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ON COMPARISON AND REDUCTION OF MAGNETIC OBSERVATIONS. 239 
Greenwich = — 0'-42. Mean for year at Kew = +0/:072, at Greenwich 
= +0'007. In horizontal force the non-cyclie effect at Kew is positive 
in all months, and similarly at Greenwich. The greatest values at Kew 
are (effect x 10° in C.G.S. units), in January, February, October, and 
November, +50, +57, +50, and +53 respectively ; and similarly at 
Greenwich, the values being +62, +53, +73, and +57 respectively. 
Mean for year at Kew = +36, at Greenwich = +40. In vertical 
force there is considerable difference, both in magnitude and sign, between 
the non-cyclic effect in different months at the two places. The mean for 
year at Kew = —8, and at Greenwich = —18. In this comparison it 
is to be remembered that the Kew results depend on the observations of 
six years, and the Greenwich results only on those of five years. 
As regards now the mean values in separate years (Table III.); of 
the five years 1890 to 1894 the non-cyclic change in declination is in the 
same direction in four of the five years at both places ; in horizontal force 
in the same direction in all years, and in vertical force in three years. 
The actual numbers are :— 
Non-cy clic change | 1890 | 1891 | 1892 | 1893 | 1894 
In declination 
At Kew . : . 2 2 . | —0'36 | +029 | +014 | +026 | +015 
At Greenwich 2 0628 | +008: | —0'-20°| 4015 | F0"24 
In horizontal force 
At Kew . - : : ; é +23 +23 +53 +40 +33 
At Greenwich +18 +37 +68 +44 +33 
In vertical force 
At Kew . z . : : : | +15 —12 — 20 —26 +12 
At Greenwich —12 — 8 —39 —24 —4 
In vertical force there is a tendency to a uniformly greater negative 
value at Greenwich than at Kew. Considering, however, the values at 
each station separately, the greatest negative values occur in the same 
two years, 1892 and 1893, at both places. 
One question that I had set myself to discuss was how far the abso- 
lute magnetic values, as, for instance, the mean monthly values, differ, as 
determined from the five ‘ quiet’ days in each month, and as determined 
from all days (excepting those of excessive magnetic disturbance). In the 
Greenwich ‘ Observations’ there are given in Tables I., III., and VII. of 
the magnetic section mean daily values of declination, horizontal force, and 
vertical force respectively throughout the year (excepting days of exces- 
sive disturbance). The means of these values for different months are 
given in Table XI. Extracting from the different tables the values for 
the adopted ‘quiet’ days, and taking the mean in each month, the variation 
of these means from the corresponding means of Table XI. gives in each 
case the deviation of the ‘quiet’ days mean from that for all days. Since 
the mean of the five selected days falls always near the middle of the 
month, the comparison, for a first inquiry, sufficiently eliminates the 
secular variation, considering it uniform, the only possible supposition. 
The excess of the ‘quiet’ day monthly mean above the all day or 
