XV. On the relation between the diurnal range of magnetic declination and horizontal force, as observed at the Royal Observatory, Greenwich, during the years 1841 to 1877, and the period of solar spot frequency

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Abstract
The equipment of the Magnetical and Meteorological Department of the Royal Observatory at Greenwich, established and organised by the present Astronomer Royal, Sir George B. Airy, K. C. B., was generally complete at the latter part of the year 1840, since which time observations have been continuously made. Until the end of the year 1847 these consisted of eye readings of the various instruments, taken at intervals of two hours. But Mr. Charles Brooke having at this time arranged a practical system of photographic registration, continuous records of the indications of the instruments have been, since the beginning of the year 1848, by this means obtained. These records form a sure basis on which to found any magnetic inquiry. The magnetic elements which have been the subject of observation are, firstly, absolute determinations of magnetic declination, of the horizontal component of the earth’s magnetic force, and of magnetic dip or inclination; and, secondly, the continuous variations to which the declination and the horizontal and vertical components of the earth’s force are subject. The absolute measures are important in combination with similar measures made at other places for determination of the general magnetic condition of the earth, and of the slow changes occurring therein. But the smaller variations of shorter period, as observed at any one place, also throw great light on the general laws and phenomena of magnetic action. Thus the variations of declination, and of the horizontal and vertical components of the magnetic force, each include a well-marked diurnal period, analogous in some degree to that of atmospheric temperature; and, speaking generally, the range of the diurnal variation at Greenwich is, in each case, greater in summer than in winter. In addition, however, to the annual inequality in the magnetic diurnal range there appears to be yet another, also of marked character but of longer period, one which resembles in its features the apparently well established eleven year sun-spot period, and which it is the object of the present paper to evolve.

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