DiuENAL Variations foe the Magnetic Declination. 



XXIX 



The least and greatest values of the probable error of an observation from the monthly mean of the hour, 

 Makerstoun in 1844 and 1845, were approximately as follow : — 



for 



Winter, Least Probable Error, 5^ a.m. = 0'-8 



Spring, yiiA.M. = 0'-8 



Summer, 6^' p.m. = 0'-6 



Autumn, 5^ p.m. = 0'-9 



Greatest Probable Error, 6^ p.m. = I'-S 



9>^ p.m. = l'-7 



1^ a.m. & 11"^ a.m. 



gi'p.M. = l'-6 



= r-1 



It is obvious, however, that even at the same place the probable error will vary with the year. In 1847, 

 the pi'obable eri'or of an observation would have been greatly increased by the excessive magnetic storms of 

 that year : neglecting these rare and excessive disturbances however, the values given above cannot be far from 

 the truth. 



Variation of Magnetic Declination with reference to the Moon's Hour-Angle. — The following Table has 

 been constructed from Table XI. 1844, p. 342, and Table IX., p. 6, of the present volume. 



Table 20. — ^Variations of Magnetic Declination with reference to the Moon's Hour- Angle for the 

 Winter and Summer Lunations, and for all the Lunations of the Years 1844 and 1845. 



Moon's 



Winter Lunations. 



Summer Lunations. 



All the Lunations. 



Hour- 





















Angle. 



1844. 



1845. 



Mean. 



1844. 



1845. 



Mean. 



1844. 



1845. 



Mean. 



h. m. 







-0-35 



-O-ll 



-0-23 



+ 003 



+ 0-36 



+ 0-19 



-0-21 



+ 0-10 



-005 



2 25 



-0-07 



-002 



-0-04 



+ 0-29 



+ 0-27 



+ 0-28 



+ 0-11 



+ 0-11 



+ 0-11 



4 20 



+ 0-30 



-003 



+ 0-13 



+ 006 



+ 0-34 



+ 0-20 



+ 0-19 



+ 0-13 



+ 016 



6 15 



+ 0-30 



+ 002 



+ 0-16 



-0-04 



+ 0-18 



+ 0-07 



+ 0-13 



+ 0-09 



+ 0-11 



8 10 



+ 0-38 



+ 0-17 



+ 0-27 



-0-18 



-0-39 



-0-29 



+ 0-10 



-0-08 



+ 0-01 



10 5 



+ 0-40 



+ 0-27 



+ 0-33 



+ 0-16 



-0-11 



+ 0-03 



+ 0-33 



+ 0-10 



+ 0-21 



12 



-1-0-49 



+ 0-28 



+ 0-36 



+ 0-40 



-0-28 



+ 0-06 



+ 0-45 



+ 0-02 



+ 0-23 



13 55 



+ 0-23 



+ 0-16 



+ 0-19 



+ 002 



-0-08 



-0-03 



+ 0-13 



+ 005 



+ 0-09 



15 50 



-003 



-0-02 



-003 



-0-49 



-0-33 



-0-41 



-0-25 



-015 



-0-20 



17 45 



-0-54 



-0-26 



-0-40 



-0-32 



-0-14 



-0-23 



- 043 



-0-21 



-0-34 



19 40 



-0-69 



-0-15 



-0-42 



+ 0-08 



-0-16 



-0-04 



-0-30 



-016 



-0-23 



21 35 



-0-35 



-0-28 



-0-32 



-0-04 



+ 0-31 



+ 0-13 



-0-24 



-0-01 



-012 



47. It appears from this Table, that the mean declination varies with the moon's hour-angle, as follows : — 



\st. In winter (when the moon is in opposition north of the equator) the maximum of westerly declina- 

 tion, for this variation, occurs when the moon is on the meridian of ll'', or about an hour before the inferior 

 transit ; the minimum occurs between 4'' and b^ before the superior transit. The group for each year gives 

 almost exactly the same result, but the range of the variation in 1844 was r-2, while in 1845 it was under 0'"6. 



2d, In summer the declination needle has a double easterly and westerly motion. 



The maximum westerly declination occurs about 2^ hours after the superior transit. 

 The minimum westerly declination occurs about 6 hours before the superior transit. 

 A maximum westerly declination occurs at the inferior transit. 

 A minimum westerly declination occurs about 8 hours after the superior transit. 



The results for the summer lunations in the two years agree to a considerable extent ; the maximum at 

 the inferior transit, however, is not nearly so distinctly marked in 1845 as in 1844. The range of the varia- 

 tions for the summer kxnations in the two years, is for 1844 = 0'-9, for 1845 = 0''8 nearly. 



3c?, The result from the lunations during the whole year, is a combination of the two results previously 

 given. The principal maximum occurs at the inferior transit, and the principal minimum about 6 hours after it. 



Several single lunations confirm the accuracy of these conclusions. See the volume for 1844, pp. 342, 343. 



