TRANSACTIONS OF THE SECTIONS. 53 



Notes on Atmospheric Electricity. 

 By Professor William Thomson, LL.D., F.R.S. 



Two water-dropping collectors for atmospheric electricity were prepared, and 

 placed, one at a window of the Natural Philosophy Lecture-room, and the other at 

 a window of the College Tower of the University of Glasgow. A divided ring- 

 electrometer was used at the last-mentioned station ; an electrometer adapted for 

 absolute measurement, nearly in the form now constructed as an ordinary house elec- 

 trometer, was used in the lecture-room. Four students of the Natural Philosophy 

 Class, Messrs. Lorimer, Lyon, M'Kerrow, and Wilson, after having persevered in 

 preliminary experiments and arrangements, from the month of November, devoted 

 themselves with much ardour and constancy during February, March, and April to 

 the work of observation. During periods of observation, at various times of day, 

 early and laie, measurements were completed and recorded every quarter-minute 

 or every half-minute ; the continual variations of the phenomenon rendering soli- 

 tary observations almost nugatory. During several hours each day simultaneous 

 observation was carried on on this plan at the two stations. A comparison of the 

 results manifested often great discordance, and never complete agreement. It was 

 thus ascertained that electrification of the air, if not of solid particles in the air 

 (which have no claim to exclusive consideration in this respect), between the two 

 stations and round them, at distances from them not very great in comparison with 

 their mutual distance, was largely operative in the observed phenomena. It was 

 generally found that after the indications had been negative for some time at both 

 stations, the transition to positive took place earlier by several minutes at the 

 tower station (upper) than at the lecture-room (lower). Sometimes during several 

 minutes, preceded and followed by positive indications, there were negative indica- 

 tions at the lower, while there were only positive at the upper. In these cases the 

 circumambient air must have contained negative (or resinous) electricity. A hori- 

 zontal stratum of air several hundred feet thick overhead, if containing as much 

 positive electricity per cubic foot as there must have been of negative per cubic foot 

 of the air about the College buildings on those occasions, would produce electrical 

 manifestations at the earth's surface similar in character and amount to those ordi- 

 narily observed during fair weather. 



Beccaria has remarked on the rare occurrence of negative atmospheric indications 

 during fair weather, of which he can only record six during a period of fifteen vears 

 of very persevering observation by himself and the Prior Ceca. On some, if not 

 all of those occasions, there was a squally and variable wind, changing about rapidly 

 between N.E. and N.W. On several days of unbroken fair weather in April and 

 May of the present year the atmospheric indication was uegative during short periods, 

 and on each occasion there was a sudden change of wind, generally from N.E. to 

 N.W., W., or S.W. For instance, on the 3rd of May, after a warm, sunny, and very 

 dry day, with a gentle N.E. 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 un protected 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.W. wind set in all round, and 

 with the aid of a lantern I found strong positive indications, which continued as long 

 as I observed. During all this time the sky was cloudy, or nearly so. That re- 

 versed 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 electricity as that of the earth's surface, since this 

 reaches a high degree of intensity on every tree -top and vegetable fibre, and must 

 therefore cause always more or less of the phenomenon, which becomes conspicuous 

 as the " light of Castor and Pollux," known to the ancients, or the " fire of St. 

 Elmo " described by modern sailors in the Mediterranean, and which consists of a 



* About six miles south of Glasgow 



