ON MAGNETIC DISTURBANCES AT TORONTO. 159 
nication to give analogous results for the years 1856 to 1862, inclusive, 
partly to shew that the diurnal and annual variations of the distur- 
bances are substantially the same in the more recent as in the earlier 
series, but: chiefly for the purpose of furnishing materials for discover- 
ing the precise character of the so-called decennial period. 
In the investigations on which the accompanying tables are based, 
those disturbances only are included which equal or exceed the limits 
employed by General Sabine ; namely, for the declination, 5’.0 ; Hori- 
zontal force, :0012; Vertical force, ‘00026 ; Total force, 0004 ; Incli- 
nation, 1’.0. 
In the process of collecting the disturbances of the Horizontal force, 
additional security has been aimed at, by employing in most instances 
the observations furnished by two, and in a few cases by three bifilars. 
The normal for each bifilar being found in the usual way, the difference 
therefrom of the disturbed readings, and expressed in parts of the Hori- 
zontal force, were placed in parallel columns. In the great majority 
of observations where one instrument gave a disturbed reading the other 
did so also, but when such was not the case, the blank was filled up 
by the difference, whatever it might be, between the observed reading 
and the corresponding normal. The means of the corresponding 
entries in the different columns being then taken, those were retained 
as disturbances which equalled or exceeded ‘0012. 
The disturbances of the horizontal and vertical components of the 
force being found, the corresponding abnormal deviation aG of the 
total force and (A@) of the inclinatiop were calculated by the formulee 
Agia pokey, AX 2 AN 
Rar ee ete + sin Oe 
AX AY 
ad = } sin} — | 
AX AY 
Where x and “y Tepresent the contemporaneous abnormal devia- 
tions of the horizontal and vertical components of the force, where one 
or both of them are disturbed. Of the resulting values of AS and Aé, 
those and those only were regarded as disturbances, which equalled 
or exceeded the limits determined on for these elements, namely, :0004 
for the total force, and 1’.0 for the inclination. 
The ratios in tables I. and II. are derived from observations in the 
seven years, commencing lst January, 1856, and endin 31st Dec. 
