1892.] Magnetic Variations, Magnetic Disturbances, fyc. 445 



II. " On the Simultaneity of Magnetic Variations at different 

 places on occasions of Magnetic Disturbance, and on the 

 relation between Magnetic and Earth Current Phenomena." 

 By William Ellis, F.R.A.S., Superintendent of the Mag- 

 netical and Meteorological Department, Royal Observatory, 

 Greenwich. Communicated by W. H. M. Christie, F.R.S., 

 Astronomer Royal. Received April 7, 1892. 



(Abstract.) 



In this paper the author refers to the ordinary variations of the 

 magnetic elements as observed at Greenwich ; the annual progressive 

 change; the diurnal variation — large in summer, small in winter, 

 and also larger when sun spots are numerous and smaller when sun 

 spots are few ; the irregular magnetic disturbances and magnetic 

 storms, and the accompanying earth currents ; phenomena which are 

 generally similar at other places. 



He then invites attention more particularly to magnetic disturb- 

 ances. Those at Greenwich may, after a calm period, arise gradu- 

 ally, or commence with great suddenness. When sudden, the move- 

 ment is simultaneous in all elements. The first indication may be 

 a sharp, premonitory, simultaneous movement, followed after a time 

 by general disturbance, or the movement may at once usher in the 

 disturbance. These initial movements are not always great in magni- 

 tude, sometimes, indeed, small, but they have a very definite charac- 

 ter, and frequently occur nearly instantaneously, as is shown by the 

 character of the photographic traces. 



It has been long known that magnetic disturbances occur at the 

 same time over wide areas of the earth's surface, but the accidental 

 comparison in past years of the times of commencement of one or 

 two disturbances at Greenwich with the times at other places has led 

 •the author to suppose that the coincidence in time is much closer 

 lihan had been before supposed, and the definite, and on occasions 

 isolated, character of the initial movement induced him to undertake 

 the collection and comparison of the times of such movements for a 

 number of days at observatories geographically widely separated. 



The times of such movements cannot be caught by eye observation 

 without continuous watching of the magnets, so that the photo- 

 graphic registers have to be relied upon, which is better, excepting 

 that the scale of time is necessarily contracted ; but, though in indi- 

 vidual measures there might be variations, it was conceived that 

 (supposing no systematic error to exist) the mean of a number of com- 

 parisons should give a good result. Seventeen days occurring in the 

 years 1882 to 1889 were selected for comparison, the observatories 



2 h 2 



