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ON COMPARING AND REDUCING MAGNETIC OBSERVATIONS. 89 
in table (A’), viz., of the numbers for the day of the entry and the four- 
teen preceding and fourteen following days. The numbers of table C 
for all the hours of a given day we may take to represent very approxi- 
mately the mean solar-diurnal variation—plus a constant—for that day, 
the average extending over the lunation of which that day is the middle 
day. They will be affected by progressive change of the values of the 
tabulations, and by disturbance within the limits. 
11. Lastly, the excesses of the numbers of table (A’) over the corre- 
sponding numbers in table C, plus a constant round number,! should be 
entered on a fourth table (D). The numbers of this table, which will be 
affected only by that part of the solar-diurnal variation which goes 
through a cycle of change in a lunation, and by disturbance within the 
limits, we may proceed to arrange in new tables with reference to the 
moon’s age and the season (or month) of the year,? and so determine 
the character of the variations which the luni-solar-diurnal variation is 
subject to. Having done this, a further elimination will put us in pos- 
session of residual numbers, the variation of which must be attributed 
solely to disturbances within the limits, and may be studied and the 
numbers be manipulated accordingly. 
12. I agree with Dr. Balfour Stewart that the time has not yet 
arrived for laying down rules for the treatment of the vertical force 
tabulations. 
XIII. Letter from the Rev. Professor 8. J. Perry, F.R.S. 
September 8, 1885. 
Dear Dr. Schuster,—I have read over the Report Dr. Stewart kindly 
forwarded, and I cannot help thinking that our first step should be to 
collect the results already obtained for the Daily Range of the Declina- 
tion, reduce the means already worked out to a common scale, and then 
distribute the whole in a tabular and in a graphical form. Much might 
be learnt from seeing these results in a collective form, and we could then 
hetter judge how far processes more laborious than those of Sir Edward 
Sabine are like to repay the labour. 
If all observations are made use of in deducing the Daily Mean Range 
the Disturbance period will certainly interfere with the Solar Diurnal 
Range, and if we pick out quiet curves in which the Daily Range is well 
marked, we are very liable to give undue weight to variations in the 
Daily Range which are independent of ordinary disturbances. 
Yours very truly, 
S. J. Perry. 
? The constant round number is added to avoid the inconvenience of having to 
deal afterwards with positive and negative numbers. 
* Ifaseparate table be allotted to each day of the moon's age, the resulting 
mean variations will be practically the same whether the hours refer to the solar or 
the lunar day ; and as the numbers available are for the exact hours of the solar day, it 
18 Convenient to let the arrangement of the table be for the solar day rather than for 
the lunar day, 
