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XXI. Theory of the Diurnal Variation of the Magnetic Needle, 
illustrated by experiments. By S. H. Christie, Esq., M. A. 
F. R. S. 
Read June 14 and June 21, 1827. 
In a Paper published in the Transactions for 1823, I stated 
my opinion, that the diurnal variation of the needle was pro- 
bably due to the influence of temperature, but that the prin- 
ciple adopted by Canton would not account for the morning 
easterly variation. In a subsequent paper,* I pointed out that 
the changes in direction and intensity appeared always to have 
a reference to the position of the sun with regard to the mag- 
netic meridian ; the direction of the needle being undisturbed 
nearly at the time the sun passed that meridian ; and the 
horizontal intensity being the least at the same time. Having 
taken this view of the subject previously to my being aware 
of Dr. Seebeck’s discovery, that magnetical phenomena will 
arise from a disturbance in the equilibrium of temperature, 
my knowledge of that discovery and of subsequent experi- 
ments, particularly those of Professor Cumming, confirmed 
me in the opinion, that temperature must have a considerable 
effect in producing some of the phaenomena of terrestrial 
magnetism, although I considered that this influence might 
be modified by the effects produced by rotation, or by pe- 
culiar influence in the sun’s rays. 
At the conclusion of the Paperf describing the experiments 
* Philosophical Transactions, 1825. 
f It appears that this Paper was read before the Cambridge Philosophical 
