Page:The American Cyclopædia (1879) Volume II.djvu/135

 AURORA BOEEALIS 123 between auroral action and disturbances of the earth's magnetism appears to have ben dem- onstrated, though doubt still remains as to the exact nature of the association. The perturba- tions of the magnetic needle undoubtedly attain their maximum extent at intervals separated by about 11 years. The researches of Sabine, La- mont, and Wolf appear to have established that fact beyond dispute. Hence we may infer that the auroral action waxes and wanes with- in the same period. A remarkable associa- tion also appears to exist between disturbances of the earth's magnetism and the occurrence of spots on the sun. It has been demonstrated that the solar spots increase and diminish in a period of about 11 years; and that this peri- odicity corresponds exactly with the periodicity of the magnetic perturbations. A great solar outburst witnessed by Carrington and Hodg- son, Sept. 29, 1859, was not only accompanied by extensive magnetic disturbances, but on the same day remarkable auroras occurred in both hemispheres. Telegraphic communication was interrupted on all the principal lines ; the ope- rators at Washington and Philadelphia received sharp electric shocks ; and the pen used in Bain's system of telegraphy was followed by a flame. Some doubt has been thrown on the supposed connection between these circum- stances and the solar outburst, in consequence of the failure of observers to obtain any corrob- orative evidence during the past 13 years; but the connection between the condition of the so- lar surface and the earth's magnetic state, and therefore the connection between the solar spot period and auroral displays, has been thor- oughly established. The following table ex- hibits the number of auroras seen in each month, in America and Europe, according to the observations of Prof. Loomis of Yale college and Kamtz of Germany. These observations, however, must not be looked upon as indi- cating the relative frequency of auroras in America and Europe, because the observations of Loomis and Kamtz range over a different number of years : January. .. Loomii. ... 173 Kamtz. 229 July LoomU. .. 244 Kamtz. 87 February. . ... 210 240 807 440 August. . . .. 233 293 217 405 267 812 236 im 1S4 215 2S5 .... June. . . .. 179 65 .. 159 2'25 In each case there is a double maximum, the two equinoxes being the epochs at which auro- ras are most frequent ; and it is noteworthy that in these mouths the solar poles are most inclined toward the earth, the southern pole in March, I the northern pole in September ; so that the southern spot zone is nearer to the centre of the sun's face in March than at any other time, while the northern spot zone holds a cor- responding position in September. As to the electrical character of the phenomenon no question can be entertained, though there are few problems of greater difficulty than the determination of the exact manner in which the electrical action is excited. It has been held by some that the aurora is due to elec- trical discharges from the earth. Through some cause the earth, regarded as a vast mag- net, becomes overcharged (according to this theory) with electrical energy, and it is as this energy is gradually dissipated that the splen- dors of the aurora are displayed. It has been noticed that whenever the earth's magnetism is unusually intense an auroral display is to be expected. As soon as the aurora has made its appearance the intensity of the magnetic force begins to diminish. The more brilliant the aurora, the more rapidly is the extra energy of the earth's magnetism dissipated. " It has also been observed by .operators of the Bain or chemical telegraph, that very singular effects are produced by the aurora upon the telegraph wires. The atmospheric electricity generated during thunderstorms passes from the wire to the chemically prepared paper, emitting a bright spark and a sound like the snapping of a pistol. It never remains long upon the wires, though it travels sometimes 40 or 50 miles before discharging itself. But the elec- tricity produced by the aurora passes along the wires in a continuous stream with no sudden discharge, effecting the same result as that by the galvanic battery. A colored mark upon the paper is made by the positive current of the aurora as by the positive pole of the bat- tery; the negative current, on the contrary, produces a bleaching effect. Preceding tho appearance of the aurora faint blue lines appear on the paper, which gradually become stronger and darker so as to burn through several thick- nesses of it. The effect then disappears, and is soon followed by the bleaching process, which entirely overcomes the artificial current of the batteries. When these effects have been ob- served, the aurora follows, and presents some of its most beautiful displays along the lines of these telegraphs ; and so familiar have the operators become with the disturbance which the aurora causes, that they can predict its ap- pearance with much certainty. They regard the electricity generated by it as precisely that of the electro-galvanic battery, which is dis- tinguished by its voluminous current without intensity of action, differing from atmospheric electricity or the kind developed by friction, which may be dissipated by placing a wire conductor leading to the ground in close prox- imity to the line of wires." Capt. McClintock observed in the arctic regions that the aurora was never visible above ice fields, but that whenever an aurora was in progress the light appeared always to be gathered over the sur- face of tho open water. Water being, as is well known, an excellent conductor of elec- tricity, while ice is a non-conductor, we may infer that the peculiarity observed by McClin- tock was due to this difference in tho conduct- ing powers of ice and water. In fact, on the theory that the aurora is due to electrical dis- charges from the earth, these discharges were