IN THE MEAN EFFECTS OF THE LARGER MAGNETIC DISTURBANCES. 359 
chronologically, showing the day, the hour, and the amount of disturbance, (i. e. the 
difference from the normal,) in scale-divisions; and on the receipt of this table from 
the Office, I proceeded to distribute the disturbances according to the years, months, 
and hours of their occurrence, separating them into disturbances increasing, and 
disturbances decreasing, the respective forces, and forming annual, monthly, and 
hourly tables ; the correctness of the distribution and of the calculations in each of 
the tables being in every case examined by a second person. 
In course of the process of marking the disturbances, it became evident that there 
were times, occasional, but by no means frequent, when the change in the mean 
monthly scale-reading, i. e. the mean of all the hours and all the days in the month, 
from one month to the next, was so considerable as to cause the regular hourly 
normals of the month to be inapplicable to its earlier or later portions. In such 
cases, the difficulty was met, and more suitable normals obtained for the earlier or 
later portions of the month, by taking the hourly means of the last fortnight of the 
one month and the first fortnight of the next ; or by a mean of the normals of the two 
months combined ; or, in a very few instances in which the departure from an 
uniformly progressive change was greatest, by normals derived from periods of less 
duration than a month. 
The Tables showing the normal values finally adopted, the periods for which they 
were employed, and the periods from which they w’ere derived, together with the 
annual, monthly, and hourly tables of the aggregate values of the larger disturbances 
of the horizontal and vertical forces in the several years, months and hours, will be 
found in full detail in the third volume of the Toronto Observations, which is now in 
the press. 
The Disturbances of the Inclination, which equalled or exceeded l'*0, and of the 
Total Force which equalled or exceeded '0004 of the whole force, (both measured from 
the respective normals at the same hour and in the same month,) were obtained from 
the observed disturbances of the Horizontal and Vertical Forces in the following 
manner: — Tables were formed, in the first column of which were placed, in chrono- 
logical order, the larger disturbances of the Vertical Force, separated as already 
described, and in the second column those of the Horizontal Force, each expressed in 
terms of the respective forces, by the conversion of the scale-divisions in which the 
disturbances were observed into parts of the respective forces by means of the scale- 
coefficient. At a large proportion of the hours of contemporaneous observation, 
when one of the two components of the force exhibited a disturbance which by its 
amount was brought into the category of the larger disturbances, the other compo- 
nent was also disturbed. In such cases, there were contemporaneous entries in both 
columns ; but when one of the components only was so affected, the entry in the 
corresponding column of the other component was blank. These blanks were now 
all filled up, by inserting for the component which did not exhibit a disturbance of 
sufficiently large amount to have been classed as a large disturbance, and separated 
3 B 2 
