HORIZONTAL FORCE OE THE EARTH’S MAGNETISM. 
397 
We perceive at once that the observed variations of daily mean horizontal force did 
not show the same regularity in the oscillations as in 1844; considerable disturbances 
seem to have occurred at irregular intervals destroying the symmetry of the movements. 
It could scarcely be expected that any calculation in which the same quantities are 
employed successively from 12 to 14 times during the year could give an approximation 
to the observed movements throughout the period. I believe, however, it will be seen 
that the resemblance of the calculated and observed results is very considerable ; and 
although at first sight the agreement of the red and black oscillations may not appear 
so satisfactory as in 1844, yet a more minute examination will show the frequent coin- 
cidences, which are all the more extraordinary that the irregularities throughout the 
year appear so marked. The variations due to each period in 1845 are projected below 
the movements for June. The maximum solar action occurs about 4 or 5 days earlier 
in 1845 than in 1844, the supposed duration of the period being 26 days; this may be 
due to a change of the solar meridian producing the maximum, or to the action of 
superposed irregular disturbances (which, there can be no doubt, have an effect on the 
mean result), or to the true period being slightly less than 26 days*. 
The lunar action has also undergone a change, since a maximum occurs both near 
new and near full moon ; this result is also affected by the irregular variations. I shall 
notice immediately the fact that the moon’s action in producing the magnetic variations 
appears to depend upon the sun. There is in 1845, as in 1844, a maximum both when 
the moon is near her greatest northerly and her greatest southerly declination. 
It is not my object here to enter into the differences of the results for each period as 
derived from each of the two years’ observations, or to seek whether these differences 
may not be due to the greater number of irregular movements which occurred in 1845 ; 
the consideration of these questions must include the study of a longer series of obser- 
vations ; yet I may add that it seems to me the least probable supposition that the 
meridian of maximum solar action which remained constant throughout 1844 had 
changed in 1845. 
The conclusion deduced from the curves for 1844, of the constancy of the solar and 
lunar actions during twelve months, is confirmed generally, I think, even with the super- 
posed sudden movements of 1845. 
Magnetic Pulsations. 
The view generally entertained of the source of the great magnetic disturbances is, 
that they are produced by some action of the sun ; this conclusion is founded on the 
diurnal law of frequency and magnitude which these movements follow. The diurnal 
law, however, appears modified by conditions of latitude ; the maximum disturbance 
which occurs near midnight in high latitudes happens near noon within the tropics. 
I have shown, however, that the great changes of daily mean horizontal intensity are 
experienced similarly at all the stations considered between 56° North and 42° South 
* See Proceedings of the Royal Society, vol. xx. p. 421. 
