472 KEPOUT— 1881. 



eclipse of the liglit for either two minutes, as in the case of Vienna, or 

 four minutes, as at Kew, Stonj'hurst, <fec. The eclipse of the light at the 

 beginning of every hoar, or of every two hours, for two minutes, which is 

 quite long enough to mark the time, might conveniently be adopted, and 

 is better tban an eclipse for a longer period, if it occurs in the middle of a 

 storm. Also at Vienna only one curve is photographed on each sheet, and 

 so the difficulty is avoided of having two or more time lines with the same 

 hours not coincident with one another, and there is no confusion arising 

 from curves crossing one another. 



In the Plates the curves have been traced with great care, and set as 

 accurately as possible to Greenwich mean time. They have been grouped 

 so as to bring out prominently the common features running through 

 them, with the object of supplying facts on which a theory of terrestrial 

 magnetic disturbances may be firmly established. 



During this August storm great difficulties were experienced iu work- 

 ing the telegraph lines, in consequence of earth- currents, and from parti- 

 culars with which I have been supplied by Mr. Preece, and from the earth- 

 current photographic records taken at Greenwich Observatory on two 

 separate wires running in a S.W. and a N.W. direction, for copies of which 

 I am indebted to the Astronomer-Royal, it is possible to trace the connec- 

 tion between these magnetic storms and earth-currents. 



The photographic records at Greenwich during this storm are bent 

 opposite ways at the same time, so that when an earth-current is running 

 on one line towards Greenwich, on the other the earth-current is away 

 from Greenwich ; thus, when there is an earth-current on the line from 

 the S.W. towards Greenwich, there is found to be a current on the other 

 line from Greenwich towards the N.W. If the cui'rent on these lines 

 were absolutely equal it would indicate that the current in the crust of 

 the earth was in a direction from south to north. If the current from 

 the S.W. is greater than the current towards the N.W. it would indicate 

 an earth-current in a direction more nearly inclined from S.W. to N.E. 

 On comparing these photographic records with the earth-current records 

 from Derby and Haverfordwest and other places, it appears that the 

 general direction of currents during this storm was from about S.S.W. 

 to N.N.E., with varying intensity, the agreement between the distur- 

 bances of the declination needle and the Blackheath and Greenwich 

 photographic record (i.e. from S.W. to Greenwich), being very close and 

 in the same direction at the same time. Earth-currents were very violent 

 from 10.30 a.m. on the 11th to about 2.30 p.m., and again from 9 p.m. 

 until midnight. They were violent on August 12, beginning at 11.30 

 a.m., the beginning of the second storm, and quieting down about 4.30 

 p.m., then beginning again at 7.30 p.m. and lasting until 9.30 p.m. Again 

 on the 13th they were recorded as strong for about one and a half 

 hours from -5 a.m., i.e. just at the end of the second magnetic storm. 



The general direction of earth-currents as observed on telegraph lines, 

 especially at Derby and Haverfordwest, as well as at the Greenwich Ob- 

 servatory, was from a S.W. to a N.E. direction. During the most violent 

 phases of the magnetic storm, the earth-current photographic record at 

 Greenwich failed in consequence of the rapidity and the extent of the 

 swings of the galvanometer needle. Intimately connected with magnetic 

 disturbances and earth-currents is the phenomenon of the Aurora or Polar 

 Light, which is an electric discharge in the upper regions of the atmosphere. 

 During the August storm, and also during another violent magiietic 



