ON CURVES OF DECLINATION MAGNETOGRAPHS, ETC. 203 



and between 10 p.m. and 12 there wei-e simultaneous disturbances at 

 Kew and St. Petersburg, but in opposite directions. 



From about 10 to 10.40 p.m. there is a disturbance of a very regular 

 kind, i.e. without much agitation, consisting of a motion of the needle 

 towards the east, followed by a motion of the needle westward for about 

 half an hour. This disturbance is strongly marked at Kew and at 

 Stonyhurst; is less strongly marked, but coincident in time, at Coimbra 

 and at Lisbon ; and also very well shown, but is small, in the Vienna 

 photographs ; but in the tracing from St. Petersburg a disturbance 

 begins at the same point of absolute time (i.e. about 10 p.m. Greenwich 

 time), with a motion of the needle towards the ivest — this motion west- 

 ward lasts for about 20 minutes, until 10.20 p.m., and is then followed by 

 a gradual motion eastward until about 10.45 p.m. 



The declination at St. Petersburg then remains nearly steady for a 

 quarter of an hour, whilst the westerly declination at the other stations 

 is regularljr increasing, and from 11.30 p.m. (Greenwich time) the dis- 

 turbances at St. Petersburg coincide in direction and in time with those 

 at Kew and at the other stations. 



Plate VII., fig. 1, represents the St. Petersburg, Kew, and Vienna 

 declination curves for March 3rd, the time being Greenwich mean time 

 for all stations. 



On referring to the Kew curves for the horizontal force, of which Mr. 

 Whipple has kindly prepared tracings for me, I find that whenever the 

 deflections of the declination needle are eastward at Kew and westward 

 at St. Petersburg at the same instant, as in this disturbance between 10 

 and 10.20 p.m., there is at the same time an increase in the horizontal 

 force ; and when the deflections are westward at Kew and eastward at 

 St. Petersburg at the same instant, as l)"t\veen 10.20 p.m. and 10.45 p.m., 

 there is at the same time a decrease going on in the horizontal force. 



This statement is borne out by the comparisons of disturbances on 

 other days throughout the month. 



Three easterly movements of the needle occurred between 3 and 8 p.m. 

 on the 5th of March. 



One began about 2.45 p.m., which is only just noticeable at Lisbon 

 and Coimbra when looked for, but which is clearly seen in the Kew and 

 Stonyhurst jjhotographs, and becomes much more important at Vienna, 

 and is much larger still at St. Petersburg ; but at all places the gi-eatest 

 easterly declination occurs at the same absolute time (at about 3 p.m. 

 Greenwich time), and there is then an increase in the westerly declination 

 iintil about 3.30 p.m. 



From 5 p.m. to 5.20 p.m. there is an easterly movement of the 

 needle, which is absolutely coincident in time and is well marked at all 

 the stations, and the amount of the disturbance is as great at Kew and at 

 Stonyhurst as it is at St. Petersburg. This is followed by a westerly 

 movement, which is also precisely similar at all the places. 



Another similar easterly movement begins about 6.20 p.m. (Green- 

 wich time) at all the stations, and lasts for a quarter of an hour, 

 followed by an equal movement westward for the next quarter of an 

 hour, thus forming a regular \/ in the photograjjhic curves. The 



second side of this \J is continued to double the length in the St. Peters- 

 burg tracings, but the following greatest eastward declination is reached at 



