Major-General Sabine on Colonial Magnetic Observatories. 301 
experiment were well selected to prove the importance of the inves- 
tigation, and thus to lead to its extension. It is only at the Colonial 
Observatories that the disturbance-variations have hitherto been made 
out ; and taking experience as our guide, we have before us the evi- 
dence of the means by which the inquiry may be further successfully 
prosecuted *. 
Periodical Variations.—The ‘anticipation expressed in the Report 
of the Committee of Physics, that for the purpose of obtaining a cor- 
rect knowledge of the regular periodical variations, it would be found 
necessary to eliminate the “casual perturbations,’ has been fully 
confirmed. Had the latter been strictly ‘casual’ (or accidental, in 
a sense contradistinguished from and opposed to periodical), a sufti- 
ciently extended continuance of observation might have occasioned 
their mutual compensation ; but now that we have learned that the 
mean effects which they produce are governed by periodical laws, and 
that these laws and those of the regular periodical variations are dis- 
similar in their epochs, it is manifest that in their joint and undivided 
effects we have two variations, due to different causes and having di- 
stinct laws, superimposed upon each other; to know the one correctly 
we must necessarily therefore eliminate the other. A striking illus- 
tration of the importance of such elimination is furnished by the solar- 
diurnal variation of the total force. It will readily be imagined that 
the question must be an important one, whether a variation, which 
is supposed to derive its origin from the sun, be a single or a double 
progression ; whether it have two maxima and two minima in the 
twenty-four hours, or but one maximum and one minimum in that 
period. When no separation is made of the disturbances, the pro- 
gression appears to be a double one, having two minima, one occur- 
ring in the day and the other in the night. With the removal of 
the disturbed observations the night minimum disappears, and we 
learn that the regular solar-diurnal variation of the total force has 
but one notable inflection in the twenty-four hours, viz. that which 
takes place during the hours when the sun is above the horizon. 
* The Colonial Observatories under my superintendence were originally four in 
number, viz. Toronto, St. Helena, Cape of Good Hope, and Hobarton. In July 
1846 the detachment of the Artillery at the Cape of Good Hope was withdrawn 
by orders from England, and the charge of the magnetical and meteorological ob- 
servations transferred to Mr. Maclear, the Government Astronomer at that station. 
The magnetical observations made at the Cape, when the magnetic observatory 
was one of those under my superintendence, were published in 1851, with a dis- 
cussion of certain of their results; and the disturbance-variation of the declina- 
tion at the Cape has since been deduced by my assistant, Captain Younghusband, 
Phil. Trans. 1853, Art. VI. Since the transfer to Mr. Maclear, Mr. Pierce 
Morton, a gentleman of considerable mathematical attainments, who has been 
added as an assistant to Mr. Maclear in that branch of the Cape observations, 
has applied himself to the investigation of the lunar magnetic influence (as derived 
from the Cape observations), with a view of presenting the results to the Royal 
Society. For this, and other deductions,—such as, for example, the laws of the 
disturbances of the inclination and total force,—he will have the entire series of 
observations, viz. those as above-stated already published, and those which have 
been made since the transfer of the Observatory, up to the present time, 
