Monthly Variations for the Magnetic Declination. 



XIX 



21. Variations of Daily Mean Westerly Declination with reference to the Moon's Age and Declination. — 

 Table 9 has been formed from tbe 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 cases, 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 the 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 probability 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 with reference to the Moon's Age and Declination. 















After 













Moon's 

 Age. 



1843. 



1844. 



1845. 



1846. 



Mean. 



Moon 

 farthest 

 North. 



1843. 



1844. 



1845. 



1846. 



Mean. 



d. d. 



- 



, 



'! 



/ 



/ 



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 



11-13 



13-68 



13-88 



12-19 



16—19 



8-91 



12-53 



14-70 



11-70 j 



11-96 



6— 9 



11-92 



14-08 



13-89 



12-96 



13-21 



20—22 



9-75 



13-16 



1210 



13-82 



12-21 



10—13 



10-42 



17-44 



12-31 



12-49 



1317 



23—26 



11-00 



1608 



12-00 



13-16 



13-06 



22. Variation of the Diurnal Range of Magnetic Declination ivith 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, — 



1st, 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 ; the mean from 

 1844 and 1845 being 14'-04. 



The value of the means of ranges is in all cases taken from 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 

 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 

 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 southei-ly declination than at the 

 intermediate periods ; being greatest at the two epochs when the body is rather north of the equator. The 

 variation of diurnal range in the lunar periods is as great as, or greater than, for the year, the means of 12 parts 

 of 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 

 Hours, with reference to the Moon's Age and Declination. — The results for the two years 1844 and 1845 from 

 Table XV., 1844, p. 347, and Table XII., p. 7 of this volume, are given in Table 11. 



The conclusions from Table 11 are almost identical with those from Table 10. 



MAG. AND MET. OBS. 1845 AND 1846. e 



