Magnetkal Observatory of Dublin during the Years 1840-43. 95 



be estabUshed by the following Table, which contains the values of '^^^' 

 corresponding to the several months of the year : 



Table XV. Annual Variation of the Mean Disturbance. 



V^ 



These numbers are somewhat irregular; but if they be combined in periods 

 of three months, taking the square root of the mean of the squares we obtain 

 the following numbers, in which the existence of an annual period is evident: 



February, March, Aiiril, . . . 2'.68 



May, June, July 3'.04 



August, September, October, . . . 2'-55 

 November, December, January, . . 1'.77 

 The mean disturbance, for the entire year, is 2'-56. 



25. It by no means necessarily follows, from the results now stated that aU 

 disturbances have a periodical character. There probably are two classes of 

 disturbances, the results of distinct physical causes, of which one observes a 

 period, while the other is wholly irregular ; and it is manifest that, in such a 

 case, the period of the former wiU necessarily be impressed upon the resultant 

 mean disturbance. We have, I think, also grounds for concluding that these 

 two kinds of disturbances are further distinguished by the important charac- 

 teristics,-that those of the former class are local (depending, as they do upon 

 the time at the place of observation), while those of the latter are universal, and 

 belong to the phenomena which have hitherto so much engaged the attention 

 of observers. 



Of the periodical disturbances the principal (if not the only one) is that 

 which occurs about 10 p.m., and which causes the north pole of the magnet to 

 deviate to the east. The epoch of the maximum of easterly deflection varies 

 however, between very wide limits, being sometimes before 8 p. m , and some- 

 times later than 1 a. m.; and hence it is evident, that its effect on the monthly 



