56 
LIEUT.-COLONEL SABINE ON THE DIURNAL VARIATION OF 
between the effect of the opposite hemispheres in the one case, and of the opposite 
seasons in the other, whilst at the same time the subordinate points of dissimilarity 
are equally conspicuous. It is not the object of this paper to enter on the discussion 
of minor points of difference, such as the non-agreement of the turning hours, which 
are somewhat earlier at St. Helena than at Toronto in the one case and at Hobarton 
in the other, and the check which appears to take place in the western elongation in 
May and September at St. Helena towards the hour of noon ; but I may permit 
myself to notice, as a minor but apparently characteristic point of resemblance, the 
circumstance that the eastern elongation at the morning hour at Toronto, as well as 
in the corresponding season at St. Helena, always precedes by an hour the western 
elongation about the same period of the day at Hobarton and its corresponding 
season at St. Helena. This feature has a further tendency to connect a hemispherical 
peculiarity of daily occurrence throughout the year in each of the two hemispheres, 
with a periodical peculiarity at St. Helena conforming strictly to the alternation of 
the seasons. 
The projections for Toronto and Hobarton in fig. 3 represent each a mean of two 
years of hourly observations ; the scale on which they are drawn has been taken for 
convenience at half the magnitude of the scale on which the St. Helena projections 
in fig. 4 are drawn. 
We have hitherto considered those portions of the diurnal variation at St. Helena 
which include the hours of the forenoon and of the first part of the afternoon. We 
have seen that from 3 a.m. to 3 p.m. the movement of the magnet is strikingly dissi- 
milar, and is even opposite for a considerable portion of those hours, in the two sol- 
stitial periods of the year. A less marked but not less systematic difference takes 
place during the remaining hours, as is shown by the projections in fig. 4, which cor- 
respond respectively to the months from May to September, and from October to 
February. The diurnal variation at these hours, from May to September, consists 
in a small but continuous and steady motion of the north end of the magnet towards 
the east, commencing at 5 or 6 p.m., and continuing without interruption through the 
night until the following morning : whereas from October to February, the motion, 
which at first is in the same direction, is more considerable, and an inferior eastern 
extreme is reached about 9 or 10 p.m., to which there is nothing analogous at the 
other period of the year ; a return then takes place towards the west (contrary to the 
direction in the opposite season), and is thenceforward continuous until the forenoon 
of the next day. In the night portion therefore of the diurnal curve as well as in 
that portion which has been more largely discussed, the horary variation at St. He- 
lena does not disappear, but continues to exhibit a diversity at opposite seasons, in 
which an analogy may still he traced to the difference in the annual projections at 
Toronto and Hobarton at the same hours, shown in fig. 3. It should be noticed 
however that the correspondence which exists during the hours of the day between 
Toronto and those months at St. Helena which form the northern summer on the 
