MR. BOTTOMLEY'S extremely interesting experiments briefly described in NATURE, vol. xxiii. pp. 218 and 243, appear to have a very important bearing on the question of atmospheric electricity; for if such high vacua are good conductors of electricity we have reason for thinking that the electrical conditions of our globe will be very different from what we have been accustomed to regard them. The layers of denser air surrounding the conducting matter of the globe will act like the glass of Mr. Bottomley's tubes in maintaining by a Leyden-jar-like action any difference of potential that there may be between their inner and their outer surface. Again, in the piercing of the glass tube by a minute spark, we have the analogue of the lightning flash between the clouds and the earth; the insulating layer in each case giving way, when, owing to an excessive increase in the surface density of the charge at any point, the dielectric stress exceeds the limits of the dielectric strength of the medium. The internal luminous effects observed by Mr. Bottomley as the result of change in the distribution of the external charge of electricity will be the physical analogues of the aurora, with this difference, that they take place in the ultra-gaseous interior, whereas in the case of our globe the luminous phenomena take place in the ultra-gaseous (i.e. highly rarefied) exterior regions of the atmosphere. It would be interesting to learn whether such discharges present any other analogies with auroral phenomena. I should be particularly interested in learning whether the conditions under which such luminous effects are obtained give any support to the theory which I think to be the only consistent one, that the aurora is due not to electrical discharges from regions of less atmospheric density to regions of a greater density (or vice versâ), but to electrical discharges in a region of pretty uniform (and small) density, and in which region differences of electric potential exist. According to this view the auroral streaks which appear to be radial should in reality lie approximately parallel to the earth's surface, and not stand (as most persons imagine) normal to it. A series of horizontal parallel lines drawn across the sky in a direction approximately north and south would necessarily appear to an observer on the earth's urface foreshortened into a set of lines diverging in fan-like forms at either the north point or the south point of the horizon. Their divergence would therefore be apparent only, like the “beams” diverging from the sun at sunset on a cloudy day, or like the beams of the rayons du crépuscule, or like the “radial streaks” which I have pointed out as frequently accompanying rainbows.
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