OF THE EARTH'S MAGNETISM. 513 
At both places the yearly mean horizontal force increased through the whole 
period, with one exception—namely, about the mean epoch July 1847 at both 
places. 
4. If we could suppose that the mean horizontal force from year to year was 
unaffected by any other cause of change than that which produces the secular 
variation, the differences of the yearly means in Table I. would show at once the 
law of variation of the secular change itself. If, however, the yearly mean hori- 
zontal force at any place, independent of the secular variation, is not a constant 
quantity, but variable with some other argument, as the positions of the planets 
or the physical state of the sun, we may expect, from sufficiently long series of 
observations, to separate the results due to the two causes, and perhaps to deter- 
mine what they are. In the present instance, I shall consider the changes from 
year to year as if they were of long period and due to one cause. 
1st, The increase of the yearly mean was a maximum at both places about 
the mean epoch, January 1844. 
2d, The increase of the yearly mean was a minimum at both places about the 
mean epoch July 1847. 
3d, The epoch of maximum increase of the horizontal force was nearly that 
which has been found to be the epoch of minimum magnetic disturbance; while 
the epoch of minimum increase of the horizontal force approaches that of the 
maximum magnetic disturbance.* 
Ath, The interval between the maximum and minimum increase was, however, 
only 3°5 years. 
5th, The variations of the yearly means agree very well at both places through- 
out the whole period, excepting before and after the mean epoch January 1845, 
and some part of the difference at this time may be instrumental. 
5. It follows from these results, that what has been usually termed the secular 
variation is not a simple linear movement, and that it had nearly the same values, 
and obeyed nearly the same laws, at Makerstoun and Hobarton. 
6. The question arises whether this similarity is to be found at all places. I 
have not been able to obtain any series of observations of sufficient length, and 
sufficiently free from instrumental errors, to answer this question completely ; 
but the following are the yearly means at those places whose observations I have 
been able to discuss :— 
* If we correct the yearly means for a regular secular change, the remaining quantities will 
show a maximum and minimum nearest the epochs of minimum and maximum disturbance, and with 
an interval of about five years. 
