100 MR BROUN ON THE RELATION OF THE VARIATIONS OF THE 
maximum is attained. It now commences to decrease slowly till 8 p.m., more 
rapidly from 8 till 9, causing another inflexion in the curve, slowly again till 
2» 20™ a.m., when there is @ minimum; the force then increases slightly till 
5» 30™ or 40" when @ maximum occurs, after which it diminishes rapidly till 
10° 20" a.m., the period of the minimum. These hours differ somewhat from 
the periods obtained at other observatories ; and while some part of these differ- 
ences may be due to errors of temperature correction, I do not think that such . 
errors will altogether account for them, but that the accurate periods of maxima 
and minima will be found to differ at different places. At Toronto in Canada, 
for example, the maximum occurred a little after 4 p.m. in 1842; and as the 
mean temperature of the magnet at the succeeding observation hour differs but 
little from that at 4", the period cannot be affected by temperature. Some 
observatories shew the maximum as late as 7 p.m. It does not, however, seem 
improbable, that the periods of maxima and minima should differ at differ- 
ent places, when it is known that these periods vary at the same place in the 
course of the year; at Makerstoun, in 1844, the afternoon maximum occurred as 
early as 3" 10™ in December and January, and as late as 6" 50" in June; the 
minimum at 10° 20™ a.m. in the winter months, and at 9" 40™ a.m. in June; the a.m. 
maximum occurs at 6" 40™ in December, and about 5" in the summer months, 
while the earliest minimum occurs nearer midnight in winter than in summer. 
In this way the periods of the principal maximum and minimum approach to 
each other, and to noon in winter, and remove from each other, and from noon in 
summer. (See Curve, No. 1.) The reverse to some extent takes place with regard 
to the periods of the secondary maximum and minimum, which remove from each 
other in winter, and approach each other in swmmer, till in June the maximum 
and minimum seem to destroy each other. 
The morning maximum is greater than the afternoon one in December ;—in 
November, January, and February, they differ but little from each other ; and in 
December, January, and February, the two minima are nearly equal. 
4. The inflexions noted in the mean curve about 3 p.m. and 9 pP.M., become 
minima in the winter months, so that there are then three or four maxima and 
minima; the smaller ones nearly compensate each other in the mean of the winter 
months, as they occur at different hours in each month.* I shall consider the cause 
of these secondary afternoon maxima and minima on another occasion. With 
regard to the 2 A.M. minimum and 6 s.M. maximum, these seem nearly to vanish 
in the summer months. In the means for the months of June and July they can- 
not be detected, excepting that the intensity decreases more rapidly after 6 a.m. 
than before it; it should not be concluded on this account that this maximum and 
minimum do not exist. Having projected the hourly observations made in each 
day of June and July, I have not found one day in ten on which the secondary maxi- 
* November, December, and January, have been taken as the three winter months. 
