. To liave accomplished what such men as Halley and Euler had left 
complete would have been an undertaking not unworthy of M. Biot; but it 
would have required the preliminary labour of collecting together, as M. 
Hansteen has since done, the great body of the facts of observation, which, at 
_ the time his essay was written, were scattered in the journals of travellers and 
| ee navigators, and in the transactions of learned societies of many countries. This 
. ibour might well in prospect have deterred him from the attempt ; but it was 
dispensable for the purpose of furnishing the basis of.a philosophical induc- 
tion of such general laws as should comprehend the whole of the phenomena. 
On no less solid foundation was it probable that phenomena should be repre- 
Sented, known to wear so complicated an aspect, and which had been the sub- 
ject of the long-continued investigation of the eminent men above noticed. 
__ * Lhave put in Italics the part of this extract to which I particularly refer. 
_ VOL. VI. 1837. F 
