ON BRITISH COLONIAL MAGNETIC OBSERVATORIES. 359 
affecting in the same way, and to the same amount at the same time stations 
thus widely separated and contrasted. 
When the lines which thus represent the differences between the mean 
diurnal variation in the year, and in each of the months of the year, are 
drawn, they are at once seen to separate themselves into two well-charac¬ 
terized groups, one consisting of the six months when the sun is north of the 
equinoctial line, and the other of the six months when he is south of the 
equinoctial line. This is found to be the case alike at stations within the 
tropics and at stations in the temperate -cone, and not in one hemisphere 
only, but in both hemispheres. 
In every case the six months from April to September, and the six months 
from October to March, forming the two distinct groups, may be repre¬ 
sented by mean semiannual lines from which the individual months com* 
posing the respective groups differ comparatively very slightly, whilst the two 
semiannual lines, us compared with each other, exhibit very marked distinctive 
characters, particularly at the hours when tbu sun is abovu the horizon; and 
these characters are essentially the same in all the varieties of geographical 
position which T have enumerated. 
For the purpose of illustrating this remarkable fact, for the knowledge of 
which we are entirely indebted to the Colonial Observatories, I have placed 
m a diagram * the corresponding phenomena at St. Helena, an island in the 
middle of the ocean and within flic tropics,—at Toronto in Canada, in the in¬ 
terior of a great continent, in 4-3° north latitude,—and (it Hobarton in Van 
Diemen Island, in 43° south latitude. The lines in this diagram are in each 
rase drawn from five years of hourly observation. The plain or black line di¬ 
stinguishes in each of these three figures the phenomena of the same period of 
th« year, namely the six months from April to September; and the dotted line 
the other semiannual period, viz. from October to March. The horizontal line 
in each figure is the mean diurnal variation in the year, and the upper side of the 
horizontal line corresponds in every case in an easterly, and the lower side 
to a westerly deviation from it. In viewing these throe figures, it is scarcely 
possible to doubt that they represent substantially fine and the Baiue pbteno- 
toonon, The magnitude and the inflections of the curves aro not indeed ab¬ 
solutely identical, but they approach so near to it, that wc may well suppose 
die small differences to be very minor modifications, which may some day 
receive their explanation in minor modifying causes. 
Now, pliffinoinena so ucarly identical in character and amount, and yet pro¬ 
duced simultaneously at stations so widely dissimilar in geographical position 
jted climatic relations, must have other causes than the very dissimilar varia- 
'°iis of temperature which take place at those stations in the course of the year, 
ituated in opposite hemispheres, the summer of Toronto is the winter of 
obarton, and vice versa ; whilst at St. Helena (in the tropics) the distinction 
0 summer and winter almost ceases; the sun being vertical twice in the 
J car, viz. in February and November, and the epochs of maximum and qri- 
1Im “ m °* temperature being very different from those at either of the extra- 
ropical stations. The sun's passages of the equator from north to south at 
e September equinox, and from south to north at the March equinox, are 
viously the approximate epochs of transition from one class of phenomena to 
ie other, alike at each of the three stations. The influence of the epoch is the 
In ®* similar effects are produced, whether the station itself be north or 
"it of the equator, and however diverse may bo its climatic or its magnetic 
motions. '['fie three stations include a difference of latitude of above 86°, 
No* 4 Rc J ro .J, C6,i hom the “ Proceedings of the Royal Society," May 18tb, 1854 (vol. 
