Annual Variations for the Magnetic Declination. 



XV 



2d, The daily mean declination departs farthest to the east of the monthly mean in the months from 

 August to February ; the means for 1844 and 1845 (which are most to be depended on for this investigation) 

 indicate November, December, and January, as the three months with the greatest average departure to the 

 cast, about 0'-80 : the least mean departures to the east occur in June and July; the average being 0''55, 

 or, by the means for 1844 and 1845, 0'-38. 



3d!, The mean Avesterly departures are most in excess of the mean easterly departures in August, Sep- 

 tember, and October, and the latter are most in excess of the former in December and January. 



4:th, The mean departures, without reference to direction, are greatest in August, September, and October ; 

 the average being 0'-80 : they are least in April, May, and June ; the average is O'-GO nearly, the means of 

 each three months in the last column of Table 4 being under consideration. This result was generalized in 

 the volume for 1844, p. 332, as follows : " The average difference of the daily means from the monthly means 

 in 1844, was a minimum when the mean westerly declination was least, and a maximum when it was 

 greatest." 



bth, The mean departure of daily mean declination from the monthly means for 1844 = 0'-56 



1845 = 0'-62 



1846 = 0'-82 



3 years = 0'-67 



The mean for 1846 is probably too high, owing to the incompleteness of the diurnal series of observa- 

 tions. 



Annual Variation of the Diurnal Ranges of Magnetic Declination. — The diurnal range of motion of the 

 declination magnet varies from month to month : the following Table contains the mean of all the diurnal 

 ranges for each month, as deduced from the usual daily observations : the means for 1844 and 1845 only are 

 comparable with each other. 



Table 5. — Mean Diurnal Kange of Magnetic Declination, as deduced from the Ordinary Daily 



Observations. 



Year. 



Jan. ■ 



Feb. 



March. 



April. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



Mean. 



1843 



5-66 



10-09 



10-12 



12-38 



11-00 



12-13 



12-30 



11-78 



12-67 



9-65 



5-85 



7-63 



10-10 



1844 



9-00 



10-28 



16-21 



15-88 



13-48 



12-41 



12-36 



14-02 



15-22 



15-69 



15-91 



11-22 



13-47 



1845 



13-98 



12-98 



14-20 



16-57 



14-07 



13-77 



1309 



16-65 



16-66 



13-51 



10-98 



10-91 



13-95 



1846 



9-10 



9-21 



13-70 



15-83 



14-70 



13-60 



14-69 



14-89 



17-03 



13-81 



10-31 



8-16 



12-92 



1843) 

 1846) 



7-38 



9-65 



11-91 



14-10 



12-85 



12-86 



13-50 



13-34 



14-85 



11-73 



8-08 



7-90 



11-51 



1844) 

 1845/ 



11-49 



11-63 



15-20 



16-22 



13-78 



13-09 



12-72 



15-33 



15-94 



14-60 



13-44 



11-06 



13-71 



Mean) 

 of all / 



9-43 



10-64 



13-55 



15-16 



13-31 



12-97 



13-11 



14-33 



15-39 



13-16 



10-76 



9-48 



12-61 



12. From the means for 1844 and 1845 in Table 5 we find, that the mean diurnal range of the hourly obser- 

 vations is greatest in the months of March and April ( = 15'- 70), and in the months of August and September 

 ( = 15'- 68) ; that it is least in the months of December and January ( = ll'-27), and in the months of June 

 and July ( =: 12'-90). This result may be stated generally thus : — The angle, including the diurnal oscilla- 

 tions of the declination magnet, is greatest immediately after the vernal and before the autumnal equinox, and 

 it is least at the winter and at the summer solstices. The means from the incomplete diurnal series of 1843 

 and 1846 indicate the same law. The quantities given in Table 5 are obtained from the ordinary daily obser- 

 vations, and while they are sufficiently comparative to indicate the variation of range with season, the daily 

 observations are in no year sufficiently numerous to give the absolute diurnal ranges : in 1844 so careful a 

 watch was kept over the smallest variations, and so many additional observations were made, as to render it 

 probable, that Table LX. for 1844, p. 400, contains very neaily the absolute ranges for all the three magneto- 

 meters : a similar remark will apply to Table L., p. 28 of this volume, for 1845, although not to the same 



MAG. AND MET. OBS. 1845 AND 1846. 



d 



