54 
LIEUT.-COLONEL SABINE ON THE DIURNAL VARIATION OF 
with gilt paper, and the boxes are kept firm in their place by screws which pass 
through the cross piece connecting the copper pillars, and resting upon the top of 
the outer box.” 
On a careful comparison of the observations before and after this change was made, 
no material difference is perceived which can be ascribed to currents of air produced 
by either form of the magnetometer box. The law of the diurnal variation is the 
same whether it be deduced from the observations of the first period of 2^ years, or 
from the second period of the same duration. The extent of the arc which the mag- 
net passes through in the twenty-four hours as a consequence of the diurnal varia- 
tion, whether it be measured by the difference of the extreme east and west positions 
of the magnet, or by the sum of the fluctuations observed from hour to hour, is not 
the same in different months of the year, and is also found to differ to a small amount in 
the same months in different years : but these are obviously real differences depending 
probably on occasional inequalities in their magnetic causes, and appear equally in 
either series considered separately. The law of the variation in each month may 
generally be derived from the observations of any one year of the series, but its 
average amount is more correctly derived from a mean of several years. I have 
therefore employed in this communication the mean of the five years, but without 
burthening the paper by inserting the details of the separate series. 
The diurnal variation which we obtain from the observations of the five years at 
St. Helena, shows that that station was well-chosen for the purpose of contributing 
to the solution of the problem in question: but the solution to which it conducts is 
of a very different character from that which was anticipated, and is one which seems 
not unlikely to assist materially in the eventual elucidation of the physical causes of 
the periodical variations. We have seen that in the northern portion of the globe the 
magnet moves to the east until seven or eight o’clock in the morning, and then re- 
turns to the west ; it does so in every month of the year ; the extent of the move- 
ment is greater in the summer than in the winter months, but the direction is always 
the same. So in the southern portion of the globe, the movement in the contrary 
direction is also constant throughout the year; it is greater when the sun is in the 
southern signs than when he is in the northern signs, but the direction is the same 
in all the months of the year ; an extreme of westerly elongation is reached in every 
month about the hour of 8 a.m., as is an extreme of easterly elongation about the 
same hour in the opposite hemisphere. At St. Helena the well-marked peculiarity of 
the diurnal variation is, that during one-half of the year the movement of the north 
end of the magnet at the hours above referred to corresponds in direction with the 
movement which is taking place in the northern hemisphere, whilst in the other half 
of the year the direction corresponds with that which is taking place in the southern 
hemisphere. The opposite movements which take place simultaneously in every day 
of the year in the same meridians in the two hemispheres, do not by their mutual 
opposition neutralise each other and thus leave the magnet stationary. On the con- 
