Simultaneity of Magnetic Variations at different Places, $c. 191 



Mj acquaintance with the literature of electricity is very limited. 

 , Others vocations have for many years diverted my thoughts and my 

 reading from electrical science, and if I have in any case advanced as 

 a novelty what has been done before I hope it will be attributed to 

 ignorance and not to intention. It was only within the last few days, 

 and after the foregoing paper was printed, that I was made aware of 

 the fact that I am not the first to use dust figures in electrical 

 research, but I am still unaware that the experiments I have de- 

 scribed have, in their results, any substantial coincidence with 

 the recorded results of other investigations.* 



" On the Simultaneity of Magnetic Variations at different 

 places on occasions of Magnetic Disturbance, and on the 

 Relation between Magnetic and Earth Current Pheno- 

 mena." By WILLIAM ELLIS, F.R.A.S., Superintendent of 

 the Magnetical and Meteorological Department, Royal 

 Observatory, Greenwich. Communicated by W. H. M. 

 CHRISTIE, F.R.S. Received April 7, Read May 5, 1892. 



The observations made in a magnetic observatory usually include 

 absolute measures of magnetic declination, horizontal force, and dip 

 (inclination) ; with photographic registration of the variations of 

 declination, horizontal force, and vertical force ; to which is added, at 

 Greenwich, a photographic registration of earth currents. 



As regards magnetic variations, the observations made at the Royal 

 Observatory indicate, for all magnetic elements : 1. A progressive 

 change of value which, when limited periods of time only are con- 

 sidered, is nearly constant from year to year. 2. A solar diurnal 

 variation, the amplitude of which is greater in summer and smaller 

 in winter, and which has also a period sympathetic with the sun spot 

 period, being greater throughout the year when sun spots are numer- 

 ous, and smaller when sun spots are few. There is also a small lunar 

 diurnal variation. 3. The occurrence of days and periods of irregular 

 magnetic disturbance or magnetic storm, which are more frequent 

 and of greater magnitude when sun spots are numerous than when 

 sun spots are few, being comparatively rare as well as insignificant in 

 character near to the times of minima of sun spots. Disturbances 

 are also in general more numerous in spring and autumn, than at 



[* The actual operation of producing dust figures by the discharges of an induc- 

 tion coil was exhibited by the author at the conclusion of the reading of his paper. 

 Two figures were produced, the one agreeing with diagram No. 15, and the other, 

 a new figure, obtained by arching the positive wire and causing it to touch the 

 plate at a succession of points.] 



VOL. LII. 



