244 
mum of declination about January 25. Finally, the annual 
curve of temperature crosses the axis of abscisse 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 
different 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 different in different 
times. 
Disturbances. 
Having examined the periodical and the secular variations 
of the declination, as deduced from the observations made at 
the Dublin Observatory, it now remains to consider those 
which, from our ignorance of their laws, we have been accus- 
tomed to call ‘* ¢rregular.” 
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 
