398 
ME. J. A. BEOTJN ON THE YAEIATIONS OE THE 
latitude where observations had been made*. When a great diminution of mean daily- 
force occurs at one station, a similar and nearly equal diminution occurs at the others. 
If this diminution be due to a solar action only, such as, it has been supposed, might 
result from some considerable movements in the sun’s envelopes, and if the moon 
has no part in the production of the resulting changes of terrestrial magnetism, the 
means obtained for the solar period will be affected by true solar actions, while those 
for the lunar periods would be affected by changes in the production of which the moon 
took no part. Were this certain there would be reasons for omitting great and sudden 
departures from the mean in the discussions for the moon’s periods, which do not exist 
in those for the sun. As the mean variations due to the moon’s action appear so consi- 
derable when compared with those due to the sun, it does not seem at all certain that 
the great magnetic disturbances are wholly solar. An effort is here made to examine 
this subject. 
We have seen that when one side of the sun is presented to the earth, the magnetic 
force of the latter is greater than when the other side is turned towards us ; we may 
even say that the intensity is greatest for a given solar meridian ; this, however, may be 
simply an integral effect resulting from the actions due to all the meridians. But can 
we suppose when a great and sudden increase or diminution of the earth’s magnetic 
force occurs that this is produced by some change occurring on a particular solar meri- 
dian \ This does not seem at all improbable. It could scarcely, however, he supposed 
that the effect produced on the earth’s magnetism could be due to the given solar 
meridian being presented to (that is, in the same plane with) the earth, unless we admit 
the idea that these solar magnetic actions are propagated only in certain definite 
directions, or that the earth has some action in the production of the solar change when 
the given meridian is presented to it. 
In order to examine the facts, all the cases were noted during the years 1844 and 
1845 in which the daily mean horizontal force diminished one thousandth of its whole 
value within an interval of three days ; they were found to be twenty-eight in number. 
If we call the solar meridian presented to us on the 1st January, 1844, the zero meridian 
(0), and consider the time of rotation to be 26 days, and that there are 26 meridians, 
we find that the solar meridians presented to us when these great movements occurred 
may be arranged in a few groups, as in the following Table. 
* Trans. Eoy. Soc. Edinb. vol. xxii. p. 545. 
