Xll 



General Results of the Makerstoun Observations. 



Mean Westerly Declination and its secular change. — The mean declination for each month in each year, 

 from August 1841 till November 1849, is given in Table I. ; it diminishes generally from month to month ; 

 in the mid-summer months, however, it frequently increases. 



4. The mean declination for each year, with the yearly value of secular change and its mean value for each 

 four years are given in Table 2 : the mean for 1841 has been deduced in assuming the change from 1841 to 

 1842 the same as that obtained from a comparison of the observations for four months of 1841 with the 

 observations for the corresponding months of 1842; and the mean for 1849 has been found similarly from 

 the comparison of the observations in the first eleven months of the years 1848 and 1849. 



5 . The mean yearly value of the secular change from the last column of Table 2 = 5''9 2. Some irregularity 

 appears in the values of the secular change from year to year, especially in those for the years 1846-1849, com- 

 pared vrith the values for the preceding years ; this marked difference, it is conceived, is not due to instrumental 

 error, because no such amount of torsion in the suspension thread of the declination magnet has existed to pro- 

 duce it ; and the observations of the bifilar and balance magnetometers indicate a similar variation in the 

 value of the secular change for the year 1847-8. In that year, great magnetic convulsions occurred, the effects 

 of which seem to have extended into the years 1848—9. 



6. The last column of Table 2 appears to exhibit the variation of the yearly value of secular change ; its in- 

 crease as the needle moves farther from its greatest westerly position. Between 1842 and 1847 the secular motion 

 from year to year is moderately equable. Tliis is not the case with the motion from month to month, which is 

 occasionally retrograde. We are induced to conclude, therefore, either that the secular motion varies from 

 month to month, while nearly constant from year to year ; or that the secular motion being uniform from 

 month to month other motions are superposed : in either case, by reducing the mean positions for the several 

 months to one epoch, the residual variations will be more clearly exposed, and it may be determined whether 

 they obey any law related to season. Since Ave are aware that the secular motion for the same place is sometimes 

 eastwards and sometimes westwards, it does not appear necessary to form any other hypothesis than that the 

 secular change is the excess of the motions in one direction over those in the other, and to determine whether 

 the amounts and directions of motion have any relation to season. 



Table 2. — Yearly Means of Magnetic Declination and the Secular Change. 







Secular Change. | 





Mean 

 W. Declination. 







Year. 



Each 



Mean of 







Year. 



4 Years. 



1841 



25 33-68 



' 



' 



1842 



25 28-45 



5-23 





1843 



25 22-85 



5-60 





1844 



25 17-06 



5-79 





1845 



25 11-32 



5-74 



5-59 



1846 



25 5-97 



5-35 



5-62 



1847 



24 59-65 



6-32 



5-80 



1848 



24 51-81 



7-84 



6-31 



1849 



24 45-12 



6-69 



6-30 



7. Annual Period of Magnetic Declination. — In the discussions for 1844 the apparent law of annual varia- 

 tion has been offered with some confidence, and that chiefly because of the considerable agreement of four years' 

 observations where the variations were of the smallest order. In the means for 1843—6, the proportional parts 

 of the yearly secular change being eliminated, the variation of the monthly means is under one minute ; since 

 the variations from month to month are so small, it is evident that, in order to detect any relation to season, 

 the greatest care must be taken to avoid all instrumental errors ; for this reason it appears proper to consider 

 at first the results from those years only (1843-6), during which a sufficient number of daily observations were 

 made to give the monthly means without any considerable error. The means for the first of these years (1843) 

 are affected to some extent with torsion of the suspension thread, which broke gradually in June ; on which 

 account the mean of May and July has been substituted for June in Table 3. 



