192 Mr. W. Ellis. On the Simultaneity of 



other parts of the year. 4. Earth currents, which, usually feeble r 

 are active and strong at the times of magnetic disturbance, remain- 

 ing active only so long as disturbance continues. In the absence of 

 magnetic disturbance there exists practically earth current calm. 



These are the broad features of magnetic variations as experienced 

 at Greenwich, and which there is reason to believe are generally 

 similar at other places. 



Having premised thus much, we will now proceed with the special 

 inquiry to which this paper, is directed. 



When magnetic disturbance is experienced at Greenwich, the 

 photographic registers show that, after a period of magnetic calm, 

 disturbance commences sometimes very suddenly, and at other times 

 with a premonitory sign or signs. In the latter case the first indica- 

 tion will not unf requently be a sudden and sharp movement occurring 

 simultaneously in all elements, moderate it may be in amount and of 

 isolated character, followed after a shorter or longer period by 

 general magnetic disturbance ; at other times the first sharp move- 

 ment ushers in at once the disturbance. In other cases disturbance 

 will arise gradually without any special premonitory indication, 

 appearing in one element before showing, in any marked degree, in 

 the others. But when there is sudden initial movement, whether 

 great or small, it is very definite in character, and appears at Green- 

 wich without exception simultaneously in the registers of declination, 

 horizontal force, vertical force, and earth currents. The instantaneous 

 movement is the really remarkable point. Much larger motions will 

 occur during the course of a magnetic disturbance, but not usually 

 movements similarly sudden. They are such as will be quite 

 familiar to all those who may be acquainted with photographic 

 registers of magnetic variations, and we shall see that similar 

 characteristics are observed at other places. 



Now it is known that any considerable magnetic disturbance or 

 magnetic storm is felt over wide areas of the earth's surface, com- 

 mencing and terminating at different places at about the same 

 absolute time. But how nearly such commencement is really simul- 

 taneous at different places, whether or not in a much closer degree 

 than has before been supposed, does not appear to have hitherto been 

 so carefully studied as seems possible. For the initial movements 

 are so definite and so suddenly manifested that it is evident, from the 

 character of the photographic traces at Greenwich, that at such times a 

 very perceptible, sometimes the whole, movement has occurred in a very 

 few seconds, the fineness and exceeding delicacy of the photographic 

 trace, as compared with that of the ordinary register, showing how 

 rapidly the spot of light has moved across the paper. The accidental 

 comparison of the Greenwich motions with those at other places in 

 one or two instances of the kind seemed to show that the corre- 



