MAGNETIC DECLINATION AT BOMBAY. 
367 
9. Method of Reduction of the Observations . — The singularly fruitful nature of the 
mode of treatment of observations of a like character with those now under discussion, 
introduced and extensively applied by Lieut. -General Sabine, P.R.S.,is such as to leave 
no doubt as to the propriety of its adoption in the present instance, especially as the 
whole series of observations, the greater part of which remains still unreduced, extends 
over nearly a quarter of a century. That mode of treatment is described by General 
Sabine as follows * : — “ The hourly directions of the magnet [in this case expressed in 
minutes of deviation of the north end of the magnet to the eastward of true north] are 
entered in monthly tables, having the days of the month in successive horizontal lines, 
and the hours of the day in vertical columns. The ‘ means ’ of the entries in each ver- 
tical column indicate the mean direction of the magnet at the different hours of the 
month to which the table belongs, and have received the name of ‘ First Normals ’ On 
inspecting any such monthly table, it is at once seen that a considerable portion of the 
entries in the several columns differ considerably from their respective means or first 
normals, and must be regarded as ‘ disturbed observations.’ The laws of their relative 
frequency and amount of disturbance, in different years, months, and hours, are then 
sought ought, by separating for that purpose a sufficient body of the most disturbed 
observations, computing the amount of departure in each case from the normal of the 
same month and hour, and arranging the amounts in annual, monthly, and hourly tables. 
In making these computations, the first normals require to be themselves corrected, by 
the omission in each vertical column of the entries noted as disturbed, and by taking 
fresh means, representing the normals of each month and hour after this omission, and 
therefore uninfluenced by the larger disturbances. These new means have received the 
name of ‘Final Normals,’ and may be defined as being the mean directions of the 
magnet in every month and every hour, after the omission from the record of every 
entry which differed from the mean a certain amount either in excess or in defect. 
“In this process there is nothing indefinite; and nothing arbitrary save the assign- 
ment of the particular amount of difference from the normal which shall be held to 
constitute the measure of a large disturbance, and which, for distinction sake, we may 
call ‘ the separating value.’ It must be an amount which will separate a sufficient body 
of disturbed observations to permit their laws to be satisfactorily ascertained ; but in 
other respects its precise value is of minor significancy ; and the limits within which a 
selection may be made, without materially affecting the results, are usually by no means 
narrow ; for it has been found experimentally on several occasions that the ratios by 
which the periodical variations of disturbance in different years, months, and hours are 
characterized and expressed do not undergo any material change by even considerable 
differences in the amount of the separating value. The separating value must necessarily 
be larger at some stations than at others, because the absolute magnitude of the dis- 
turbance-variation itself is very different in different parts of the globe, as well as its 
comparative magnitude in relation to the more regular solar-diurnal variation ; but it 
* Proceedings of the Loyal Society, vol. x. pp. 624-626. 
