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and the declination diminishes, but at a slower rate than 

 it has increased. This diminution continues until about 

 10 p.m., when the declination is a maximum. The whole 

 diurnal range, between 1 p, m. and 10 p. m., is about 9.4 

 minutes. 



3. During this increase, and subsequent decrease, the de- 

 clination twice reaches its mean value, as deduced from the 

 results of the entire day. The epochs of the mean declination 

 are 9h. 30m. a. m., and 6h. p. m. nearly. 



4. There is a second, but much smaller oscillation of the 

 magnet during the night and morning, the north pole moving 

 slowly to the west, from 10 p.m. until 3 a.m., nearly, when 

 there is a second maximum, and then returning to the east 

 until 7 A.M., when there is a second minimum. 



When the means corresponding to the several hours of 

 observation, taken for the six summer months, and the six 

 winter months, separately, are examined, it is found that the 

 course of the diurnal variation is very different in the two sea- 

 sons. The following are the distinguishing circumstances : 



1. The range is much greater in summer than in winter. 

 The mean summer range amounts to 12-2 minutes; the mean 

 winter range to 90 minutes. 



2. In summer, the morning minimum is greater than the 

 evening minimum, which is not the case in the curve for the 

 entire year; and, consequently, the greatest range is between 

 7 A.M. and 1 P.M. In winter, on the other hand, the morn- 

 ing minimum, and the small preceding maximum, disappear 

 altogether; and the course of the diurnal change presents 

 only a single oscillation. 



If now the inquiry be extended from the seasons to the 

 separate months, it is found that the course of the diurnal 

 change, in each of the six months from April to September 

 inclusive, has all the characters of the mean summer variation, 

 both as respects the law of the change and its amount. In like 

 manner the diurnal variation, in each of the six winter months, 



VOL. III. u 



