Professor W. Thomson y [May 18, 



Even in fair weather, the intensity of the electric force in the air 

 near the earth's surface is perpetually fluctuating. The speaker had 

 often observed it, especially during calms or very light breezes from 

 the east, varying from 40 Daniel's elements per foot, to three or four 

 times that amount during a few minutes ; and returning again as 

 rapidly to the lower amount. More frequently he had observed 

 variations from about 30 to about 40, and back again, recurring in 

 uncertain periods of perhaps about two minutes. These gradual 

 variations cannot but be produced by electrified masses of air or 

 cloud, floating by the locality of observation. Again, it is well known 

 that during storms of rain, hail, or snow, there are great and some- 

 times sudden variations of electric force in the air close to the earth. 

 These are undoubtedly produced, partly as those of fair weather, by 

 motions of electrified masses of air and cloud ; partly by the fall of 

 vitreously or resinously electrified rain, leaving a corresponding de- 

 ficiency in the air or cloud from which it falls ; and partly by disruptive 

 discharges (flashes of lightning) between masses of air or cloud, or 

 between either and the earth. The consideration of these various 

 phenomena suggested the following questions, and modes of observa- 

 tion for answering them. 



Question 1. How is electricity distributed through the different 

 strata of the atmosphere to a height of five or six miles above the 

 earth's surface in ordinary fair weather ? To be answered by electrical 

 observations in balloons at all heights up to the highest limit, and 

 simultaneous observations at the earth's surface. 



Q. 2. Does electrification of air close to the earth's surface, or 

 within a few hundred feet of it* sensibly influence the observed electric 

 force ? and if so, how does it vary with the weather, and with the time 

 of day or year ? The first part of this question has been answered very 

 decidedly in the affirmative, first, for large masses of air within a few 

 hundred yards of the earth's surface, by means of observations made 

 simultaneously at a station near the seashore in the island of Arran, 

 and at one or other of several stations at different distances, within six 

 miles of it, on the sides and summit of Goatfell. After that it was 

 found, by simultaneous observations made at a window in the Natural 

 Philosophy Lecture Room, and on the College Tower of the University 

 of Glasgow, that the influence of the air within 100 feet of the earth's 

 surface was always sensible at both stations, and often paramount at 

 the lower. Thus, for example, when in broken weather, the superficial 

 electrification of the outside of the lecture room, about 20 feet above 

 the ground, ip a quadrangle of buildings, was found positive, the super- 

 ficial electrification of the sides of the tower, about 70 feet higher, 

 was often found negative or nearly zero ; and this sometimes even 

 when the positive electrification of the sides of the building at the 

 lower station equalled in amount an ordinary fair weather negative. 

 This state of things could only exist in virtue of a negative electrifica- 

 tion of the circumambient air, inducing a positive electrification on the 

 ground and sides of the quadrangle, but not suflficient to counter- 



