Prof. W. Thomson's Notes on Atmospheric Electricity. 363 



the discharge not nearer on any occasion than about four or five 

 miles. There were besides many smaller impulses ; and most 

 frequently I observed several of these between one of the larger 

 and the thunder with which I connected it. The frequency of 

 these smaller disturbances, which sometimes kept the needle in 

 a constant state of flickering, often prevented me from identify- 

 ing the thunder in connexion with any particular one of the im- 

 pulses I had observed. They demonstrated countless discharges, 

 smaller or more distant than those that gave rise to audible 

 thunder. On none of these occasions have I seen any lightning. 

 The absolute potential at the position of the burning match was 

 sometimes positive and sometimes negative; and the sudden 

 change demonstrated by the impulses on the needle were, so far 

 as I could judge, as often augmentations of positive or diminu- 

 tions of negative, as diminutions of positive or augmentations of 

 negative. This afternoon, for instance (Thursday, June 28), I 

 heard several peals of thunder, and I found the usual abrupt 

 changes indicated by the electrometer. For several minutes the 

 absolute potential was small positive with two or three abrupt 

 changes to somewhat strong positive, falling back to weak posi- 

 tive, and gathering again to a discharge. This was precisely 

 what the same instrument would have shown anywhere within a 

 few yards of an electrical machine turned slowly so as to cause a 

 slow succession of sparks from its prime conductor to a conductor 

 connected with the earth. 



I have repeatedly observed the electric potential in the neigh- 

 bourhood of a locomotive engine at work on a railway, some- 

 times by holding the portable electrometer out a window of 

 one of the carriages of a train, sometimes by using it while 

 standing on the engine itself, and sometimes while standing on 

 the ground beside the line. I have thus obtained consistent 

 results, to the effect that the steam from the funnel was always 

 negative, and the steam from the safety-valve always positive. I 

 have observed extremely strong effects of each class from carriages 

 even far removed from the engine. I have found strong nega- 

 tive indications in the air after an engine had disappeared round 

 a curve, and its cloud of steam had dissolved out of sight. 



In almost every part of a large manufactory, with steam-pipes 

 passing through them for various heating purposes, I have found 

 decided indications of positive electricity. In most of these 

 localities there was some slight escape of high-pressure steam, 

 which appeared to be the origin of the positive indications. 



These phsenomena seem in accordance with Faraday's obser- 

 vations on the electricity of steam, which showed high-pressure 

 steam escaping into the air to be in general positive, but negative 

 when it carried globules of oil along with it. 



2B2 



