158 ON MAGNETIC DISTURBANCES AT TORONTO. 
The existence and general charactar of this periodicity was exhibit- 
ed by the approximate methods employed in the earlier volumes of the 
colonial observations, but it was by the more accurate system first de- 
veloped by General Sabine, in the 3rd volume of the Toronto obser- 
vations, and since applied by him to the observations of other stations, 
that the periodic laws were rendered definite and precise. 
In the method referred to, the disturbed values of an element under 
discussion, are confined to those which differ from the normal value 
of that element proper to the hour by an amount equal or exceeding 
a certain definite limit, such normal being the average of the values of 
the element for that hour, during a month or some other suitable 
group of consecutive days, excluding all the disturbed values and in- 
cluding all others; the magnitude of the disturbance being measured 
by the difference between the actual and the normal value of the 
element. 
The disturbance limit for an element, determined on with reference 
to the amplitude of its regular periodic variations, is generally dif- 
ferent at different stations; but for the sake of inter-comparison must 
be constant at the same station. 
_ Of the facts revealed by discussing the disturbances at several stations, 
the following are among the most prominent :— 
(1,) The frequency and amount of disturbance of the declination, 
inclination, and force, have a diurnal and an annual period. 
(2,) The disturbances of the elements without regard to sign, the 
disturbances in which the needle is deflected to the east, and those in 
which it is deflected to the west of its normal position, as well as the 
disturbances which increase, and those which decrease the force and 
inclination, have all distinct and often different periodic laws. 
(3,) The periodic variations at different stations, though possessing 
the same general characters, exhibit in their epochs of maximum and 
minimum, very great diversities. 
(4,) In addition to the diurnal and annual periods, the yearly aggre- 
gates of disturbance for each element and at every station are subject to 
a periodic increase and diminution, occupying a cycle of about ten 
years, which corresponds both in its length and in the epochs of maxi- 
mum and minimum, with a periodic variation in the number of groups 
of spots on the surface of the sun. The disturbances discussed, and 
the results announced by General Sabine, in the 38rd volume of the 
Toronto observations, relate to the hourly observations from Ist July, 
1843, to 30th June, 1848. It is my purpose in the present cormmu- 
