PHILOSOPHICAL TRANSACTIONS 
I. On the Luni-Solar Variations of Magnetic Declination and Horizontal Force at 
Bombay, and of Declination at Trevandrum. 
By Charles Chambers, F.R.S., Superintendent of the Coldba Observatory, Bombay. 
Received March 24,—Read April 1, 188G. 
[Plates 1-5.] 
In the early attempts to investigate the influence of the moon upon terrestrial 
magnetism, the observations dealt with extended over periods so limited that little 
was possible beyond determining the average character of the lunar diurnal variation. 
This was mainly because magnetic disturbance tends—and especially in extra-tropical 
regions—to mask the minute variations that depend upon the moon. The series of 
observations made at the Colaba Observatory, Bombay, and discussed in the present 
paper, extending over twenty-five years in the case of the declination and over 
twenty-six and a half years in the case of the horizontal force, possesses therefore the 
double advantage of being originally affected by only the relatively small disturbances 
of a tropical station, and of being lengthy enough to secure an approximate elimina¬ 
tion of such disturbance as is involved in it, even by combination of portions only of 
the whole body of observations. 
2. The instruments used at Colaba were made by Grubb, of Dublin, and are like 
those described in the report (of 1840) of the Committee of Physics of the lloyal 
Society, the magnets being fifteen inches long. An account of them and of their 
history will be found in the ‘ Appendices to the Bombay Alagnetical and Meteoro¬ 
logical Observations, 1879 to 1882,’ pages [84] and [138] : and to this account reference 
may be made for particulars as to the adjustments and determination of scale 
coefficients of both the declination and horizontal force magnetometers, and as to the 
determination of the temperature coefficient of the latter instrument. The following 
extract is, however, given in full, the matter of it being essential to an understanding 
of the principal object of this paper. 
3. Method of Reduction of the Observations. —The mode of treatment adopted is 
that which was introduced by General Sir Edward Sabine, and which is described 
by him in the ‘Proceedings of the Boval Society,’ vol. 10, pp. 624-626, in the 
following words:—“The hourly directions of the magnet are entered in monthly 
MDCCCLXXXVII.-—A. B 2.5.87 
