Vertical Component of Magnetic Force. 



379 



Diurnal Variation of the Vertical Component. — The following is the mean result for the year 1844: — 

 The vertical component is a maximum at 5 h 20 m p.m., after which it diminishes with considerable rapidity till 

 midnight, the principal minimum occurring about l h 30 m a.m. ; it then increases till 9 h 10 m a.m, when it is 

 a secondary maximum, becoming a secondary minimum at 10 h 40 m a.m., after which it increases to the princi- 

 pal maximum at 5 h p.m. The secondary maximum and minimum are distinctly marked, but the variation from 

 the former to the latter is inconsiderable, being only 0-000010, while the variation from the principal maximum 

 to the principal minimum is 0-000420, the whole vertical component being unity. 



The form of the diurnal variation of the vertical component varies more with season than that of the 

 diurnal variation for either of the elements already discussed. In winter the diurnal variation is single, the mini- 

 mum occurring about 5 h a.m. and the maximum about 6 h p.m. In the months near the equinoxes the range 

 of the variation is greatly increased, and its form is similar to that for the year given above ; the secondary 

 maximum and minimum become gradually more marked as the epoch approaches the summer solstice, till in 

 the months of June and July the principal minimum occurs about noon, the range of the diurnal variation hav- 

 ing diminished at the same time to nearly the same value as at the winter solstice. The following are the ap- 

 proximate times of maxima and minima in each month of the year, the principal maximum being indicated by + 

 and minimum by — 







Jan. 



Feb. 



March. 



April. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 







h. in. 



h. id. 



b. m. 



h. m. 



b. m. 



h. m. 



h. m. 



b. m. 



b. m. 



h. m. 



b. m. 



h. m. 



Max. 



P.M. 



+ 17-10 



6-10 



5-10 



610 



510 



6-10 



5-10 



6-10 



5-10 



3-10 



4-30 



6-10 



Min. 



a.m. 



-15-10 



3-10 



210 



1210 



1-10 | 



1-30 



1-10- 



| 110 



12-10 



4-10 



12-10 



510 



Max. 



A.M. 









9-10 



8-10 



8-0 



5-10 



8-10 



9-40 









Min. 



A.M. 









10-10 



11-10- 



11-20 



0-0 | 



o-o 



(0-50) 



P.M. 



... 





The principal maximum occurs earliest at the equinoxes : the principal minimum occurs farthest from 

 midnight at the winter solstice and nearest it as we approach the summer solstice. The principal minimum 

 actually occurs near noon at the summer solstice, but the minimum near midnight differs little in value. 



Adopting the method already pursued (see page 339; for the purpose of considering the diurnal variation 

 when freed as far as possible from the effects of irregular causes, the following Tables have been formed. 



TABLE XLIV. — 'Hourly Means of the Balance Magnetometer Micrometer Readings for the Ten 



Days least disturbed in each Month of 1844. 



Mak. 

 M. T. 



Jan. 



Feb. 



March. 



April. 



May. 



June. July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



| Year. 



li. 



Mir. Div. 



Mia, Div. 



Mi.-. l>iv 



Micnir, 



Mic Div. 



Mir. Div. Mic. Div. 



Mir. D,v. 



Mir. Div. 



Mic. Div 



Mic. Div. 



Mic. Div. 



1 Mic. Div. 



12 



780-9 



746-7 



728-3 



732-8 



696-0 



681-9 



6510 



626-3 



615-3 



610-8 



609-8 



598-4 



673-2 



13 



781-6 



746-3 



726-7 



730-2 



697-9 



681-3 



650-9 



625 



6 



616-4 



609-2 



609-7 



597-5 



672-8 



14 



781-4 



747-9 



729-8 



734- 1 



700-3 



681-5 



649-8 



627 



2 



619-3 



608 



5 



609-0 



597-6 



673-9 



15 



7790 



747-6 



730-3 



7371 



702-8 



685-0 



653-9 



630 



4 



621-5 



608 



4 



609-3 



597-7 



675-3 



16 



777-8 



747-6 



727-8 



7371 



705-6 



686-1 



657-6 



635 



2 



621-2 



609 







609-6 



597-5 



6760 



17 



777-9 



746-4 



728-5 



738-5 



707-9 



689-6 



658-8 



639 



7 



624-9 



609 



1 



609-6 



597-4 



677-3 



18 



777-1 



744-8 



729-7 



744-3 



709-5 



691-6 



664-6 



640 



6 



628-3 



609 



6 



6100 



5961 



678-8 



19 



7770 



742-2 



734-3 



743-3 



7090 



693-3 



664-0 



642 



8 



631-3 



613 



5 



610-6 



596-6 



679-9 



20 



780-3 



741-8 



737-8 



7460 



709-7 



6931 



6611 



642 



3 



634-3 



616 



8 



611-5 



599-4 



681-2 



21 



782-4 



739-9 



736-8 



746-3 



705-7 



686-3 



657-9 



638 



1 



633-3 



617 



3 



614-5 



599-6 



679-8 



22 



784-4 



739-6 



7311 



741-6 



700-2 



680-4 



655-1 



632 



7 



6291 



614 



1 



614-3 



599-3 



676-8 



23 



787-1 



739-6 



728-9 



735-4 



697-1 



672-6 



646-9 



627 







622-5 



611 



2 



615-5 



599-9 



673-6 







786-1 



742-8 



7280 



726-2 



696-1 



674-6 



644-3 



623 



5 



620-6 



610 







615-3 



600-5 



672-3 



1 



786-0 



745-9 



729-8 



731-3 



701-3 



672-2 



645-3 



626 



5 



625-2 



611 



1 



617-6 



6011 



674-5 



2 



788-2 



747-3 



733-2 



738-6 



705-0 



6770 



646-3 



633 



1 



631-9 



614 



6 



620-8 



602-9 



678-2 



3 



787-6 



750-3 



7370 



739-1 



708-6 



680-7 



652-3 



637 







634-7 



619 



3 



6230 



604-5 



681-2 



4 



786-5 



7520 



746-2 



741-7 



712-2 



688-2 



657-9 



641 



5 



636-5 



621 



7 



622-6 



604- 1 



684-3 



5 



786-2 



748-2 



748-9 



743-4 



715-0 



691-4 



6610 



641 



6 



638-0 



619 



2 



620-3 



6030 



684-7 



6 



785-8 



746-3 



744-5 



743-7 



714-4 



693-3 



661-8 



640 



8 



633-5 



616 



8 



621-3 



602-6 1 



683-7 



7 



786-6 



747-7 



739-7 



742-4 



715-5 



691-3 



661-6 



637 







6311 



615 



9 



620-9 



602- 1 



682-7 



» ! 



789-1 



746-3 



738-3 



738-9 



716-1 



688-7 



661-6 



635 



7 



630-3 



615 



1 



622-1 



602-8 



682-1 



9 



790-7 



749-5 



739-6 



739-1 



7120 



687-9 



655-9 



633 



3 



624-4 



610 



6 



621-5 



603-1 1 



680-6 



10 



786-9 



748-5 



733-1 



735-8 



707-3 



683-5 



654-3 



630 



6 



620-9 



610 



8 



617-8 



602-9 1 



677-7 



11 



783-9 



744-7 



729-7 



733-9 



701-5 



682-2 



650-0 



629-2 



6151 



605-8 



614-4 



599-8 j 



674-2 



