400 
ME. J. A. BEOUN ON THE VAEIATIONS OE THE 
If any doubt existed as to the possibility of these being mere accidental coincidences, 
it would be removed, I think, by a consideration of the marked succession occurring 
between July 31st and December 11th, 1845. It will be observed that though consi- 
derable magnetic changes occur successively near the same solar meridian, yet that they 
do not always occur in successive rotations ; thus a great disturbance commenced at 
+ 8, March 28, 1844, but no other above the limit appears till July 8th at the same 
solar meridian. For this reason the succession mentioned merits particular consideration. 
If we neglect the two cases of July 31 and August 26, which commence at -f-6 and +5 
respectively, we have five cases of successive solar rotations in which the diminutions of 
intensity began on the +8 day*. This exact recurrence at the end of 26 days of the 
marked diminution of force proves, it seems to me, that the actions are all due to the sun, 
whose time of rotation must be very nearly 26 days. 
An examination of the projected means will show that the sudden diminutions of 
terrestrial magnetic force are in nearly every case preceded by a sudden increase. It 
seems extremely difficult to explain these regular pulsations, which are felt all over the 
earth f, by any theory of distribution of magnetism on the solar surface, or by any 
variations of temperature on the earth, or to understand why the sudden increase should 
have ended and the rapid diminution of magnetic force should have begun exactly when 
the solar meridian +8 arrived at the same position relatively to the earth , without 
admitting that the earth itself had some part in their production — a part which might 
be due, however, merely to her passage through some ray-like, electrical emanation 
from the sun. 
Lunar Disturbing Action. 
Has the moon, then, no part in the production of these disturbances'? Since the 
earth’s magnetic force varies during the solar rotation, we may suppose that the moon’s 
magnetic intensity varies from the same cause ; but we can scarcely conclude that a 
change of the moon’s intensity with a period of 26 days could produce the comparatively 
large results found for periods of 29 - 5 and 2 7 ‘3 days. 1 have shown elsewhere that 
the moon’s action on the diurnal variation of the easterly horizontal force near the 
equator depends on the sun’s position. The moon when on a given terrestrial meridian 
draws the north end of a needle to the east when the sun is furthest north, and to the 
west when he is furthest south ; the lunar action on the needle is much the greatest 
during the day (whether the moon be above the horizon or below it), and the lunar 
* See Plate 38, where the diminutions in the latter half of 1845 at the +8 meridian are indicated by thick 
lines. It will he seen that considerable diminutions of intensity occurred near the + 8 meridian from the 
beginning of 1844 to the end of 1845, hut that marked diminutions near the zero meridian began in the last 
days of 1844. 
t See plate xxvii. Trans. Eoy. Soc. Edinb. vol. xxii., where the variations are given for Makerstoun, Trevan- 
drum, Singapore, andHobarton. It should be remarked that the projections in that plate are for the Gottingen 
astronomical day, whereas in the Plate illustrating this paper they are for the Makerstoun civil day. 
