ELECTRICITY. 



483 



Descriptive 

 Electricity. 



General re- 

 lulu. 



Diurnal va- 

 riarnns in 



Descriplfve 

 Electricity. 



It appears from the first 18 observations of the prece- 

 ding Table, which were made in an interval of '2t- or 

 25 hours, during which the sky was perfectly serene, 

 that the electricity of the atmosphere was very strong 

 about nine o'clock in tlie morning ; that it diminished 

 gradually till six o'clock in the evening, when the first 

 mini ntuii took place ; that it afterwards increased till 

 eight o'clock, when it reached its second maximum ; 

 that it then diminished' again, making some oscillations, 

 till six o'clock of the following morning, when it reach- 

 ed its second minimum ; and then increased again till 

 ten o'clock in the evening, when it was again at a maxi- 

 mum. The weather, however, being now cloudy, the 

 periods bec.;ine le-s regular. 



In summer, the electricity of clear weather is much 

 weaker than in winter. Saussure has observed the pith 

 balls diverge two lines in winter in an open country; 

 whereas in summer their greatest divergency did not 

 exceed a line, at lea.st when the sky was jicrfeetly se- 

 rene, for in the time of a storm they often diver-/ 

 remote as the threads permitted them. The weakness 

 of the electricity of clear weather in Bummer renders 

 the diurnal period less regular and less distinct ; tiir 

 since its fundamental quantity is very small, accidental 

 causes, such as winds, an'l different quantities of humid 

 vajrour and dry exhalations that exist in the air, <! 

 the regularity of the diurnal period, ami make the maxi- 

 mum and minimum fall in points opposite to what they 

 would have done in cloudy wcath>r. 



In summer, when the ground has been dry for some 

 days, the electricity of a dry and warm day generally 

 increases from the rising of the sun, when it is almost 

 insensible, to Uiree or four o'clock in the afternoon, 

 when it acquires its greatest force. It then gradually 

 diminishes till the fall of the dew, when it recovers its 

 strength ; and afterwards Ix-comes almost ini|.eiv. |>tiole 

 luring the night. \Ve have .Ir.-.uly -ecu that this diur- 

 nal period was first noticed by Le Monnier. It was af- 

 terwards determined with more exactness by Beccaria, 

 and l)r Garniiii obtained a similar result. None of 

 these writers, however, have noticed the diurnal period 

 in winter which we have just described from Sau^ure. 

 It is very remarkable, also, that this period is the same 

 in summer in those clear days which follow rainy days, 



the electricity diminishing towards the middle of the 

 day. 



In serene weather the electricity of the atmosphere is Electricity 

 invariably positive, both in summer and in winter, du- of clear 

 ring the day and night, at the rising of the sun and at weather al- 

 the falling of the dew, and in short at every time when " a >' s P **" 

 there are no clouds in the sky. It is impossible, there- Uvc * 

 fore, not to conclude with Volta, that the electricity of 

 the atmosphere is essentially poiilh'r, and that when 

 negative electricity does exhibit itself in snow, rain, and 

 in storms, it arises from the clouds being exposed to 

 the pressure of the electric fluid contained in the upper 

 regions of the atmosphere, or in clouds more elevated 

 than themselves. These clouds discharge a part of their 

 electricity into the earth or into other clouds, and are 

 thus electrified negatively by the effect of an electricity 

 originally positive, in the sar.ie manner as an electrome- 

 ter acquires a permanent negative electricity when it is 

 touched at the instant that the air is positively electri- 

 fied 



The interesting results obtained by Saussure, of which irjpcri- 

 we have given a very full account, cannot be considered tmntsof 

 as of general application to climates different from that Rcniyatin 

 of Geneva. Hence it Ix-comes a matter of import;! nee to 

 have observations made in different parts of the globe, in 

 order to ascertain the local causes by which the electri- 

 city of the atmosphere may be affected. Mr Thomas 

 Honayne made a series of observations on the electricity 

 of the atmosphere in Ireland, and obtained the following 

 results: He found that the air in Ireland was always 

 charged with positive electricity in winter, at a proper 

 distance from buildings, musts of ships, &c. The 

 electricity was diminished in frosty, foggy, or misty 

 weather, and in calm and cloudy weather it was still 

 perceptible. In summer he was never able to discover 

 any electricity, excepting when a fog came on in the 

 cool of the evening, oral night, -md then it was always 

 positive, but weaker than in winter fogs. When a fog 

 became very thick, Mr Ronayne remarked that the elec- 

 tricity diminished, but again recovered its strength 

 when the fog returned to its former state. When rain 

 fell in foggy weather, tin- i-ieclricity became impercep- 

 tible, but reappeared with the fog. When fogs, float- 

 ing near the earth, bccajne very dense, the electricity ia 



