Diurnal Variations for the Magnetic Declination. 



XXI 



Table 12. — Diurnal Variations of "Westerly Declination for each Month, as deduced from the Regular 

 Daily Observations made during the Four Years 1843 to 1846. 



Mak. 

 Mean 

 Time. 



Jan. 



Feb. 



March. 



April. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



h. m. 



12 10 



0-96 



0-74 



0-26 



1-90 



1-90 



4-21 



2-35 



1-53 



1-57 



0-99 



1-22 



0-94 



13 10 



1-02 



1-82 



0-73 



1-08 



2-18 



4-00 



1-97 



1-55 



1-23 



1-19 



1-84 



1-00 



14 10 



0-91 



1-99 



1-56 



0-32 



2-77 



3-56 



1-67 



2-07 



1-48 



1-50 



3-09 



1-92 



15 10 



Ml 



2-16 



0-45 



0-88 



2-69 



301 



2-43 



1-69 



0-74 



2-54 



2-79 



2-41 



16 10 



1-49 



1-83 



0-54 



1-78 



2-11 



1-79 



1-61 



0-53 



M3 



2-65 



2-36 



2-57 



17 10 



1-68 



1-78 



0-89 



1-05 



1-21 



0-68 



0-48 



0-30 



1-55 



2-89 



2-44 



2-13 



18 10 



2-46 



2-21 



1-19 



0-73 



0-22 



0-00 



0-24 



0-00 



2-13 



3-40 



2-57 



2-53 



19 10 



2-62 



2-51 



0-95 



0-00 



0-00 



0-32 



0-00 



0-12 



2-06 



3-00 



2-90 



2-47 



20 10 



2-54 



2-86 



0-68 



0-02 



1-31 



1-13 



1-04 



1-23 



3-01 



2-52 



301 



2-68 



21 10 



3-00 



3-50 



1-40 



1-70 



2-93 



311 



2-62 



2-85 



4-41 



3-49 



3-45 



2-81 



22 10 



4-00 



4-63 



3-43 



3-96 



5-70 



5-79 



502 



5-41 



6-82 



5-72 



4-92 



3-71 



23 10 



4-96 



6-23 



6-13 



7-37 



8-60 



8-92 



7-90 



8-32 



9-26 



8-07 



6-43 



4-94 



10 



5-53 



6-88 



8-18 



10-40 



10-33 



10-83 



9-82 



10-47 



10-45 



9-50 



7-55 



5-97 



1 10 



5-85 



7-11 



9-28 



11-29 



11-02 



11-61 



10-60 



10-83 



10-18 



9-41 



7-10 



6-02 



2 10 



4-89 



6-53 



8-02 



10-32 



10-30 



11-21 



9-76 



9-79 



8-89 



8-30 



6-44 



5-27 



3 10 



4-16 



5-03 



6-48 



8-44 



8-67 



9-80 



8-63 



7-36 



6-54 



6-65 



5-20 



4-03 



4 10 



3-76 



4-08 



4-77 



6-57 



7-17 



8-05 



7-14 



5-59 



4-34 



4-65 



4-28 



3-48 



5 10 



2-69 



3-31 



2-37 



4-24 



5-65 



6-45 



6-08 



3-77 



3-28 



3-43 



3-63 



2-77 



6 10 



1-97 



2-28 



1-31 



2-82 



4-98 



5-53 



4-97 



2-03 



1-87 



2-71 



1-96 



1-89 



7 10 



1-55 



1-88 



1-39 



1-76 



4-21 



5-26 



4-08 



2-28 



0-69 



2-39 



1-38 



1-41 



8 10 



0-83 



0-70 



0-77 



1-56 



3-32 



5-02 



3-45 



2-30 



000 



0-64 



0-48 



0-01 



9 10 



0-28 



0-32 



0-27 



1-52 



3-30 



4-65 



2-80 



1-54 



0-68 



0-74 



0-18 



0-29 



10 10 



0-36 



0-00 



0-51 



1-51 



3-14 



4-38 



2-57 



1-38 



0-95 



0-29 



000 



0-23 



11 10 



0-00 



0-89 



0-00 



Ml 



2-42 



4-04 



2-29 



1-81 



0-94 



0-00 



0-30 



0-00 



28. From Table 12 we find that the north end of the declination magnet is most westerly throughout the year 

 between 0^ 20™ p.m., and 1"^ 25™ p.m., the epochs in apparent time for each month being as follow : — 



Jan. 

 0'' 50™ 



Feb. 

 Qh 50™ 



March. 

 0^1 55™ 



April. 

 Ih W 



May. 

 Ih 15m 



June. 

 Ih 25m 



July. 

 Ih Qra 



Aug. 

 Qh 50™ 



Sept. 

 Qh 30™ 



Oct. 

 Qh 45m 



Nov. 



Qh 35™ 



Dec. 

 0^ 50™ 



It appears, therefore, that the maximum westerly declination occurs farthest after apparent noon in the months 

 of April, May, and June ; and that it occurs soonest after apparent noon in September, October, and Novem- 

 ber. 



29. The north end of the declination magnet is most easterly from April to August, from 6*^ to S'^ in the 

 morning, and from September to March from S** to 11"^ in the evening; the approximate epochs in apparent 

 time are as follow : — 



Jan. Feb. March. April. May. June. July. Aug. Sept. Oct. Nov. Dec. 



10'» 50™ 9^ 45™ 10*' 50™ 19^ 40™ 18'' 50™ 18"^ 20™ 19^ 5™ 18^ 25™ 8^ 10™ 11^^ 10™ 10i» 20™ 9^ 50™ 



30. These epochs are considerably less certain than those for the maximum, especially when they occur be- 

 I tween 9^ 10™ and I?'' 10™, as they depend upon only two years' observations. The principal minimum occurs 



between 8'' and ll'' p.m. in the months from September till March, in the latter month the westerly declina- 

 tion at 8** A.M. differs little from that at 11'' p.m. : in the remaining months the minimum occurs between 



'; 6'' 20™ and 7^ 40™ a.m. The morning minimum occurs earliest in June and August ; the evening minimum 



I occurs earliest in September. 



31. Secondary maxima and minima of westerly declination are shewn with moderate distinctness in some 

 months, but the epochs vary so much from one month to the next as to render it doubtful whether they are other- 

 'wise than accidental : clearer results may be expected from the combinations of the means for two or three 

 months, if sufficient care be taken that only those months are combined which exhibit' separately similar cha- 

 racteristics. A careful examination of the projected means, seems to shew the combinations employed for the 

 following Table, as those best fitted for exhibiting distinctly the changing character of the diurnal variation. 



