344 
Mr. Christie's theory of the 
dipping needle.* According to this, if equal disturbing forces 
act at places where the dips are different, the sines of the 
* I first stated this law of the dependance of the deviations of the horizontal needle 
on those of the dipping needle, in the form of an hypothesis, in the Cambridge Philo- 
sophical Transactions for 1820; but without adverting to that hypothesis, it may be 
considered as a most convenient method of embracing in one view, various phaenomena 
observed with the horizontal needle. From whatever cause deviation in the direction 
of the horizontal needle may have arisen, except, possibly, in cases where the length 
of the needle bore a very sensible ratio to the distance of the disturbing body, I have 
met with no instance that was not quite consistent with this law ; nor was I aware that 
any such was said to exist, until I heard the circumstance stated in a paper of Mr. Bar- 
low’s, recently read before theRoyal Society. Not being in possession of Mr.BARLow’s 
experimental results, however far I may be from being convinced by the arguments 
he has adduced against the law itself, independent of any hypothetical views, I will 
not venture to point out the nature of their fallacy : but as far as I could collect the 
facts during the reading of the paper, I am of opinion that they would result from 
my view of the subject. Thus supposing the centre of the shell, in his experiments, 
to be placed in the equator of the imaginary dipping needle passing through the 
centre of the horizontal needle, so that, if the branches of this needle were of equal 
intensity, no deviation would, according to my view, take place; then, if either 
branch be deteriorated, the centre or pivot of the needle will no longer be the centre 
of its magnetism, and a point in the horizontal needle, remote from its pivot, must 
now be considered as the centre of the dipping needle ; and the centre of the shell 
being now above, or below the equator of this dipping needle, according to the cir- 
cumstances of deterioration, unequal action will take place on the branches of the 
dipping needle, and consequently, according to the law in question, deviation of the 
horizontal needle will ensue. Although this law has been called in question, I will 
here mention an instance of its advantage in connecting different phaenomena. By 
supposing the effect of a disturbing force to be produced on the dipping needle, and 
that in consequence the poles describe, in a certain time, a circle round their un- 
disturbed places, connecting the observations on the dip, in this manner, with those 
on the horizontal needle, I found, as I have stated in my paper in the Cambridge 
Philosophical Transactions, 1820, that the resulting variations will agree, within 
less than half a degree, with the observed variations of the horizontal needle in 
London, during a period of 200 years. These results I never published ; but I found 
ver\ shortly alterwards, that, on this view, the same approximation n ight be made 
to the variations of the horizontal needle observed at Paris, during the same period. 
