392 Mr. F. Chambers* The EartKs Magnetic Force [Nov. 20, 



I V. " 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." By 

 FREDERICK CHAMBERS, Acting Superintendent of the Colaba 

 Observatory, Bombay. Communicated by C. CHAMBERS, 

 F.R.S., Superintendent of the Colaba Observatory. Re- 

 ceived October 9, 1884. 



(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 ulti- 

 mately 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 255,000 hourly observa- 

 tions. 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 df adopt- 

 ing, 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 varia- 

 tion, exhibit all those variations of the magnitude of the diurnal varia- 

 tion 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 varia- 

 tions 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. 



2. It is also subject to an annual variation, whose amplitude is 



