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 Coldba 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 elimination 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 Colába were made by Grubb, of Dublin, and are like those described in the report (of 1840) of the Committee of Physics of the Koyal Society, the magnets being fifteen inches long. An account of them and of their history will be found in the * Appendices to the Bombay Magnetical and Meteorological 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.