Magnetic Variations at different Places, $c. 211 



ties, in regard to which there is yet much to learn. It would tend to 

 help this, as well as other inquiries concerning earth current phe- 

 nomena, if at some of the magnetic observatories in different parts of 

 the world, the magnetic registers were supplemented by a registra- 

 tion of earth currents, as a regular part of the daily work. 



The comparison of the times of magnetic impulses with those of 

 the corresponding earth currents, Table IV, gave, as a mean result, 

 that, at Greenwich, the magnetic impulse, on the whole, followed the 

 earth current by some few seconds. To determine more exactly and 

 with greater certainty, and especially in individual cases, what may 

 really be the interval, if appreciable, between the phenomena would 

 require apparatus designed to indicate simultaneously and measure 

 accurately much smaller intervals of time than is at present possible. 

 This leads us to mention a difficulty that affects the registration of 

 magnetical and meteorological phenomena of all kinds, and one not 

 readily overcome. To obtain continuous registration of any element 

 a contracted time scale is necessary, otherwise the accumulated 

 registers would become overwhelming, neither is an extended time 

 scale, under ordinary circumstances, required. But on occasions of 

 disturbance or storm, whether magnetic or atmospheric, it becomes 

 desirable to possess the power of greatly increasing the extent of the 

 time scale, in order to obtain, not only a more accurate time indica- 

 tion, but also a better record of the details of phenomena, The 

 difficulty becomes increased because of the uncertainty of the advent 

 of any special phenomenon of which fuller particulars would be 

 desirable. Still, if it were possible to provide means ot easily 

 changing the speed of movement of a register, much valuable infor- 

 mation might be gained. 



In recapitulation of the results which may be considered to have 

 been arrived at in this paper, we may remark that, though it was 

 known that magnetic storms were felt generally at the same time 

 over wide areas of the earth's surface, it had not been ascertained 

 that any magnetic movements were so entirely coincident at different 

 places as now appears, at any rate so far as concerns the initial 

 magnetic impulses that precede disturbance, and which, it would 

 seem, really occur at the same instant of time, or nearly so, over 

 the whole earth. It appears also, in addition, that the change that 

 takes place at such times in the earth's magnetic condition is, on all 

 occasions, in great measure of like character. A definite magnetic 

 effect is produced suddenly and simultaneously, in which the varia- 

 tions of the magnetic elements, whilst different at different places, 

 are, on different occasions, locally similar, forming thus a type of 

 the magnetic phenomena that, repeating itself usually in the same 

 way, generally presages or introduces a magnetic disturbance or 

 storm. 



