AND ANNUAL VARIATION OF THE TERRESTRIAL MAGNETIC FORCE. 211 
of the determinations of secular change of the inclination and horizontal force. 
Viewed in this light, the probabilities are in favour of the existence of a small annual 
decrease in the total force, as the legitimate conclusion from the portion of the series 
of absolute determinations in progress at Toronto which has been received in this 
country, and is here discussed : whilst it is obvious that the groundwork is laid of a 
positive conclusion, admitting of no uncertainty, attainable by steady perseverance 
in the prolongation of the series ; avoiding as far as possible, upon all occasions, all 
changes in the instruments employed or in the methods of observation. It will be 
shown in the sequel that certainty in respect to the question, whether the total 
force at Toronto is at the present epoch increasing or decreasing, may have a very 
considerable theoretical importance. 
Annual Variation . — We may now proceed to a consideration of the inferences 
which the observations will afford in regard to annual variation ; but in entering on 
this investigation, we must remember, in the first place, that fifty-two months consti- 
tute but a short period from which to derive an annual variation ; and in the second 
place, that we are as yet unable to eliminate the effects of the irregular disturbances 
from the residual errors, which consequently remain charged with them to the last ; 
and that if these effects are not themselves the sole cause of an annual variation, by 
reason of their greater frequency or magnitude at certain seasons of the year than at 
others, we must be prepared to expect that they will embarrass the research, by 
rendering the effects of other causes less apparently systematic than they would 
otherwise have been found. In the horizontal force particularly we may have reason 
to apprehend the influence of disturbances, because that element is greatly affected 
by them at Toronto, and their average effect appears to be to depress the force at the 
periods of their occurrence below its mean value. 
We have shown in the preceding pages the fifty-two monthly results of the obser- 
vations of the inclination and horizontal force, and their arithmetical means consti- 
tuting the mean values of those elements on the 1st of March, 1847 ; we have also 
derived from the observations the most probable values of the secular change of the 
elements during the pei’iod of observation. When each of the fifty-two results has 
received a correction for secular change proportioned to the interval of time elapsed 
between the date to which it refers and the 1st of March, 1847, and the differences 
are taken between the fifty-two results thus corrected and the arithmetical means, 
we obtain a suite of residual quantities, by which the influence of annual variation, 
if it exists, might be expected to be indicated. The following Table exhibits the mean 
difference in each month of the observed results, (when corrected as above noticed for 
secular change,) from the arithmetical means. 
2 E 2 
