244 



mum of declination about January 25. Finally, the annual 

 curve of temperature crosses the axis of abscissae in two 

 points, which correspond to May 1, and October 10, — the 

 corresponding epochs in the curve of declination are May 10, 

 and October 15. 



Secular Variation. 



The westerly declination is at present diminishing from 

 year to year in these countries, and has been so since the year 

 1818, which was the time of the maximum. The present 

 rate of the secular decrease in Dublin, as deduced from four 

 years' observations, is 5.0 minutes annually. 



With respect to the physical cause of the secular change, 

 Dr. Lloyd said that he had been led to form an opinion very 

 diiferent from any of those heretofore held. From the re- 

 markable relation which had been shown to exist between the 

 annual and the secular changes, he was driven to conclude 

 that they depended (ultimately at least) upon a common cause ; 

 and that thus the sun was the cause of the secular, no less than 

 of the periodical changes, although not only the magnitude, 

 but even the direction of the effect were diiferent in different 

 times. 



Disturbances. 



Having examined the periodical and the secular variations 

 of the declination, as deduced from the observations made at 

 the DubUn Observatory, it now remains to consider those 

 v^hich, from our ignorance of their laws, we have been accus- 

 tomed to call " irregular." 



Professor Kreil seems to have been the first to notify the 

 remarkable fact, that magnetic disturbances occur more fre- 

 quently at certain hours than at others ; and, that the direc- 

 tion, as well as the frequency, of these movements, has a 

 dependence upon the time of the day. Colonel Sabine has 

 since made a more complete and elaborate examination of this 



