Annual Variations for the Magnetic Declination. 



XVI 1 



The difference is greater for March and September than for the winter months, and it is least for June. 

 From this then we conclude (as in the volume for 1844, p. 334) that those irregular disturbances which render 

 the mean diurnal range greater than the range of the mean diurnal variation, have their maximum effect about 

 the equinoxes, and their minimum at the summer solstice. We are still ignorant of the law of disturbances as 

 affecting the position of the declination magnet at all portions of its diurnal motion ; to determine this, we may 

 consider the differences of the positions of the magnet at each hour, from its mean position at the same hour 

 for each month. 



Annual Variation of the Mean Difference of a Single Observation of Magnetic Declination from the Monthlt/ 

 Mean at the corresponding Hour. — These differences have been obtained for the years 1844 and 1845 only, and 

 the means for each month (from Table XIV., 1844, p. 346 ; and Table XIII., p. 8 of this volume) are as 

 follow : — 



Table 7. — Mean Difference of a Single Observation of Magnetic Declination from (1) the Hourly 

 Means of all the Observations, and from (2) the Hourly Means of Observations on Days selected 

 free from disturbance. 



Series. 



Year. 



Jan. 



Feb. 



March. 



April. 



May. 



.Tune. 



July. 

 1-43 



Aug. 



Sept. 



Oct. 



hov. 



Dec. 



Mean. 

 1-71 



■{ 



1844 



1-34 



1-40 



2-22 



1-87 



1-60 



M2 



1-71 



1-93 



2-26 



2-30 



1-40 



1845 



202 



1-82 



1-76 



1-63 



1-46 



1-20 



1-41 



1-86 



2-06 



1-66 



1-75 



1-63 



1-69 



Mean 



1-68 



1-61 



1-99 



1-75 



1-53 



116 



1-42 



1-78 



1-99 



1-96 



2-02 



1-51 



1-70 



( 



1844 



1-31 



1-42 



2-15 



1-90 



1-63 



M8 



1-51 



1-77 



1-S8 



2-15 



215 



1-34 



1-70 



M 



1845 



200 



1-72 



1-87 



1-56 



1-52 



MS 



1-40 



1-88 



2-11 



1-58 



1-78 



1-55 



1-68 



1 



Mean 



1-65 



1-57 



2-01 



1-73 



1-57 



M8 



1-45 



1-82 



1-99 



1-86 



1-96 



1-44 



1-69 



16. The mean differences from both series give nearly the same result, which is, on the whole, similar to that 

 deduced from the diurnal ranges ; it may be stated thus ; — the mean departure of the declination magnet from 

 its normal position for any hour is greatest near the equinoxes, and least at the summer solstice. When we 

 examine the means for the separate years, we find that those for 1844 give the result with considerable dis- 

 tinctness, the chief difference consisting in the occurrence of the autumnal maximum in October and November ; 

 in 1845, on the contrary, the spring maximum is ill defined at best (as in the 2d series,) while January is a 

 month of considerable disturbance. It appears evident that two years' observations are too few to exhibit a 

 law of this character free from all irregularity. The year 1844 appears to have been remarkably adapted for 

 exhibiting all the usual laws of magnetic and meteorological variation ; it is on this account, that it will be 

 found generally in these discussions, that the combination of another year's observations, does not serve to make 

 the results already obtained in the volume for 1844 more regular. 



Annual Variation of the Number of Observations of the Magnetic Declination luhich were Positive (West) 

 of the Monthly Means for the Corresponding Hours. — The following Table contains the numbers for each month 

 of 1844 and 1845, with reference (1) to the hourly means of all the observations, and (2) to the hourly means 

 of days selected free from irregular disturbance. 



Table 8. — Number of Observations of Magnetic Declination in 100 to the "West of their Hourly Means 

 as obtained (1) from all the Hourly Observations, and (2) from those for Selected Days. 



Series. 



Year. 



Jan. 



Feb. 



March. 



April. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct, 



Nov. 



Dec. 



Mean. 



( 



1844 



52-7 



51-8 



51-4 



48-4 



49-7 



52-8 



50-0 



49-9 



530 



46-0 



48-9 



55-3 



50-8 



1 \ 



1845 



54-5 



530 



54-6 



54-6 



53-2 



47-8 



49-2 



51-9 



521 



47-8 



48-0 



51-2 



51-5 



1 



Mean 



53-6 



52-4 



530 



51-5 



51-4 



50-3 



49-6 



50-9 



52-5 



46-9 



48-4 



53-3 



51-2 



( 



1844 



51-5 



55-3 



59-8 



49-5 



5M 



52-2 



45-5 



50-8 



54-7 



47-2 



58-0 



60-4 



530 



M 



1845 



62-2 



55-4 



58-8 



61-2 



45-7 



46-7 



48-0 



46-8 



48-2 



53-9 



50-0 



50-3 



52-3 



1 



Mean 



56-8 



55-4 



59-3 



55-3 



48-4 



49-4 



46-8 



48-8 



51-4 



50-6 



54-0 



55-4 



52-6 



