Horizontal Component of Magnetic Force. 



361 



TABLE XXVII. — Diurnal Variations of the Horizontal Component of Magnetic Force in 1844. 



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

 The horizontal component is a minimum at IC' 10™ a.m., after which hour it increases with its greatest 

 rapidity, attaining its principal maximum at t>^ 30™ p.m.; it then hegins to diminish, and continues to do so 

 with considerable regularity till 2'" 10'" a.m., when there is a secondary minimum ; it again increases slightly, 

 an<l becomes a secondaiw maximum at S*" 30'" a.m., after \vhieh it diminishes with its greatest rapidity to tlie 

 principal minimum at 10'' 10™ a.m. The range of the mean (hurnal variation is rather more than two thou- 

 sandths of the whole horizontal component (0-0022). An examination of the monthly means will shew, that 

 the two maxima and the two minima are each of nearly equal value in the winter months, the morning 

 maximum being the principal maximum in the month of December ; in the summer months, on the contrary, 

 the secondai-y maximum and minimum nearly or altogether disappear. There are several irregularities, 

 especially in the autumnal and winter months, which are evidently due to disturbances. The following are 

 the approximate times of the principal minimum ( — ) and maximum ( + ), and of the secondary minimum 

 and maximum : — 



— Min. A.M. 



+ Max. P.M. 



Min. A.M. 



Max. A.M. 



Jan. 



10''20'" 

 310 

 2 

 610 



Feb. 



210 



1 



6 



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



lOiilOm 9>'30n' ICIO™ 9140™ 10''0™ gi'SO™ lO'^lO™ Q'-SO"' ICSO"' 10''30™ 

 6-10 7-0 6-40 5-10 4-10 5-0 510 3-20 



6-10 



1 



6 



5-20 



3 



5 



4 

 5 



3 

 4 



1-30 

 3 



11 P.M. 



6-10 



1 A.M. 



6-40 



On the whole, it appears that the principal minimum occurs rather earlier in the day in the summer 

 months than in the winter months, and that the principal or afternoon maximum occurs rather later in the 

 day in the former than in the latter. 



In order to examine the diurnal variation when freed, as far as possible, fi-om the effects of iri-egular 

 causes, the hourly observations on the days previously selected. Table VII., have been used. The following 

 Tables contain diurnal variations for the ten days and for the five days in each month least affected by 

 disturbances. 



MAG. AND MET. OBS. 1844. 



4y 



