240 AUEOEA BOREALIS. 



current from the northeast to the soxTthwest, and such a current has been posi- 

 tively indicated in a most decisive manner — 



1. By observations on the telegraph lines of England with a galvanometer 

 needle* 



2. By observations on the telegraph lines of the United States and other 

 countries.t 



3. By simultaneous observations of the magnetic declination made in 1836 to 

 1841 at numerous stations scattered over Europe.f 



4. By similar observations at several stations in Xorth America, made from 

 1840 to 1S!42.§ 



This current of electricity does not, however, flow steadily and uninterrupt- 

 edly from northeast to southwest, but alternates at short intervals with a cur- 

 rent in the contrary direction. This fact was distinctly noticed in September, 

 1859, upon the telegraph lines of the United States. It was also distinctly 

 observed and measured upon the telegraph lines in Switzerland, where a north- 

 erly current continued for two or three minutes and then slowly declined, when 

 it was succeeded by a southerly current of less intensity, which continued for 

 sixty or ninety seconds and then declined, to be succeeded by another current 

 from the north — the northerly current having' a double intensity and a double 

 duration ; the other, proceeding from south to north, having a less intensity and 

 a less duration. 1 1 



In England, the northerly currents are also generally stronger 4;han the south- 

 erly, and they continue for a longer time, but the difference js less than was 

 observed in Switzerland.^ 



Such currents of electricity must produce a continual disturbance of the mag- 

 netic needle, and they seem sufficient to account for the disturbances actually 

 observed. Mr. 0. V. Walker has compared magnetic observations made at 

 Greenwich and Kew, and has discovered that the deflections of the magnet;3 

 there observed were such as should be produced by the electric currents ob- 

 served on the telegraph wires ** 



15. Effect of the aurora upon telegraph wires. — The effect of the aurora upon 

 the telegraph wires is similar to that of electricity in thunder-storms, except in 

 the intensity and steadiness of its action. During thunder-storms the elec- 

 tricity of the wires is discharged instantly with a flash of lightning, while during 

 auroras there is sometimes a steady flow for a few minutes, which may even be 

 employed as a substitute for the voltaic battery, in transmitting telegraph 

 messages. 



16. The geographical distribution of auroras. — The geographical distribu- 

 tion of auroras appears to depend chiefly upon the relative intensity of the 

 earth's magnetism in different latitudes. The circumstances favorable to a 

 grand display of the auroras appear to be — 1. The upper portion of the atmo- 

 sphere must b(! highly charged with electricity. 2. The atmosphere must be 

 filled with particles of condensed vapor, probably in the form of minute crystals 

 of ice. 3. This condensed vapor must form an imperfect conductor of great 

 extent, for the passage of the electricity from one portion of the heavens to 

 another, and from the upper atmosphere to the earth. According to the experi- 

 ments of De la Hive with artiflcial magnets, the electric light should be most 

 noticeable in the neighborhood of the magnetic pole, but not directly over the 

 pole, since the electric light tends to form a ring around the pole, and at some 



' London Pliil. Trans. 1861, p. lOG. 



t American Journal of Science, n. s., v. 32, p. 325. 



t American Journal of Science, n. s., v. 32, p. 334. 



$ Ibid., V. 34, p. 39. 



II Comptes Rendus, t. 49, p. 662. 



H Lond. Phil. Trans. 1861, pp. 128, 129. 



"^ Ibid., pp. Ill, 112. 



