ON COMPARING AND REDUCING MAGNETIC OBSERVATIONS. 69 
the undisturbed normals for each hour, for each month. I agree with 
Sir J. H. Lefroy in thinking that the best plan of investigating disturb. 
ances is, in the first place, to obtain the various departures of individual 
observations from their respective normals for that month and hour. It 
would be desirable to embody these departures in a fresh table, in which 
{except for those who are colour-blind) the negative departures might be 
given in red ink and the positive in black. 
8. In this table, at the right of the twenty-four departures for the 
various hours of the day, I should represent the mean departure for that 
whole day either in red or black. It would thus be seen, at a glance, 
whether the average of the whole day was affected by disturbance, in 
what direction, and to what extent. 
9. li is here assumed that, during the month in question, no alteration 
of scale value or other instrumental change has taken place. Never- 
theless at stations which have a considerable secular variation of decli- 
nation, and for which this is known, it might be desirable to introduce, 
say to the extreme right, a column embracing a small residual correction, 
applicable to each day’s departures, on account of secular change. 
10. I imagine that a monthly table, constructed after the method 
which I have described, will afford a full and satisfactory basis for the 
discussion of disturbances. 
11. It is probable that the smaller departures will follow the law of 
the ordinary solar diurnal variation, and, in that case, there should be as 
many black as red sums in these minor departures, or, in other words, the 
algebraic sum of these should be zero, while the sum taken without 
respect to sign or colour should represent the amount of oscillation or 
disturbance obeying the ordinary law, this being a point which it is of 
interest to determine. No doubt the larger disturbances will obey some 
other law, and it will be necessary to separate them into two categories, 
those increasing and those diminishing the declination. Here I should 
follow Dr. Buys Ballot’s advice, and allow the observations themselves to 
determine where the one law ends and the other begins. It is just possible 
that sometimes the day’s mean may be decidedly different from what it 
ought to be, and yet the diurnal variation for that day be as nearly as 
possible the same as for undisturbed observations. A table, such as that 
now described, will show, at a glance, whether such a state of things ever 
takes place. 
Horizontal and Vertical Force. 
12. The horizontal and vertical force magnetographs are different 
from the declination magnetograph, inasmuch as their indications are 
affected by change of temperature, by loss of magnetism, and possibly, 
in the case of the vertical force instrument, by other circumstances not 
well understood. 
13. It will be noticed that, in treating the deciination resulis by 
Sabine’s method, we perform oar operation upon the individual declina- 
tion values. Now it might be said, why not (your object being to find 
the solar diurnal variation) take the departure of the individual hours of 
a day from the mean of that day, and treat each month’s departures by 
Sabine’s method ? 
14. The reply would be that the mean of a day is more likely to be 
affected by disturbance than the monthly mean of an hour. For disturb- 
ances, when they come, generally affect several consecutive hours, thus 
