IV. On the variations of the mean diurnal inequality of the horizontal component of the earth’s magnetic force at Bombay, and their relations to the sun-spot period

Abstract

The investigation with which this paper deals was suggested by a remark made by Dr. Balfour Stewart, in “Nature,” vol. xxiii, p. 238, to the effect that well selected magnetic observations might ulti­mately be found to indicate variations of solar heat more quickly and with greater certainty than any other kinds of indirect observations. The observations selected are those of the variations of the horizontal magnetic force, recorded at the Colaba Observatory, Bombay, between the years 1846 and 1880, comprising about 265,000 hourly observations. The mean diurnal variations for each month, calculated from all the observations, without any exclusion of disturbances, were extracted from the records of the Observatory ; but instead of adopt­ing, in the usual manner, the extreme range of these variations as the subject for further treatment, the mean of all the twenty-four hourly deviations, regardless of signs, was adopted; the objects aimed at in departing from the usual rule being to give due weight to all the observations, and to eliminate, as far as possible, the effects of rapidly fluctuating disturbances, without rejecting any of the observations. The monthly number obtained in the manner above described may be called the mean diurnal inequality for the month. The series of such monthly numbers should, after elimination of the annual varia­tion, exhibit all those variations of the magnitude of the diurnal varia­tion of the earth’s magnetic force which may possibly depend on absolute variations of solar heat. Indications may therefore be thus obtained of all real variations of solar energy whose periods lie between one month and thirty-three years, if such there be. Several magnetic variations are shown to exist, and these are compared with the varia­tions of the sun-spots. The following is a brief summary of the principal results of the investigation:— 1. The mean diurnal inequality of horizontal force at Bombay is subject to a periodical variation, whose duration is almost exactly eleven years.