2 
MR. C. CHAMBERS OH LUNI-SOLAR VARIATIONS OF 
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 vertical 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 out 
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 by 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’s 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 characterised 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 disturbance 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 must be a constant quantity throughout at one and the same 
station, or it will not truly show the relative proportion of disturbance in different 
years and different months.” The words “ directions of the magnet ” in the extract 
must be taken when applied to the Colaba declination observations to imply those 
directions as expressed either by the original scale readings or those readings con¬ 
verted into minutes of easterly declination ; and, when applied to the horizontal force 
observations, to imply the directions as expressed by the original scale readings after 
reduction to a uniform temperature. 
4. The period of the declination observations is from 1846'0 to 1871'0, and that of 
the horizontal force observations from 1846'5 to 1873‘0. With the exception of 
Sundays and eight or ten complete days in each year, the observations were taken 
continuously at hourly intervals throughout these periods. The entries in the 
monthly declination Tables of the years 1846 to 1865 were in minutes of arc, in those 
of the years 1866 to 1870 in original scale readings, and the “separating value ” made 
