417 
INEQUALITIES OE TERRESTRIAL MAGNETISM. 
DIURNAL INEQUALITIES IN THE HORIZONTAL PLANE. 
I will now call attention to the curves which represent the laws of diurnal inequalities 
of force in the horizontal plane. And, first, the curves (Plate XXXIII.) which represent 
the inequalities in different months (the same nominal months in all the years being 
combined, and their means being taken), for the period 1858-1863, agree very closely 
with those for the period 188-1857. But, secondly, the curves (Plate XXXIV.), which 
represent the diurnal inequalities in different years, have undergone in the course of the 
period 1858-1863 a most striking change. In the years 1858 and 1861 they are similar 
to those of the winter months; in the years 1859, 1860, 1862, 1863 they resemble those 
of the summer months. And, on comparing these curves with those in the former paper, 
the following order of changes will be seen. From 1841 to 1847 the magnitude and 
summer character of the annual curves had slightly increased; but from 1848 to 1857 
they rapidly diminished, giving the smallest and most winter-like curves in 1856 and 
1857. The curves in the present paper have risen to the summer form steadily (with 
the exception of a little irregularity in 1861), and have at length sensibly attained to 
their original character. Thus 
1858 
nearly resembles 1856 
1859 
55 
„ 1851 
1860 
„ 1850 
1861 
55 
„ 1851 
1862 
„ 1847 
1863 
„ 
„ 1841. 
In other words, the magnetic action of the sun, which had during several years 
greatly diminished, has now increased till it has attained sensibly the same energy as 
before. 
DIURNAL INEQUALITIES OF VERTICAL FORCE. 
I now advert to the changes in the diurnal inequality of Vertical Force. In these, 
perhaps, the uncertainty connected with temperature-correction is even greater than for 
the Horizontal Force; but as the same instrument and the same numerical correction 
are adopted throughout, little uncertainty can attach to the comparisons of different 
years. On comparing the month-curves (Plate XXXV.) of the period 1858-1863 with 
those of the period 1848-1857, I think it appears that the curves of autumn are not 
now quite so bold as in the former period ; there is not, however, any remarkable dif- 
ference. The times, however, of nodal passage (or points where the actual Vertical 
Force = mean Vertical Force for the day) are not quite the same, as will appear from 
the following Table, in which the hours &c. are all referred to Greenwich Time. 
3 k 
MDCCCLXIX. 
