54 REPORT — 1860. 



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 

 inwards 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 portable electrometer, during day thunder- 

 storms. I commenced the observation on each occasion after having heard thunder, 

 and I perceived 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 de- 

 gree 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 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 identifying 

 the thunder in connexion with any particular one of the impulses 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 dimi- 

 nutions 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 some- 

 what strong positive, falling back to weak positive, 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 neighbourhood of a loco- 

 motive engine, at work on a railway, sometimes by holding the portable electrometer 

 out of a window of one of the carriages of a train, sometimes by using it while stand- 

 ing 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 negative 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 elec- 

 tricity. 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 phenomena seem in accordance with Faraday's observations on the electricity 

 of steam, which showed high pressure steam escaping into the air to be in general, 

 positive, but that it was negative when it carried globules of oil along with it. 



Note on the Dispersion of the Planes of Polarization of the Coloured Rays 

 produced by the Action of Magnetism. By M. Verdet, Paris. 

 The researches in which I have been for some years engaged upon the magnetic 

 relations of the plane of polarization, discovered as we all know by Mr. Faraday, 

 have naturally led me to examine how these relations vary with the nature, or, using 

 theoretical language, with the length of the undulation, of the light. The experi- 

 mental method which I have employed is the general method introduced into science 



