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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. 
By William Ellis, F.R.A.S., Superintendent of the Magnetical and Meteorological 
Department, Royal Observatory, Greenwich. 
Communicated by Sir George Airy, K.C.B., F.R.S., Astronomer Royal 
Received April 23,—Read May 8, 1879. 
[Plates 22-24.] 
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 continu¬ 
ous 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 
* For information in reference to this subject, see papers by Mr. Brooke in the Philosophical Trans¬ 
actions for the years 1847, 1850, and 1852 ; and also the Addendum to the introduction to the ‘ Green¬ 
wich Magnetical and Meteorological Observations ’ for 1847. 
MDCCCLXXX. 4 A 
