Monthly Vaeiations for the Magnetic Declination. 
XIX 
21. Variations of Daily Mean Westerly Declination with reference to the Moon's -dge and Declination. — 
Table 9 has been formed from tlie Tables given in the former volumes of observations, and in this volume, 
pages 2 and 29 ; the means of groups of 3 or 4 days are given positive when west, and negative when east of 
the mean. 
From Table 9 it appears, — 
1st, That the westerly declination is greatest about two days after full moon. 
2d, That it is greatest when the moon is farthest north. 
In both eases, the epoch of maximum only is well marked ; in the 3 or 4 days before it and after it, the 
declination is most westerly ; in all the rest of tlie periods the variations are inconsiderable. When the small- 
ness of the variations and the large effect of considerable disturbances are considered, the results of the separate 
years are sufficiently consistent, to give a considerable ])robability to the truth of the conclusions : in each 
year, the declination was more westerly about the time of the moon's greatest north declination, than for any 
other time. 
Table 10. — Diurnal Range of Magnetic Declination veith reference to the Moon's Age and Declination. 
Moon's 
Age. 
1843. 
1844. 
1845. 
1846. 
Mean. 
After 
Moon 
farthest 
North. 
1843. 
1844. 
1845. 
1846. 
Mean. 
d. d. 
'1 
d. d. 
14—16 
9-73 
16-22 
14-80 
13-41 
13-54 
27— 1 
10-84 
11-22 
13-99 
11-56 
11-90 
17—20 
9-73 
15-90 
16-20 
15-12 
14-24 
2— 5 
12-60 
15-48 
16-83 
12-16 
14-27 
21—24 
9-25 
11-59 
13-64 
12-94 
11-85 
6— 8 
9-21 
14-11 
14-69 
13-42 
12-86 
25—28 
11-20 
10-13 
12-44 
11-20 
11-24 
9—12 
9-36 
12-39 
13-47 
15-10 
12-58 
29— 1 
9-68 
11-53 
13-15 
12-56 
11-73 
13—15 
10-19 
12-05 
12-16 
14-01 
12-10 
2— 5 
10-07 
1113 
13-68 
13-88 
12-19 
16—19 
8-91 
12-53 
14-70 
11-70 
11-96 
6— 9 
11-92 
14-08 
13-89 
12-96 I 
13-21 
20—22 
9-75 
13-16 
12-10 
13-82 
12-21 
10—13 i 
10-42 
17-44 
12-31 
12-49 ! 
1317 
23—26 : 
11-00 
16-08 
12-00 
13-16 t 
13-06 
22. Variation of the Diurnal Hange of Magnetic Declination with reference to the Moon's Age and Declina- 
tion. — The means for groups of days given in Table 10 have been deduced from the tables in the present and 
former volumes. It results from Table 10, — 
\st, That the diurnal range is greatest (from the mean of 1844 and 1845, = 16'-05) about 2 or 3 days 
after the sun and moon are in opposition : that it is least about 3 days before they are in conjunction ; the 
mean of 1844 and 1845 giving for that epoch ll'-28, 
2d, That the diurnal range is greatest about 4 days after the moon is farthest north ; the mean at that 
epoch for 1844 and 1845 being 16'-15 : that it is least when the moon is farthest north, and about three days 
after it is farthest south ; the value from the means of 1844 and 1845 in both cases being about 12'-30 : that 
a secondary maximum of diurnal range occurs about 3 days before the moon is farthest north ; tlie mean from 
1844 and 1845 being 14'-04. 
The value of the means of ranges is in all cases taken fi'om the observations for 1844 and 1845, as the 
value for 1843 and 1846 is imperfect. 
23. The means for 1843 and 1846 give the same results as the means for 1844 and 1845 for both arguments. 
The observations for 1844 exhibited these laws (see No. 16.) with remarkable distinctness: several single 
lunations shewed the law with reference to the moon's age very clearly ; from these it appeared, that the varia- 
tion of the diurnal range was greatest for the lunations about the equinoxes and, in connection with the first of 
I the conclusions above, when the sun and moon were in opposition near the equator. (See volume for 1844 
|P. 336.) The conclusions from the 2d part of Table 10 are analogous to those already found. No. 12. for the 
I sun's declination, and the results for both bodies may be stated thus : — The diurnal range of magnetic declina- 
tion is less when the body (sun or moon) has its greatest northerly and southerly declination than at the 
{intermediate periods ; being greatest at the two epochs when the body is rather north of the equator. The 
j/ariation of diurnal range in the lunar periods is as great as, or greater than, for the year, the means of 12 parts 
|)f the period in the former being compared with those for the 12 months of the latter. 
! 24. Variation of the Mean Difference of a Single Observation from the Monthly Mean for the corresponding 
jlours, with reference to the Moon's Age and. Declination. — The results for the two years 1844 and 1845 from 
pable XV., 1844, p. 347, and Table XII., p. 7 of this volume, are given in Table 11. 
1 The conclusions from Table 11 are almost identical with those from Table 10. 
MAG. AND MET. CBS. 1845 AND 1846. 6 
