368 
ME. C. CHAMBEES ON THE SOLAE VAEIATIONS OF 
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.” 
In two instances only were days of a certain month included, in consequence of inter- 
ruption of the observations, in the monthly Tables of a different month, viz. : — the 1st 
and 2nd of July with the month of June 1861, and from the 1st to the 6th of August 
with the month of July 1861 ; and the only incomplete months were those of August, 
October, and November 1861. The first step to be taken was to fix upon a suitable 
separating value , and in doing this regard was paid to the desirableness of having the 
results derived for Bombay comparable with those for some one of the stations where 
similar observations had been made and similarly treated. Now the ordinary reductions 
of the Bombay observations already made showed that the station amongst those which 
it most nearly resembled was St. Helena ; for whilst both are equatorial stations in the 
sense of the ordinary solar-diurnal variation being at certain seasons overridden by the 
semiannual inequality of that variation, Bombay has very nearly the same preponderance 
of the character of a north-latitude station that St. Helena has as a south-latitude 
station. The separating value adopted by General Sabine in the reduction of the 
St. Helena observations was l'*T8, or a disturbing force equal to - 0029 English units; 
a force of this magnitude acting at Bombay would produce a deviation of the declination- 
magnet of l' - 25. The value finally fixed upon was l' - 4, which gives the ratio of the 
number (3516) of disturbed observations to the total number (50736) of observations in 
the seven years as 1 to 14'4, the similar ratio for St. Helena having been 1 to 13*7. 
10. Trials were also made with separating values l'*0 for the years 1859 and 1860, 
and P‘7 for 1860, the results of which (being further confirmatory of the position taken 
above that “ the ratios by which the periodical variations of disturbance in different 
hours are characterized and expressed, do not undergo any material change by even 
considerable differences in the amount of the separating value ”) are shown in the sub- 
joined Tables, and by Curves Nos. 1 to 4 of Plate XXV. 
