542 MR. W. ELLIS ON THE RELATION BETWEEN MAGNETIC DIURNAL 
the general laws and phenomena of magnetic action. Thus the variations of declina¬ 
tion, and of the horizontal and vertical components of the magnetic force, each include 
a well-marked diurnal period, analogous in some degree to that of atmospheric tem¬ 
perature ; and, speaking generally, the range of the diurnal variation at Greenwich is, 
in each case, greater in summer than in winter. In addition, however, to the annual 
inequality in the magnetic diurnal range there appears to be yet another, also of 
marked character but of longer period, one which resembles in its features the 
apparently well established eleven year sun-spot period, and which it is the object of 
the present paper to evolve. 
This is not by any means the first time that the relation alluded to has been dis¬ 
cussed. The investigations of General Sir E. Sabine, Professor Balfour Stewart, 
and Mr. Broun in our own country, and those of Professor Lamont and Dr. Wole 
among' foreign workers, will immediately occur to all who may be in any way acquainted 
with the literature of the subject. But it appeared to me that the long series of Green¬ 
wich observations might be applied as a valuable independent test of the accuracy of 
the generally received relation. For (as regards the results made use of in this paper) 
the observations at Greenwich have been throughout made on the same general plan, 
and with the same instruments ;* the results are therefore well adapted for use in an 
inquiry of the kind, and the conclusion arrived at is one in which it is reasonable to 
suppose that considerable confidence may be placed. 
To proceed now with the subject. The indications of the declination magnet are 
not directly affected by temperature, but those of the horizontal force magnet are so 
affected. The correction applicable in the case of the latter instrument has been at 
various times determined by different processes with fair general accordance of results, 
and no error of importance is likely to have been by this cause introduced. Moreover, 
as regards the present inquiry, since the effect of any such small error would simply 
be to only slightly raise or depress parts of the horizontal force curve as figured in the 
diagrams, the general deductions of this paper would in no degree be affected. The 
indications of vertical force are for the present object not very manageable; several 
different instruments have been employed during the period under discussion, and the 
results present some anomalies which are possibly in part instrumental. Our attention 
at present is therefore confined to a discussion of the inequalities of declination and 
horizontal force. 
The mean diurnal range of declination in each individual month is taken to represent 
(relatively to other months) the magnetic energy of the month. And similarly for 
* It is true that a new magnet (precisely similar in dimensions to the old one) was brought into use at 
the beginning of the year 1865 for photographic registration of the variations of declination; but its 
indications are compared four times daily with those of the old magnet (still used for determination of the 
absolute declination), so that the complete correspondence of the whole series of observations is thereby 
assured. Eor horizontal force the same identical magnet was used throughout. 
