MAGNETIC AND METEOROLOGICAL OBSERVATORIES. 297 
the mean amount of diurnal change is él, when taken over the whole year; 
but that on any particular day in the year it has a determinate magnitude, 
which passes through an annual periodicity, with opposite characters in 
opposite seasons; and that for a station in middle latitudes the mean 
diurnal fluctuation is not zz/, but such as during every part of the year to 
exhibit an easterly deviation in the morning hours, and a westerly in the 
evening hours, for stations north of the magnetic equator, and vice versa 
for those south of it; but that the amount of this deviation, or the amplitude 
of the diurnal fluctuation, varies with the seasons, being exaggerated or 
partially counteracted by the alternate conspiring and opposing influence of 
the sun’s declination during the summer and winter seasons. 
As regards the irregular disturbances, though arbitrary and capricious in 
extent and in the moments when they may be expected individually, this 
does not prevent their obeying with great fidelity the law of averages when 
grouped in masses and treated separately from those of the former class. 
So handled, they are found to conform in the average effect, at each of the 
twenty-four hours of the day, and on each day of the year, to the very same 
rules as regards the sun’s daily and annual movement,—with one remarkable 
point of difference: viz., that their hours of maxima and minima are not 
identical with those of the regular class, but that each particular station 
has, in this respect, its own peculiar hours, analogous to what is called the 
“establishment” of a port in the theory of the tides; and that, in con- 
sequence, the superposition of these two systems of diurnal fluctuation gives 
rise to a series of compound variations analogous to the superposition of two 
undulations having the same period but different amplitudes and different 
epochal times, and that, by attending to this principle, many of the most 
complex phenomena, such as that of a double maximum and minimum, with 
the occurrence of a nightly as well as a daily movement, are explained in a 
satisfactory manner. 
The discussion of the observations already accumulated has further 
brought into view, and in the opinion of your Committee fully established, 
the existence of a very extraordinary periodicity in the extent of fluctuation 
of all the magnetic elements, and in the amplitude and frequency of their 
irregular movements especially, which connects them directly with the 
physical constitution of the sun, and with the periodical greater or less 
prevalence of spots on its surface,—the maxima of the amount of fluctuation 
corresponding to the maxima of the spots, and these, again, with those of 
the exhibitions of the Aurora Borealis, which appears also to be subject to 
the same law of periodicity—a law which, as it does not agree with any of 
the otherwise known solar, lunar, or planetary periods, may be considered as, 
so to speak, personal to the sun itself. And thus we find ourselves landed 
in a system of cosmical relations, in which both the sun and the earth, and 
probably the whole planetary system, are implicated. 
That the sun acts in influencing the earth’s magnetism in some other 
manner than by its heat, seems to be rendered very probable by seveya] 
features of this inquiry; and the idea of a direct magnetic influence ext the 
to the earth is corroborated by the discovery of a minute fluctuation in 
magnetic elements, having for its period not the solar but the lunar day, and 
therefore directly traceable to the action of the moon. The detection of 
this fluctuation by Mr. Kreil, from a discussion of the Prague observations, 
has been confirmed by the evidence afforded by those of our Colonial 
observatories, and appears to be placed beyond all question by the recent 
deductions for the horizontal force and the declination, extending over three 
ee observation at the Cape of Good Hope, which General Sabine has 
8. x 
