164 



The Astronomer Royal on the 



[Dec. 10, 



curves also, the nodal passage in both periods is earliest in the summer 

 months ; but it is not quite certain whether the curves in autumn, in the 

 period 1858 to 1863, are quite so bold as those in 1848 to 1857 ; the dif- 

 ference, however, if any, is inconsiderable. After this, the monthly means 

 of every solar hour are taken through each year, giving the mean diurnal 

 inequality of each year ; and here a very remarkable change is observable. 

 To explain this, it is necessary to refer to the former paper, where it is 

 shown that 'the curves for diurnal inequality of the horizontal forces had 

 very slightly increased from 1841 to 1847, but had rapidly diminished 

 from 1848 to 1857, giving the smallest and most winter-like curves in 1856 

 and 1857. Now it is found that from 1858 to 1863 the curves have in- 

 creased, with a little irregularity in 1861, till they are sensibly as large as 

 they were at first. Thus — 



1858 nearly resembles 1856 



1859 „ „ 1851 



1860 „ „ 1850 



1861 „ „ 1851 



1862 „ „ 1847 



1863 „ „ 1841 



With regard to the diurnal inequality of vertical force, it appears that the 

 curves gradually increased in boldness to 1855, and have gradually dimi- 

 nished to 1862. The nodal passages, it was remarked in the former paper, 

 had been much accelerated in the hour of the day, from 1842 to 1857. 

 Now, from 1858 to 1863, the hours of nodal passages have been retarded, 

 till in 1863 they are again nearly the same as in 1848. In all these re- 

 markable changes there is no appearance of cycle. 



The author then proceeds to the treatment of lunar inequalities from 

 1848 to 1863. The bases of their treatment were thus obtained: the 

 exact time of moon's transit was laid down on the time-scales of the 

 photographic sheet, and the intervals were divided into lunar hours, and a 

 new system of ordinates, corresponding to the lunar hours, was measured 

 to the pencil curves. The system of grouping was precisely similar, mu- 

 tatis mutandis, to that for the solar inequalities. First, for the menstrual 

 inequalities. The declination seems to exhibit a distinct lunar menstrual in- 

 equality, with + maximum about the fifth day of lunation ; the horizontal 

 force seems to show a lunar semimenstrual equation with — maximum 

 about the second day ; the vertical force shows nothing certain, proving 

 only that, if there is anything, it is very small. Secondly, for the luno- 

 diurnal inequalities. The luno- diurnal inequalities in declination and hori- 

 zontal force on the mean of 1858 to 1863 agree so closely with those on 

 the mean of 1848 to 1857, as to leave no doubt of their existence and law 

 as luno- semidiurnal inequalities, with no trace of luno-diurnal or other 

 inequality. 



Remarking the singular difference for different years which has presented 



