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phenomena connected with the diurnal movements. Tlie 

 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 rfay, 

 or in a 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 westward, 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, 



