ADDRESS. liii 
cennial magnetic period coincides, both in its duration and in its epochs of 
maxima and minima, with the decennial period observed by Schwabe in the 
solar spots ; from which it is to be inferred that the Sun exercises a magnetic 
influence upon the Earth, dependent on the condition of its luminous 
envelope. 
We are thus in the presence of two facts, which appear at first sight 
opposed, namely, the absolute simultaneity of magnetic disturbances at all 
parts of the Earth, and their predominance at certain local hours at each 
place. General Sabine accounts for this apparent discrepancy by the cir- 
cumstance, that the hours of maximum disturbance are different for the 
different elements; sa that there may be an abnormal condition of the 
magnetic force, operating at the same instant over the whole globe, but 
manifesting itself at one place chiefly in one element, and at another place 
in another. I would venture to suggest, as a subject of inquiry, whether the 
phenomena which have been hitherto grouped together as “ occasional” 
_ effects, may not possibly include two distinct classes of changes, obeying 
separate laws—one of them being strictly periodic, and constituting a part of 
the regular diurnal change, while the other is strictly abnormal, and simul- 
taneous. If this be so, it would follow that we are not justified in separating 
the larger changes from the rest, merely on the ground of their magnitude ; 
and that a different analysis of the phenomenon will be required. 
The effects hitherto considered are all referable to the Sun as their cause. 
Professor Kreil discovered, however, that another body of our System— 
namely, our own satellite—exerted an effect upon the magnetic needle; 
and that the magnetic declination underwent a small and very regular varia- 
tion, whose amount was dependent on the lunar hour-angle, and whose period 
was therefore a lunar day. This singular result was subsequently confirmed 
by Mr. Broun, in his discussion of the Makerstoun Observations ; and its 
laws have since been fully traced, for all the magnetic elements, by General 
Sabine, in the results obtained at the Colonial Magnetic Observatories. 
~The foregoing facts bear closely upon the debated question of the causes 
of the magnetic variations. It has been usual to ascribe the periodical 
changes of the Earth’s magnetic force to the thermic action of the Sun, 
operating either directly upon the magnetism of the Earth, or affecting it 
_ indirectly by the induction of the thermo-electric currents. Here, however, 
we have a distinct case of magnetic action, unaccompanied by heat; and 
the question is naturally suggested, whether the solar diurnal change may 
not also be independent of temperature. 
_ The most important fact, in its bearing upon this question, is the existence 
of an annual inequality in the diurnal variation, dependent on the Sun’s 
declination, recently pointed out by General Sabine. If we deduct the 
ordinate of the curve, which represents the mean diurnal variation for the 
entire year, from those for the summer and winter half-yearly curves re- 
_ spectively, the differences are found to be equal and opposite ; and the curves 
