Diurnal Variations for the Magnetic Declination. 



XXV 



Throughout the year, therefore, the effect of disturbance in increasing the westerly declination is greatest be- 

 tween 8" A.M. and 2^ p.m. ; being near the latter hour for the months about mid-summer and mid-winter, and 

 n6ar the former hour for the intermediate months. 



2d, That the greatest effect of disturbance in decreasing the westerly declination occurs 



In Dec. Jan. Feb. March, April. 

 About 8^ P.M. 12'^ P.M. 6^ P.M. ll** P.M. 



May, June. July; -Aug. Sept. Oct. Nov. 

 11^ P.M. 12*^ P.M. 9^ P.M. IQl^ P.M. 8^ P.M. 11*> P.M. 



Sd, That the effect on the hourly mean westerly declination is zero 



In Dec. Jan. Feb. March, April. May, June. Jiily? Aug. Sept. Oct. Nov. 



About 5 J'^ A.M. and S;^*^ P.M. 41^ a.m. and S'' p.m. S** a.m. and 6|^'' p.m. 4^^ a.m. and 5^*^ p.m. 4^^ a.m. and 5^'' p.m. 



Diurnal Variation of frequency of Positive and Negative Excursions from the Hourly Mean Position. — The 

 number of observations which were to the west of the hourly mean for each month in 1844 and 1845 having 

 been obtained, the following Table was formed, containing the numbers per cent, for quarterly groups of months. 



Table 16. — Numbers of Excursions of the Declination Magnet in 100 which were to the West ; 

 1st, of the Hourly Means, as deduced from all the Hourly Observations in each Month of 1844 

 and 1845 ; and, 2d, of those deduced from the Selected Days. 



Mak. 

 Mean 



With reference to Mean of all. 



With reference to Mean of Selected 



Days. 



Nov. 



Feb. 



May. 



Aug. 





Nov. 



Feb. 



May. 



Aug. 





Time. 



Dec. 



March. 



June. 



Sept. 



Year. 



Dec. 



March. 



June. 



Sept. 



Year. 





Jan. 



April. 



July. 



Oct. 





Jan. 



April. 



July. 



Oct. 





h. m. 



12 10 



63-9 



66-7 



551 



601 



61-4 



39-9 



47-1 



36-1 



32-3 



38-8 



13 10 



67-7 



63-4 



60-8 



59-5 



62-8 



43-7 



490 



310 



31-6 



38-8 



14 10 



57-6 



60-8 



57-6 



53-8 



57-4 



46-2 



51-0 



40-5 



42-4 



45-0 



15 10 



58-9 



59-5 



47-5 



48-7 



53-6 



53-8 



51-0 



44-9 



42-4 



48-0 



16 10 



50-6 



50-3 



44-3 



500 



48-8 



46-8 



53.6 



36-1 



38-0 



43-5 



17 10 



53-2 



49-7 



42-4 



35-4 



45-1 



46-2 



55-6 



43-0 



47-5 



48-0 



18 10 



34-8 



45-8 



43-7 



26-6 



37-6 



58-2 



66-7 



50-6 



56-2 



57-7 



19 10 



37-3 



45-1 



42-4 



32-9 



39-4 



66-5 



65-4 



47-5 



58-2 



59-3 



20 10 



33-5 



39-9 



411 



361 



37-6 



67-7 



59-5 



56-3 



62-0 



61-4 



21 10 



36-7 



42-5 



43-7 



37-3 



400 



71-5 



70-6 



65-2 



61-4 



67-1 



22 10 



36-7 



34-6 



47-5 



43-7 



40-7 



70-9 



64-7 



69-6 



58-2 



65-9 



23 10 



40-5 



42-5 



46-8 



45-6 



43-9 



60-8 



621 



61-4 



66-5 



62-7 



10 



38-6 



39-2 



45-6 



45-6 



42-3 



67-7 



65-4 



65-2 



68-4 



66-7 



1 10 



40-5 



43-1 



44-3 



47-5 



43-9 



63-3 



68-0 



58-2 



65-8 



63-8 



2 10 



36-1 



431 



44-9 



41-8 



41-5 



64-6 



64-7 



63-3 



641 



64-1 



3 10 



46-2 



39-2 



47-5 



450 



44-5 



65-8 



66-7 



58-2 



53-8 



6M 



4 10 



43-7 



41-8 



49-4 



45-6 



45-1 



62-0 



70-6 



56-3 



57-0 



61-4 



5 10 



53-8 



536 



48-7 



50-6 



51-7 



59-5 



52-3 



41-8 



53-2 



51-7 



6 10 



65-2 



68-6 



47-5 



63-3 



611 



58-2 



49-0 



43-0 



53-8 



51-0 



7 10 



70-9 



69-3 



57-6 



63-9 



65-4 



48-7 



47-1 



38-6 



50-6 



46-3 



8 10 



72-8 



68-6 



61-4 



63-9 



66-7 



38-0 



45-1 



34-2 



38-6 



38-9 



9 10 



69-6 



65-4 



67-1 



690 



67-8 



54-4 



44-4 



40-5 



34-2 



43-4 



10 10 



69-0 



634 



61-4 



67-7 



65-4 



39-2 



44-4 



38-6 



29-1 



37-8 



11 10 



67-1 



60-8 



63-3 



67-7 



64-8 



36-7 



471 



35-5 



41. 1 



40-0 



40. The following are the epochs of maximmn and minimum frequency of the positive or westerly excursions. 



Nov. Dec. Jan. 

 Feb. Marcb, April. 

 May, June, July, 

 Aug. Sept. Oct. 



From the Means of all the Observations. 



From tl 



Min. 



Max. 



Min. 



8^ A.M. 



8^ P.M. 



llh P.M. 



10^' A.M. 



6^^ P.M. 



9^ P.M. 



6^^ A.M. 



9^ P.M. 



8^ P.M. 



6" A.M. 



9^^ P.M. 



\0^ P.M. 



From the Means of the Undisturbed Days. 



-1*^ A.M. 

 -l^' A.M. 



Max. 

 91t A.M. 



9^ A.M. 4*' P.M. 



10'' A.M. 



0^' Noon. 



