AURORA BOREALIS. 237 



result of a current of electricity travelling in the direction of tlie axis of the 

 beams. » 



7. The slaty appearance of the sky, which is remarked in all great auroral 

 exhibitions, arises from the condensation of the A'apor of the air ; and this con- 

 densed vapor probably exists in the form of minute spiculse of ice or flakes of 

 snow. In the Arctic regions fine flakes of snow have been repeatedly observed 

 lo fall during the exhibition of auroras, and this snow only slightly impairs the 

 transparency of the atmosphere, without presenting the appearance of clouds.* 

 The presence of these minute flakes of snow produces that turbid appearance of 

 the atmosphere which invariably attends bright auroras, and causes that dark 

 bank or segment which in the United States rests on the northern horizon. 

 This turbidness is more noticeable near the horizon than it is at great elevations, 

 because near the horizon the line of vision traverses a greater depth of this hazy 

 atmosphere, while the effect is increased by contrast with the light above it. 

 "When the aurora covers the whole heavens, as in the neighborhood of Hudson's 

 bay, the entire atmosphere is filled with this haze; and if the attrora goes far 

 beyond the zenith of the observer towards the south, he sees for the same rea- 

 son a dark segment resting on the southern horizon. 



8. W/iat is the sov,rcc of the clcctririty (yf the atmosphere ? — Philosophers are 

 by no means agreed as to the origin of atmnspheric electricity. It has been 

 ascribed successively to friction, combustion, and vegetation, but these causes 

 seem entirely inadequate to account for the enormous quantities of electricity 

 sometimes present in the atmosphere. 



Evaporation is probably the principal source of atmospheric electricity. The 

 following experiment shows the production of electricity by evaporation. If 

 upon the top of a gold leaf electrometer we. place a metallic vessel containing 

 salt water, and drop into the water a heated pebble, the leaves of the electrometer 

 will diverge. The vapor Avhich rises from the water is chargt'd with positive 

 electricity, while the water retains negative electricity. The water used in this 

 experiment must not be perfectly pure, but must contain a little salt or some 

 foreign matter. The evaporation of the water of the ocean must, therefore, fur- 

 nish a large amount of electricity : and fresh water must also furnish some elec- 

 tricity, for the water of the earth is never entirely pure. The vapor that rises 

 from the sea, therefore, constantly carries away positive electricity, while the 

 solid part of the earth must be charged with negative electricity. 



9. The vapor which rises from the ocean in all latitudes, but most abundantly 

 iu the equatorial regions of the earth, carries into the upper regions of the 

 atmosphere a considerable quantity of positive electricity, while the negative 

 electricity remains in the earth. This positive electric :ty, after rising more or 

 less vertically with the ascending currents of the atmosphere, would be conveyed 

 towards either pole by the tropical current of the upper regions of the atmos- 

 phere. This tropical curKmt, setting out from the equiitor where it occupies the 

 most elevated regions of the atmospliere, descends iu proportion as it advances 

 towards the higher latitudes, until in the neighborhood of the poles, Avhere it 

 approaches the earth's surface. 



The earth and the rarefied air of the elevated atmospheric regions may be 

 regarded as forming the two conducting plates of a condenser, of which the 

 insixlating stratum is the inferior portion of the atmosphere. The two opposite 

 electricities must then be condensed by their mutual influence in those portions 

 of the atmosphere and of the earth to which they are nearest; that is, in the 

 regions near the poles, and there neutralize themselves in the form of dis- 

 charges whenever their tension reaches a certain limit. When the air is 

 humid, it becomes a partial conductor between th* upper regions of the atmos- 

 phere and the earth, by which means a portion of the electricity of the atmos- 



* Fraukliu's First Expedition, pp. 583 and 600. 



