239 
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 rv. 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. Pp. 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 122 minutes; the mean 
winter range to 9-0 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.and1 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 
 inelusive, has all the characters of the mean summer variation, 
both as respects the Jaw of the change and its amount. In like 
manner the diurnal yariation, in each of the six winter months, 
VOL, 111, U 
