ELECTRICITY. 



485 



Descriptive 

 Electricity. 



Read's ob- 

 ervations 

 on atmo- 

 tphcrical 

 electricity. 



was pulling in the string, told me that he had received 

 one or two slight shocks in his arms, and that if he 

 were to feel one more, he would certainly let the string 

 go ; upon which I laid hold of the string, and pulled 

 the kite in as fast as I could, without any farther ob- 

 servation, being then ten minutes after four o'clock." 



By means of the atmospherical electrometer, with 

 which Cavallo made experiments several times a-day, 

 during many months, he obtained the following addi- 

 tional results : 



1 . The electricity of fogs is always positive, like that 

 of the atmosphere. 



2. The electricity is always strongest in thick fogs, 

 and in frosty weather, and weakest when it is cloudy and 

 warm, and about to rain. 



3. The electricity is strongest in elevated situa- 

 tions. 



A series of interesting observations on the electricity 

 of" the atmosphere, were made by Mr John Read, at 

 Knightsbridge, in the year 1789, 1790, and 1791, by 

 means of an ingenious apparatus which we shall after- 

 wards describe.* The monthly results of his expe- 

 riments in the year 1790, are given in the follow- 

 ing Table, which was formed principally from the 

 electrical signs afforded by the pith balls within his 

 apartment. When the balls collapsed, and were not 

 attracted by the finger, he wrote down that there were 



no signs of electricity. When they were attracted Descriptive 

 on the approach of the finger, but were not so strong- Electricity, 

 ly charged as to produce a divergence, he wrote down s ~ "V"'' 

 weak signs of electricity. When the balls diverged Kead - 3 ob _ 

 and collapsed on the approach of excited glass, he servations. 

 wrote down that the electricity was positive ; but when 

 the balls diverged still more, by the application of the 

 excited glass, he wrote down that they were electrified 

 negatively. When the divergency of the balls was one 

 inch and upwards, visible sparks were drawn from a 

 brass ball connected with the apparatus. The utmost 

 limit of regular divergency were about 5 or 6 indies, 

 above which it was very unsteady. The pith balls were 

 nearly -,%ths of an inch in diameter, and were suspend- 

 ed by very fine flaxen threads 5 inches in length. In 

 carrying on these experiments, Mr Read was seldom 

 absent one hour, excepting during the time of sleep ; 

 but whenever he left the apparatus for the evening, he 

 examined the state of the electricity, and when the 

 roil was unelectrified, he connected with it a Leyden 

 phial, and next morning if the bottle was charged, he 

 marked down the kind of electricity with which it was 

 charged. In Mr Read's journal, is marked the state of 

 the wind, the barometer, and thermometer for every 

 day in the year, and the electric spark, the kind of elec- 

 tricity exhibited by pith balls, and the state of the 

 balls themselves, and of the weather. 



TABLE I. Electrical State of the Atmosphere in 1790. 



It appears from Mr Read's Journal, that there were 

 only 7 days in the year in which the electricity was 

 imperceptible, and only 73 in which weak signs of 

 electricity appeared. The days in which no electrici- 

 ty appeared, were the 14th, 15th, and 23d of Novem- 



ber, and the 6th, 15th, 17th, 21st, and 22d of Decem- 

 ber. 



The following Table contains the summary of Mr 

 Read's observations in 1791. 



* This instrument will be described in Part II. of the present article. 



