EARTH’S MAGNETISM TO THE SOLAR AND LUNAR PERIODS. 103 
cludes from the Toronto observations for 1842 (corrected by the usual method), that 
there is a maximum in June, and a minimum in December. I have projected 
the monthly means of the Toronto observations as corrected by the usual method. 
From these Colonel Saprne draws his conclusion. Under it I have projected the 
temperature of the magnet in a broken line, and below both, the means from the 
two-hourly observations, as corrected approximately by myself. (See Curves, 
No. 5, Plate IV.) These will shew how much depends on the accuracy of the cor- 
rection in arriving at sound conclusions. I conceive that the consistency of the 
results at which I have arrived, independently of other considerations, will leave 
little doubt as to which method of obtaining the corrections should be adopted. 
12. It has been already mentioned (10), that the apparent secular change con- 
sists of a considerable increase of horizontal intensity. Throughout the wholeperiod, 
the rapidity of increase has been diminishing, and it is much less in 1845 than in 
any of the previous years. Of all the puzzling problems in terrestrial magnetism, 
that of connecting the secular change with some known or observed phenomenon 
has been the most difficult; any fact, therefore, tending to this, will have interest. 
One of the first questions which I proposed to myself, connected with it, was 
whether all hours of the day were equally affected by the secular change? In order 
to answer this more distinctly, the annual period was eliminated from the monthly 
means, or, which is nearly the same thing, the mean of each month was reduced to 
the straight line passing through January and December 1844.* I then found 
that the mean horizontal force in the first six months of the year 1844, was almost 
constant one hour after the period of the morning maximum, and also that it was 
almost constant for the last six months, one hour before the period of the even- 
ing maximum. When the diurnal curve for each month was projected, I found 
the curves for the first six months to pass through a space of 3-4ths of a scale 
division in the ordinate of 64 40" a.m., with the exception of the curve for Feb- 
ruary, which is very irregular there. The curves for the last six months pass 
through a space of 13 scale divisions, in the ordinate of 4° 40" p.m.; the increase 
of horizontal force from January till December was 18 scale divisions. I next elimi- 
nated all the larger disturbances from the monthly means of each hour, but this 
neither affected the periods of the nodes, nor the values of the ordinates in which 
they were contained. In this way, then, the horizontal force in its secular pro- 
eress, seems to rest one foot during the first half of the year about an hour after 
* The line should have been drawn through January 1844 and January 1845, but that there is an 
irregularity in the progress of the horizontal force from December 1844 to January 1845, compared 
with the previous years. I have, however, also reduced the means to the line passing through Janu- 
ary 1844 and January 1845, and find the ordinate of the morning node slightly increased, but that 
for the evening node diminished. 
VOL. XVI. PART IL. 2D 
