144 



Royal Society, 



and condensed form the means of the whole eight years. These are 

 exhibited in the next table, and we perceive at the first view that 

 the increase of west declination is progressive in each month of the 

 year without a single exception. If we desire to examine further 

 the degree of approximation which these values present to a progres- 

 sion absolutely uniform, we may apply an aliquot portion of the an- 

 nual value (7'-93) to each of the monthly means corresponding to 

 the difference in time from the mean epoch (December 1) . These ali- 



Months. 



Mean 



Declina< 



tion. 



Correction 



for secular 



change to 



Dec. 1. 



Mean Declina- 

 tion in the year. 



Differences 



(^/'-^/'0. 



June 



July 



August ... 



September 



October ... 



November 



December 



January ... 



February... 



March 



April 



May 



23-42 

 24-45 

 24-91 

 25-30 

 26-32 

 27-07 

 27-73 

 28-29 

 29-23 

 29-76 

 30-21 



+3-64 

 +2-97 

 +2-31 

 + 1-65 

 -^0-99 

 +0-33 

 -0-a3 

 -0-99 

 -1-65 

 -2-31 

 -2-97 

 -3-64 



23 27-06 = 

 23 27-42 = 

 23 27-22= 

 23 26 95 = 

 23 27-31 = 

 23 27-40= 

 23 27 40= 

 23 27-30 = 

 23 27-58 = 

 23 27-45 = 

 23 27-24 = 

 23 2705 = 



+0-22 

 -0-14 

 +006 

 +033 

 -0-03 

 -012 

 -0-12 

 -0 02 

 -0-30 

 -017 

 +0-04 

 +0-23 



Mean, corresponding to Dec. 1 23 27-28 



23 27-2S = ^ 



quot portions are sho^ni in the second column, and it will be seen 

 by the third column, containing the mean declinations of the year 

 deduced severally from the observation-values in the different months, 

 with the correction for secular change assumed uniform applied, how 

 very nearly the results derived from the several months approximate 

 to one and the same value. The small differences which are shown 

 in the last column are for the most part such as would probably dis- 

 appear by a longer continuance of the observations ; but we may 

 notice, by the character of the signs, that there is also visible amongst 

 them the indication of a comparatively very small semi-annual affec- 

 tion, depending on the sun's position on either side of the equator, 

 which will be reverted to when treating of superimposed effects. 



*' The same features of regularity and uniformity are manifested if 

 the examination be further pursued into shorter periods, by comparing 

 with each other the twenty-six fortnightly means in the year; but 

 enough has been already stated to show the magnitude, the regula- 

 rity, and the systematic character of the changes called secular, 

 which are thus produced by forces in constant operation at the sur- 

 face of our planet. In our entire inability to connect these changes 

 with any other of the phaenomena of nature, either cosmical or ter- 

 restrial, we appear to have no other alternative than to view them as 

 a constituent feature of the terrestrial magnetic force itself, and as 

 one of its most remarkable characteristics, not to be overlooked by 

 those who would seek to explain the phaenomena of that force by 



