350 Mr. Christie’s theory of the 
the diurnal variation at different times of the year. It is by 
no means improbable, that it is owing to circumstances of 
this kind that we have a maximum daily variation in April, 
and another in August. The situation of the poles will also 
be influenced by the elevation of the land to which the sun is 
vertical ; and when it passes over the plains of Hindoostan, 
with the Himaleh mountains covered with perpetual snow, 
and the high and cold plains of Tartary to the north ; and 
again, when it crosses the Andes ; the situations of these 
poles, with respect to the place to which the sun is vertical, 
will, I consider, be different from their positions with regard 
to it, when the sun is vertical to the centre of Africa, or of 
the Pacific ocean. The great disturbances which take place 
in the atmosphere in the intertropical regions, and which 
when of considerable extent must influence the situation of 
the place of heat, will also become a source of anomalous 
action on the needle. All these circumstances, admitting the 
correctness of the hypothesis, must greatly modify the 
regular effects that would otherwise be produced, and must 
influence considerably the nature and extent of the daily 
variation when observed at different parts of the earth ; and 
probably they are the causes of some apparent discrepancies 
in the preceding results. 
On commencing the experiments with the compound plate 
of bismuth and copper, I had no expectation of reducing the 
deviations of the needle to so simple a law as that resulting 
from a polarising of the plate in a particular direction ; but 
I considered that it would be essential to determine, whether 
the deviations near the outer edge of the copper were of the 
same character as those near the line of junction of the 
