362 Prof. W. Thomson's Notes on Atmospheric Electricity. 



breeze and slight easterly haze in the air, I found about 8*30 p.m. 

 the expected positive atmospheric indication. After dark (nearly 

 an hour later) it was so calm that I was able to carry an unpro- 

 tected candle into the open air and make an observation with my 

 portable electrometer. To my surprise I found a somewhat 

 strong negative indication, which I observed for several minutes. 

 Although there was no sensible wind in the locality where I 

 stood*, I perceived by the line of smoke from a high chimney at 

 some distance that there was a decided breeze from W. or S.W. 

 A little later a gentle S."\V. wind set in all round, and with the 

 aid of a lantern I found strong positive indications, which con- 

 tinued as long as I observed. During all this time the sky was 

 cloudy, or nearly so. That reversed electric indications should 

 often be observed about the time of a change of wind may be 

 explained, with a considerable degree of probability, thus . — 



The lower air up to some height above the earth must in 

 general be more or less electrified with the same kind of elec- 

 tricity as that of the earth's surface ; and, since this reaches a high 

 degree of intensity on every tree-top and pointed vegetable 

 fibre, it must therefore cause always more or less of the phe- 

 nomenon which becomes conspicuous as the " light of Castor and 

 Pollux," known to the ancients, or the " fire of St. Elmo " de- 

 scribed by modern sailors in the Mediterranean, and which 

 consists of a flow of electricity of the kind possessed by the 

 earth into the air. Hence in fair weather the lower air must be 

 negative, although the atmospheric potential, even close to the 

 earth's surface, is still generally positive. But if a considerable 

 area of this lower stratum is carried upwards into a column over 

 any locality by wind blowing inw r ards from different directions, 

 its effect may for a time predominate, and give rise to a negative 

 potential in the air, and a positive electrification of the earth's 

 surface. 



If this explanation is correct, a whirlwind (such as is often 

 experienced on a small scale in hot weather) must diminish, and 

 may reverse, the ordinary positive indication. 



Since the beginning of the present month I have had two or 

 three opportunities of observing electrical indications with my port- 

 able electrometer during day thunder-storms. I commenced the 

 observation on each occasion after having heard thunder, and I per- 

 ceived frequent impulses on the needle which caused it to vibrate, 

 indicating sudden changes of electric potential at the place where 

 I stood. I could connect the larger of these impulses with thunder 

 heard some time later, with about the same degree of certainty 

 as the brighter flashes of lightning during a thunder-storm by 

 night are usually recognized as distinctly connected with distinct 

 peals of thunder. By counting time I estimated the distance of 

 * About six miles south of Glasgow. 



