14 REPORT—1845. 
inspection of these numbers, or, still better, by projecting them in a curve, that 
the mean disturbance follows a law of remarkable regularity, as depending upon the 
hour of the day. During the day, i. e. from 18 to 6 hours, it is nearly constant. 
At 6 hours, i. e. at the time of mean sunset, it begins to increase; it arrives at a 
maximum a, little after 10 hours; it then decreases with the same regularity, and is 
reduced to its constant day value, at about 18 hours, i. e. at sunrise. The maximum 
value at night is about double of the constant day value. 
The function whose values have been hitherto considered is independent of the 
direction of the disturbance. If, however, we take the sum of the squares of the 
easterly and westerly deviations separately, it is found that the easterly disturbance 
preponderates during the night, and the westerly during the day, the former being 
much more considerable than the latter, and the difference reaching a maximum 
about 10 hours. 
It thus appears that the tendency to disturbance observes a regular period, both in 
magnitude and direction, connected with the diurnal movement. In order to perceive 
their relation to the regular diurnal variations, it will be necessary to regard the latter 
in a somewhat different point of view from that in which they have been usually 
considered. From the very small amount of the regular change of declination, which 
takes place during the night, and from the manifest connexion of the day movement 
with the position of the sun, Dr. Lloyd said, that he was led to consider the position 
of the magnet during the night as its normal position, from which it was made to 
deviate during the day by the influence of the sun. In this point of view, the re- 
gular diurnal progression may be described, in its main features, as a westerly devia- 
tion of the north end of the magnet, commencing about an hour after sunrise, reach- 
ing its maximum a little after 1 p.m., and thence diminishing until a few hours after 
sunset, when the magnet returns nearly to its normal position. Now the mean 
disturbance, it will be remarked, observes a period nearly the reverse of this, both in 
magnitude and direction ; its value being nearly constant during the day, while it is 
largely developed during the night, in a direction opposed to that of the regular day 
movement. From these remarkable relations, which hold also between the changes, 
regular and irregular, of the horizontal intensity, it seems evident that the two 
classes of phenomena are physically connected. Without entering into the question 
of the mode of this connexion, Dr. Lloyd said that he regarded the disturbance of the 
two elements (in part at least) as an irregular reaction from the regular day move- 
ment, and dependent upon it both for its periodical character and for its amount. 
If this hypothesis be a just one, it will of course follow that the magnitude of the 
mean disturbance will vary in some direct proportion to the daily range, and should 
therefore be greater in summer than in winter. Now this (which is contrary to the 
results deduced by Prof. Kreil and Col. Sabine, with reference to the frequency of 
disturbances exceeding a certain limit) appears to be the fact. If we calculate the 
mean disturbance of the declination for the several quarters of the year, we fiad it to 
be as follows :— 
Spring. Summer. Autumn. Winter. 
2"66 3!-02 2!-52 180. 
From these results, it appears that the mean disturbance observes an annual as well 
as a diurnal period; its maximum occurring in summer, its minimum in winter, while 
in spring and autumn its values are nearly equal. This important relation appears 
to confirm, in a remarkable manner, the views above given. 
It by no means necessarily follows, from the results above stated, that the perio- 
dical character necessarily belongs to all disturbances. It may be that there are two 
classes of disturbances, the results of distinct physical causes, of which one observes 
a period, while the other is wholly irregular; for it is obvious that on such an hy- 
pothesis, the period of the former would necessarily be impressed upon the resultant 
disturbance, and that the latter would have no effect in effacing it, provided the ob- 
servations from which it was inferred were sufficiently numerous. There are many 
circumstances which seem to render this supposition highly probable; and if it be 
established, the next step in the investigation will be to distinguish these two kinds 
of disturbances by their external characters, and to resolve the complex resultant, 
where they happen to be combined, into its more simple elements. Dr. Lloyd stated 
that he had commenced a series of observations in Dublin, upon a plan which seemed 
