III. On the luni-solar variations of magnetic declination and horizontal force at Bombay, and of declination at Trevandrum

The materials described in this paper are twenty-five years of declination observations, and twenty-six and a half years of horizontal force observations, taken at the Colaba Observatory, Bombay, and some results of ten years declination observations taken at the Trevandrum Observatory. A consideration of the lunar diurnal variations derived from these observations for different seasons and phases of the moon, leads the author to form the hypothesis that these variations are, properly speaking, combinations of solar diurnal variations that run through a cycle of change in a lunation. The characteristics of the variations that give rise to the hypothesis are (1) that generally the great movements occur in them, as in the mean solar diurnal variations for full lunations, in the solarday hours , whilst the night hours are relatively quiescent; and (2) that they have generally the same character and range at intervals of half a lunation, and opposite characters at intervals of a quarter of a lunation.