640 
LIEUT.-COLONEL SABINE ON THE ANNUAL VARIATION OF THE 
from a similar reason, may also be grouped together, the other extremity, — in the 
early morning hours, and until from 8^* to 10^* a.m., about which time the two groups 
are respectively transferred, each towards the opposite quarter to that which it pre- 
viously occupied. The period at which this transfer takes place is somewhat earlier 
at Toronto and St. Helena than at the Cape and Hobarton. 
The comportment of the two equinoctial months, March and September, at the 
Cape of Good Hope and St. Helena, presents a contrast to that of the two solstitial 
groups which have been just described, and at the same time the two months are 
remarkably contrasted with each other. At the Cape, September is found exclusively 
on the east side of the mean line throughout the twenty-four hours, except at the 
observation hours of 34’“ and C 34'“ ; and March as exclusively on the west side 
of the mean line throughout the twenty-four hours, excepting only at the observation 
hour of 2*’ 34’“. At St. Helena also September is found on the east side of the mean 
line at all hours of the twenty-four, except for one hour before and one hour after 
noon, when it is slightly on the west side; and March is found on the west side of 
the mean line with the exception of the hours from noon to 6 p.m. April participates 
with March in the peculiarity thus described both at the Cape and St. Helena. April 
is on the west side of the mean line at the Cape at all hours without exception, and 
at St. Helena at all hours with the exception of 11 a.m. and noon. 
I abstain from dwelling on other particulars, fearing that, not possessing any 
clue towards an explanation of these remarkable phenomena, I may do harm rather 
than good by contributing to give an undue significance to what may not prove the 
most important features. I will only permit myself therefore to point out one or two 
practical considerations suggested by the facts now under notice. 
1. Recent researches in meteorology have shown us, that when we confine our 
attention to the mean annual and mean diurnal values (of the temperature for example) 
in different parts of the globe, we derive but a small part of the instruction which the 
observation of nature is capable of affording. A similar remark applies with equal 
justice to the consideration of the variations of the magnetic elements. In the Annual 
Variation now before us, even a very cursory inspection is suflScient to show, that as 
the same months occupy positions on opposite sides of the mean line at different 
parts of the twenty-four hours, the mean Annual Variation, or that whieh is shown by 
the mean values in each month taken from all the observation hours, must be merely 
a residual quantity ; and that consequently natural features must be more or less 
masked in deduetions in which only mean values are brought into view. In fact, 
as has been shown in the published volumes of the observations at St. Helena and 
Hobarton, the mean annual variation at those stations is so small as to be scarcely 
sensible, more particularly at Hobarton, where its whole range amounts to not more 
than a small fraction of a minute of arc. But when we resolve these mean results 
into their respective constituents, viz. the Annual Variation at each of the observa- 
tion hours, there is then at once disclosed to us an order of natural phenomena. 
