IN THE MEAN EFFECTS OF THE LARGER MAGNETIC DISTURBANCES. 
125 
influence its position, and taken on an average of a sufficient number of days to 
make the figure a fair mean representation. This is the “diurnal variation,” as 
shown by observation at different seasons and jat different parts of the globe, which 
it has been Mr. Faraday’s object, in papers recently read before the Society, to ex- 
plain physically. But the variation thus observed is in fact constituted by two vari- 
ations superimposed upon each other, having different laws, and bearing different 
proportions to each other in different parts of the globe. At tropical stations the 
influence of what have been hitherto called the “irregular disturbances” is compara- 
ratively feeble, and their periodical effects may be therefore less deserving of atten- 
tion ; but it is otherwise at stations situated as are Toronto and Hobarton, where 
their influence is both really and proportionally greater, and already amounts to a 
clearly recognizable part of the whole diurnal variation ; and it must be very far 
otherwise at stations which still more nearly approach those localities on the globe, 
where disturbances of this class become much greater than either at Hobarton or 
Toronto. As these localities are approached, the mean diurnal variation must be 
expected to partake more and more of the law by which the effects of those disturb- 
ances are regulated ; and if that law be different, as it will be shown to be in the 
instances adduced in this paper, from another law also shown to exist, and by which 
another portion of the diurnal variation is governed, it may become necessary, and 
most probably will become necessary, at such stations, to separate, approximately at 
least, the effects so regulated by different laws in order to study their respective 
physical causes. 
The Instructions of the Royal Society, which were written before the periodical 
character of the so-called “irregular” variations was suspected, contemplate that by 
a sufficiently long continuance of the observations the effects of the “ irregular va- 
riations” will be self-compensatory. “The observations,” it is said, “ must be long- 
continued at the same hours, before we can be assured that the irregularities do not 
sensibly affect the mean results.” But since the true character of the disturbances is 
now more correctly known, it becomes obvious that no continuance of the observa- 
tions will render their influence insensible; and the simplification of the problem, in 
the solution of which Mr. Faraday is engaged, will require the separation, as far as 
may be practicable, especially in the higher latitudes, of the two classes of variation 
which have distinct laws, and therefore probably distinct immediate causes. I ven- 
ture therefore to hope for the indulgence of the Royal Society in an endeavour to 
establish the fact of a distinct law of periodical action in the disturbances of occa- 
sional occurrence and unusual amount, — to show its general and remarkably analo- 
gous character at two stations separated from each other nearly as widely as two 
stations can be on the surface of the globe, and approximately at least to separate 
the mean diurnal variations at those two stations into their respective components. 
The stations of Toronto and Hobarton are well-situated to furnish observations to 
be employed in the investigation which forms the subject of this paper. Both stations 
