Magnetical Observatory of Dublin during the Years 1840-43. 89 



greater at the close of the year than at the commencement. The difference 

 was the yearly amount of the secular change. 



The observations of Cassini were made during five years, viz. from 1784 to 

 1788, inclusive. Although the annual variation at Paris was then greater than 

 it is now at Dublin, the final means are less accordant ; and M. K^emtz deduces 

 from them the existence of a double oscillation. This conclusion, however, has 

 arisen from what appears to be an erroneous mean value in the month of 

 October, and is therefore not a legitimate interpretation of the results. 



18. When we compare the course of the changes observed by Cassini with 

 those observed at Dublin, we find that the movements are precisely opposite. 

 But, it is to be observed, the directions of the secular changes at the two 

 periods are likewise opposed ; and, putting together these facts, we are led to 

 generahze tlie law as follows : 



From a little after the vernal equinox until a little after the summer solstice, the 

 movement of the north pole of the magnet is reteograde, or opposite in direction to 

 the secular change; and during the remaining nine months of the year it is direct. 



The remarkable relation between the annual and secular chanses, here 

 stated, may be observed on comparing the observations of Bowditch, in 1810, 

 with those of Cassini. At this time the westerly dechnation was diminishing 

 at Salem, in Massachusetts, by about two minutes annually ; and, in accordance 

 with the preceding law, the direction of the annual movements is the inverse 

 of that observed by Cassini at Paris, in 1786, and agrees with that observed at 

 Dublin at the present time. M. Aeago, who notices these observations {An- 

 nales de Chimie, torn, xvi.), draws from them a different conclusion, and infers 

 (although with an expression of doubt) that when the westerly declination 

 diminishes from year to year, the period of Cassini is transported from Spring 

 to Autumn. 



It further appears probable, that, at a given place, the amount of the annual 

 variation is related to that of the secular change, and vanishes when the latter 

 vanishes. This conclusion has been drawn by Arago, from the observations 

 of Gilpin at London, in 1787-1793, and those of Beaotoy in 1818-1820, as 

 compared with those of Cassini. At the former period, in fact, the secular 

 change was only + !'• annually at London, and the annual variation was pro- 

 portionally small ; while, at the latter, both changes appeared to be evanescent. 



VOL. XXII. N 



