102 MR BROUN ON THE RELATION OF THE VARIATIONS OF THE 
the monthly means, as deduced from the observations of horizontal force made at 
Toronto in 1842, at the following hours:—First, the hours corresponding to the 
Makerstoun hours; this could not be done exactly, as the observations were 
made at Toronto every two hours only, and for each of two of the Makerstoun 
hours, the mean of two of the Toronto hours had to be taken; thus, one of the 
observation hours at Makerstoun was 7 a.m.; the mean of the Toronto observations 
at 6 and 8 a.m. was taken as equivalent (see the open curve, No. 3, Plate III.) 
Second, from the whole two-hourly observations (see the open curve, No. 5, Plate 
_ IV.) The monthly means from both these methods, gave the same, or nearly the 
same, annual period as the Makerstoun observations. 
10. [have since then verified this period by the monthly means of the observa- 
tions made at Makerstoun in the years 1843, 1844, and 1845. I may first refer 
to the Makerstoun and Toronto curves for 1842, which were exhibited to the 
Physical Section of the British Association at Cambridge. (See Curves, No. 3, 
Plate III.) From these I concluded that the horizontal force of the earth’s mag- 
netism has an annual period, consisting of a maximum at each solstice, and a mi- 
nimum at each equinox ; both curves present a curious inflexion in March, which 
{ then considered due to some irregularity. The monthly means for the years 
1842, 3, 4, and 5, have also been projected together ; (see Curves, No. 4, Plate III.) 
the whole speak the same language, excepting that the inflexion in March 1842 
does not occur in the other years, unless it may be said to do so in 1845. All the 
curves shew a considerable secular change, the horizontal intensity increasing 
throughout the whole period. Whether this be really an increase of the earth’s 
horizontal intensity, or due to some instrumental cause, cannot be positively 
stated; it is not at all likely to be due to an increase of the free magne- 
tism of the bar, which is suspended at right angles to the magnetic meridian ; the 
only apparent and possible cause is a stretch of the suspension wires; it must 
be a matter for consideration, whether such a cause is likely to operate for such 
a period, and to nearly an equal amount for two years. Whether a secular 
change of horizontal intensity, or due to a stretch of the wires, it is evident that it 
may be considered as a regular increase throughout the year. Upon this hypothesis 
I have eliminated this increase from the monthly means of the last three years, 
and projected the mean below the others. ‘This curve shews more strikingly the 
annual period of solstitial maxima and equinoctial minima. The minima have 
nearly the same value; the summer maximum is greater than the winter maxi- 
mum, but so little, that an error of a thirtieth in the amount of the temperature 
correction, would account for the difference. The annual range from the mean of 
the three years is 0°000724, or about the mean diurnal range for the three 
winter months. 
11. M. HansTeEeEn concluded from his observations, that there was a maximum 
of horizontal intensity in December, and a minimum in June. Colonel SaBinE con- 
