ON THE MAGNETIC INTENSITY OF THE EARTH, 83 
magnetic poles themselves. In the northern hemisphere, these 
are far more ample and exact than at any former period, owing 
in great measure to the interest excited by the publication of 
M. Hansteen’s theory. But the corresponding observations in 
the southern hemisphere are yet wanting; and until these are 
supplied, we cannot advance beyond an anticipation, more or 
less confident, of the eventual accordance of the hypothesis, 
when the correct elements of calculation shall have been ob- 
tained ; and in this view, we may at least say thus much in re- 
gard to the general accordance of the hypothesis with the ob- 
servations of intensity, that if we omit the consideration of the 
higher latitudes, where the contemporaneous and correct posi- 
tions of the magnetic poles are most essential, the formula, 
even with the elements derived from the earlier and less perfect 
observations, both represents all the leading features of the 
system, and shows a fair approximation in individual cases. 
_ The method in which this science has progressively advanced 
is strikingly illustrative of a passage in Mr. Playfair’s writings, 
in which the distinct offices of theory and experiment, and the 
value of their co-operation in inductive investigation, are well 
described. ‘‘In physical inquiries the work of theory and ob- 
servation must go hand in hand, and ought to be carried on at 
the same time, more especially if the matter is very complicated, 
for then the clew of theory is necessary to direct the observer. 
Though a man may begin to observe without any hypothesis, 
he cannot continue long without seeing some general conclusion 
arise; and to the nascent theory it is his business to attend, 
because by seeking either to verify or to disprove it, he is led 
to new experiments and new observations. He is led also to 
the very experiments and observations that are of the greatest 
importance; namely, to those tmstancie crucis that naturally 
present themselves for the test of every hypothesis. By the 
correction of his first opinion a new approximation is made to 
the truth, and by the repetition of the same process certainty 
is finally obtained. Thus theory and observation mutually 
assist one another; and the spirit of system, against which 
there are so many and so just complaints, appears nevertheless 
as the animating principle of inductive investigation. The 
business of sound philosophy is, not to extinguish this spirit, 
_ but to restrain and direct its efforts. It is therefore hurtful to 
_ the progress of physical science to represent theory and obser- 
vation as standing opposed to one another.” 
The earlier observations of terrestrial magnetism were made 
_ without reference to theory. As facts accumulated general 
- conclusions arose. ‘Their elaborate examination conducted to 
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