262 Proceedings of the British Association. 



The mean daily disturbance, deduced in a similar manner from 

 the mean disturbance corresponding to the several hours, is 2'-56. 

 It will be at once seen, from the mere inspection of these numbers — 

 or, still better, by projecting them in a curve — that the mean dis- 

 turbance 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 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, 

 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 in- 

 dependent of the direction of the disturbance. If however, we take 

 the sum of the squares of the easterly and westerly deviations sepa- 

 rately, 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 maxi- 

 mum about 10 hours. It thus appears, that the tendency to dis- 

 turbance 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 connection 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 regular 

 diurnal progression may be described, in its main features, as a 

 westerly deviation of the north end of the magnet, commencing about 

 an hour after sunrise, reaching 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 distur- 

 bance, 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, 



