Page:Popular Science Monthly Volume 24.djvu/493

Rh (he does not say how), gave rise, at some distance from the earth, to the phenomena of the aurora.

A large library would hardly be sufficient to hold all the memoirs and notices that have been published during the past sixty years on the subject of the aurora borealis, to say nothing of the numerous treatises on physics, meteorology, and astronomy which have devoted one or more chapters to it. Some authors have limited themselves to the simple description of what they have perceived, or to a mere exposition of their theories, while others have done more. Alexander von Humboldt has drawn in his "Cosmos" an excellent outline of the ideas which science entertained on the subject in his time; and the "Popular Astronomy" of Arago contains valuable details, well classified and arranged, on the same question.

About 1850, M. de La Rive, a Genevese physicist, endeavored to found a definite theory of the aurora borealis, and with this view artificially reproduced the phenomenon with considerable success. A prime point, which is still far removed from being fixed, is the approximate height of the meteor above the ground. Sometimes two observers, in the neighborhood of a thousand miles apart, will affirm that they have seen the same aurora at the same time and under the same aspect; at other times, the phenomenon is visible only within a radius of a few leagues. Mairan, basing his calculations on data that are not without value, concluded an elevation of two or three hundred leagues; Bravais proposed one hundred and fifty kilometres as a mean value. Other authors have supposed that the highest flashes soar to an elevation of eight hundred kilometres.

M. de La Rive has made a table of all former data, and represents that the auroræ boreales, very low in reality, hardly pass beyond the zone of clouds. They have been perceived (by Parry) projected on the flanks of mountains. Contradictions of this view are also not wanting. In support of his opinion that the meteor is low in height, M. de La Rive cites the well-established cases in which sounds have been heard during the manifestations. Sometimes a sulphurous odor has been perceived. The crackling occasioned sometimes by slow electric discharges and the odor of electrified oxygen or ozone are quite analogous. Explorers and aëronauts have pretended, according to M. de La Rive, to have gone through the aurora or through the mist that gives rise to it.

Arago had conceived the electric nature of the meteor, and assumed to predict its appearance by consulting the compass. Other facts, proving a connection between auroras and magnetic phenomena, are abundant. Jessan, in 1878, sailing on the Venus, relates that during a fine aurora all the compasses of the vessel were disordered, and they