OF FERRUGINOUS BODIES. 227 
advance. According to this view, the period of 
revolution of the magnetic poles, in their respec- 
tive orbits, would depend upon the magnitude of 
the diurnal increments; and if these lessen by 
time, the periodic times of revolution would 
lengthen in the same ratio. Hence, since accu- 
rate chronological tables of the extent of the 
diurnal variation for a long series of years, are 
the only data that can furnish the necessary infor- 
mation, no available means should be left unem- 
ployed to collect data of such essential importance. 
47. I hope shortly to submit to the considera- 
tion of this society, other novel facts at present 
in my possession; from which I have every reason 
to believe, that if there be no sustaining force, 
the zntensity of terrestrial magnetic action is con- 
siderably less than it was at an early period of 
the world. 
Note added.—In the Report of the 12th Meet- 
ing of the British Association for the Advancement 
of Science, printed in 1843, is the following state- 
ment. ‘It would appear (if earlier observations 
can be relied on) that the line of least intensity on 
successive meridians, is travelling rapidly north- 
wards.” (P.2). May not this transplacement of 
the poles be owing to solar heat ? 
