1863.] 663 



XIV. "-On the Magnetic Disturbance which took place on the 

 14th of December 1862." By BALFOUR STEWART, M.A., 

 F.R.S. Received May 19, 1863. 



On the 14th of December 1862, a magnetic disturbance occurred 

 about 6 o'clock in the afternoon, and was registered by means of the 

 Kew magnet ographs. As usual it was accompanied by an auroral 

 display and by earth- currents, and the latter phenomena were ob- 

 served at Greenwich by means of a system of telegraphic wires which 

 had recently come into the possession of the Astronomer Royal. 



By the kindness of Mr. Airy, the Kew Observatory has been 

 favoured with a copy of the curves which represent the earth-cur- 

 rents collected at Greenwich during the progress of this disturbance ; 

 and a comparison of these with the Kew magnetograph curves will 

 form the subject of the following paper. It will, however, first be 

 necessary to state the arrangement adopted at Greenwich. There 

 are two wires proceeding from the Observatory, one ending near 

 Croydon and the other near Dartford (nearly at right angles) ; and 

 these are well insulated throughout their entire length, and have 

 good earth-connexions at their extremities. 



From these, by means of galvanometers, the intensity of earth- 

 currents is recorded for two very favourable directions ; and from 

 these again the intensity of those currents which flow in the magnetic 

 meridian, and in a direction perpendicular to it, may be very easily 

 deduced. The standard for direction is the current which flows 

 through the wire of a battery from the zinc pole, and which is called 

 the zinc-current. With this explanation the earth-current diagrams 

 appended to this paper will be quite intelligible ; and with regard to. 

 the magnetic curves, it is only needful to remark that increasing or- 

 dinates denote decreasing declination and decreasing horizontal force, 

 and that the normal lines, which have been furnished through the 

 kindness of General Sabine, denote the position which the curves 

 would have occupied had no disturbance supervened. But before 

 proceeding to compare together the two sets of curves, it will be 

 necessary to advert to a peculiarity of disturbances which enters as 

 an essential element into all such discussions. It has been found by 

 General Sabine that if the disturbances of declination be divided into 

 two categories, easterly and westerly, these obey very different laws 



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