586 



variation in the relative amount of disturbance in different years, 

 contituting a cycle of about ten terrestrial years, which has been 

 found to correspond, both in the duration of the period and in the 

 epochs of maxima and minima, with a periodical variation in the 

 appearance of spots on the solar disk. 



In the introductions prefixed to the several volumes containing 

 the observations made at the colonial observatories, the concurrent 

 testimony of the disturbances of the three magnetic elements to 

 these conclusions is fully exhibited ; and in reference particularly to 

 the third, viz. the decennial variation, a resume has been given in 

 the second St. Helena volume, pages cxxii to cxxxvi. 



In that rtsumt, the particular form of the previously announced 

 decennial variation is more fully traced, and, from the analysis of the 

 observations, shown to be of the following character. If we begin 

 with the part of the cycle to which the maximum of disturbance be- 

 longs, we find, first, three consecutive years in each of which the 

 aggregate amount of disturbance (measured from a constant value) is 

 nearly the same ; then, two years of diminished disturbance ; and 

 then, three years in each of which the aggregate amount is nearly 

 the same, but is considerably less than in the two preceding years, 

 and very considerably less than in the three commencing years. 

 The three years of minimum are then succeeded by two of medium 

 disturbance, and these by the recommencement of three years of 

 maximum amount. Thus, for example, referring to the years in 

 which the colonial observatories were in action, 1841 and 1842 were 

 years of medium disturbance ; 1843, 1844, and 1845 years of mini- 

 mum, differing little from each other; 1846 and 1847 years of me- 

 dium, and 1848, 1849, and 1850 years of maximum. The general 

 analogy of these particular features with Schwabe's observations 

 of the solar spots, commenced in 1826 (showing, on the one hand, 

 the number of groups of spots, and on the other hand, the num- 

 ber of days free from spots in each year), may be examined by a 

 reference to the table in the third volume of * Cosmos' (English 

 translation), page 292, and is as satisfactory as, from the nature of 

 the subject, could well be expected*. 



* The variation in the aggregate amount of disturbance in the years of mini- 

 mum, 1843, 1844, and 1845, compared with the two years of medium immediately 

 preceding, 1841 and 1842, and with the two years of medium immediately 



