AURORA BOREALIS. ' 233 



These irregular deflections of tlie mngnetic needle do not occur everywhere 

 simultaneoui^ly. From n comparison of a very large number of observations 

 made in the years 1836 to 1841, at twenty-seven stations scattered over Europe 

 from latitude 45° to 60° N , I have discovered that they are propagated over 

 the surface of Europe in a direction from N. 28° E. to S. 28° W., at the rate of 

 about 100 miles per minute.* From a similar comparison of observations made 

 at "Washington, Philadelphia, Cambridge, and Toronto in the years 1840 to 

 1842, I have discovered that in North America those irregular deflections of 

 the magnetic needle are propagated in a direction from N. 68° E. to S. 68° W., 

 at the rate of about 100 miles per 'minute. t Mr. 0. V. Walker has determined 

 that the direction of this motion in England was from N. 42° E. to S. 42° W. 



Influence of the aurora upon tJte tdcgraph wires. — Auroras exert a remarka- 

 ble influence upon the wires of the electric telegraph. During the prevalence of 

 brilliant auroras the telegraph lines generally become unmanageable. The 

 auiora develops electric currents upon the wires, and hence results a motion of 

 the telegraph instruments similar to that which is employed in telegraphing; 

 and this movement being frequent and irregular, ordinarily renders it impossible 

 to transmit intelligible signals. During the aurora of September 2, 1859, the 

 currents of electricity on the telegraph wires of the United States were so steady 

 and powerful that, on several lines, the operators succeeded in using them for 

 telegraph purposes as a substitute for the battery ; that is, telegraph messages 

 were transmitted from the auroral influence alone, without the use of any vol- 

 taic batteiy.| This result clearly proves that the aurora develops on the tele- 

 graph wires an electric current similar to that of a voltaic battery, and differing, 

 only in its variable intensity. 



These electric currents during the auroras of August 29 and September 2, 

 1859, moved alternately to and fro over the earth's surface, their average direc- 

 tion being probably from about N. 45^ E. to S. 45° W. 



Similar effects were noticed upon the telegraph lines of Europe. In Switzer- 

 land the intensity of the currents was measured by a galvanometer, and was 

 found to be three-fold the ordinary current employed in telegraphing. Two 

 currents were found to succeed each other, having a general direction nearly 

 along a meridian line ; the one proceeding from north to south having a double 

 intensity and a double duration, the other proceeding from south to north hav- 

 ing a less intensity and a less duration. 



From careful observations of galvanometers upon the telegraph lines of Eng- 

 land during the auroras of August 29 and September 2, Mr. C. V. Walker dis- 

 covered that there was a stream of electricity of indefinite width drifting across 

 the country, moving to and fro along a line directed from N. 42° E. to S 

 42° W. 



THEORY OF THE POLAR LIGHT. 



1. Some have ascribed the polar light to a rare nebulous matter occupjnng 

 the interplanetary spaces, and revolving round the sun at such a distance that 

 a portion of this matter occasionally falls into the upper regions of the atmo- 

 sphere with a velocity sufficient to render it luminous, from the condensation of 

 the air before it. But we can see no reason why matter, reaching the earth 

 from such a source, should be confined to certain districts of the earth, and be 

 ■wholly unknown in other portions. During a single month, or possibly an en- 

 tire year, the fall of such matter might be limited to certain parts of the earth ; 

 but that certain portions of the earth should alwaijs be exempt from such visits 

 whfle other portions receive them uninterruptedly from night to night, is quite 



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



t American Journal of Science, n. s., v. 34, p. 38. 



+ American Journal of Science, a. s., v. 29, pp. 92-97. 



