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phenomena connected with the diurnal movements. The 
results now laid before the Academy are chiefly those of the 
four years just mentioned. The observations have since been 
continued upon a different and reduced scale, and with a view 
to other classes of phenomena. 
The changes which the magnetic declination undergoes, 
at a given place, may be reduced to three classes, namely, 1. 
periodical variations; 2. secular variations, which are either 
continually progressive, or else return to their former values 
in long and unknown periods; and 3. irregular variations, 
which observe, apparently, no law. The periodical variations 
hitherto noticed are those which depend upon the position of 
the Sun with respect to the horizon, or with respect to the 
equator, and which, therefore, complete their course in a day, 
or ina year. The author commences with the first of these. 
Diurnal Variation. 
In order to determine the laws of the diurnal changes, the 
observations of each month are combined separately, and the 
means of the results corresponding to the same hour taken. 
The observations having been continued for four successive 
years, there are thus four groups of mean results for each 
month, the means of which are then taken. And, finally, 
the mean values for the separate months are grouped toge- 
ther, so as to obtain the mean yearly, as well as the mean 
summer and winter course of the variation. ‘The total num- 
ber of individual observations thus combined exceeds 14,000. 
When the mean results at each hour of observation, for 
the whole year, are examined, it is found that the course of 
the diurnal variation is regulated by the following laws : 
1. About 7h. 30m. a.m., the north pole of the magnet 
begins to move to the wesiward, and, therefore, the declina- 
tion increases. This increase continues until about 1 P.M., 
when the declination attains its maximum. 
2. The north pole of the magnet then moves to the east, 
