Inquirks concernmg ilie Magnetika of the Earth, 
minute appearances ; and though the whole remains incapable of 
a satisfactory explanation, and many points are still quite unin« 
telligible, the experiments of three centuries have at least been 
submitted to arrangement, and put in a state to receive fresh 
augmentation and correction. 
It is obvious, that if our acquaintance with any of the three 
classes of phenomena above alluded to, with the intensity, the 
dip, or the variation, were accurate and extensive enough, k 
would be sufficient, of itself, for enabling us fully to ascertain 
the magnetic condition of the earth, and to exhibit, « 
the various modifications of the other two. Hence, it is an im- 
portant restriction of the problem in its actual state, that every 
attempt directed to account for one particular branch, shall be 
rejected if it fail to comprehend them all ; while, on the other 
hand, an hypothesis possessing this quality, acquires a propor- 
tionable increase of probability. It farther deserves to be re- 
marked, that of those three properties, the variation is the one 
concerning which we are furnished with the most abundant data: 
it was first detected, experiments with Regard to it are most ea- 
sily performed, least liable to error, and immediately applicable 
to navigation. To this property, accordingly, the attention of 
inquirers has in general been chiefly directed ; and Mr Han- 
steen, without overlooking the intensity and dip, has naturally 
grounded his main conclusions on the aspect of the variation. 
Whoever has attempted to collect observations of this sort, or 
even cast his eye on a magnetic chart, will at once be sensible of 
the extreme uncertainty attending speculations on the subject, 
and of the need there is, not only for immense industry to ac- 
cumulate materials, but also for the most vigilant discrimination 
to separate the correct from the erroneous, and the ambiguous 
from the decisive. How far Mr Hansteen has succeeded in this 
enterprize, will be variously determined : his theory must stand 
or fall by the conformity of its predictions to events ; and in the 
mean time, it demands the inspection of naturalists, because it 
is founded on a very large induction of facts, and because the 
consequences, if true, would be of the very highest import- 
ance. 
On looking over the chart of variations for 1787, which we 
have chosen as the most perfect specimen of Mr Hansteen’s 
H 9 , 
