302 Royal Society :— 
The night minimum is in fact the mean effect of the occasional dis- 
turbances. It is probable that the nocturnal inflection of the solar- 
diurnal variation of the Declination may be ascribed to the same 
cause, namely to the superposition of two distinct variations. 
A careful analysis of the solar-diurnal variations of the Declination 
at the Colonial Observatories has brought to light the existence 
at all these stations, of an annual inequality in the direction of the 
needle concurrent with changes in the sun’s declination, having 
its maxima (in opposite directions) when the sun is in or near the 
opposite solstices, and disappearing at or near the epochs. of the 
equinoxes. An intercomparison of the results of the analysis at 
these stations has shown, that this inequality has the remarkable 
characteristics of having notably the same direction and amount in 
the southern as in the northern hemisphere, and in the tropical as in 
the temperate zones. An ingenious explanation of these phenomena 
has been suggested by Dr. Langberg of Christiania (Proceedings of 
the Royal Society, vol. vii. p. 434); but whether this explanation be 
or be not the correct one, the theoretical importance of the facts can- ~ 
not be doubted, inasmuch as they appear to be wholly irreconcileable 
with the hypothesis which would attribute the magnetic variations 
to thermic causation. We may ascribe to the general and almost 
exclusive prevalence of the thermic hypothesis, and to its influence 
on magnetic reasonings, that the well-known erroneous opinion was 
so confidently promulgated by a deservedly high magnetic authority *, 
that a line must exist surrounding the globe, in which the needle 
would be found to have xo diurnal variation. We have now, on the 
contrary, reason to be assured, by the facts of the annual inequality 
thus discovered, that there is no such line ; but that everywhere in 
the regions of its supposed existence a diurnal variation subsists, 
having opposite characteristics in opposite parts of the year as in- 
fluenced by the sun’s position on either side of the equator, and dis- 
appearing only at the epochs when the sun passes from south to 
north or from north to south Declination. 
Lunar Variation.—But if thermic relations have failed to supply 
a connecting link between the sun and those magnetic variations 
which are, without doubt, referable to the swn as their primary cause, 
the failure of that hypothesis is made still more obvious by the ex- 
istence of variations governed by the moon’s position relatively to the 
place of observation. We are indebted to M. Kreil, now holding 
the same position in Austria that I have filled in England, for the 
first suggestion of the existence of a lunar-diurnal variation of one 
of the elements, viz. of the Declination, founded on observations at 
Milan and Prague; and in the Phil, Trans. for 1856, Art. XXII, 
will be found an exposition of the facts of the moon’s diurnal influ- 
ence on each of the three magnetic elements at Toronto, viz. on 
the Declination, Inclination, and Total Force. In the case of this 
investigation, notwithstanding the smallness of the values concerned, 
the instrumental means supplied to the Colonial Observatories have 
* Arago, Annuaire, 1836, p. 284, 
