338 



ATMOSPHERIC ELECTEICITY. 



the same when fogs obscured the air. The latter, on the contrary, ap- 

 peared by their presence to increase the positive tension of the appar- 

 atus, as nearly all have remarked who have made observations on 

 atmospheric electricity. It was in thick fogs which did not terminate 

 in rain that Th. Ronayne,* in Ireland, and Achard,t at Berlin, ob- 

 served the strongest electricity. The latter of these scientists twice 

 noticed that the divergence of the balls of Canton's electrometer 

 ceased when the fog fell in the form of very fine rain^ and wholly 

 disappeared in less than a quarter of an hour. This phenomenon has 

 some connexion, perhaps, with the fact, frequently observed, that the 

 electricity of fogs sometimes changes from positive to negative when 

 they are resolved into rain. 



Volta;j: and De Saussure§ have both noticed the great tension of 

 electricity during fogs; according to the former it was especially fogs 

 attended by an odor which exhibited the most powerful action on the 

 instruments; in tliis case, also, he frequently saw his electrometer 

 exhibit a divergence which reached 100°, and sometimes gave out 

 sparks at the approach of a conducting body. Schiibler'sH observa- 

 tions also agree with those which we have mentioned, but the latter 

 remarked besides that, during fogs, the variations of atmospheric 

 electricity were wholly irregular, though its force varied with the sea- 

 sons, as it occurred in clear weather, the electrometer presenting the 

 greatest divergence in the winter months during the coldest days. 

 The following table gives the mean force of this electricity fur every 

 'month in the year : 



Months. 



June, ISll 



July, 1811 



August, 1811 



September, 1811. 

 October, 1811 ... 

 November, 1811. 

 December, 1811 . 



January, 1812 



February, 1812.. 



March, 1812 



April, 1812 



-May, 1812 , 



Means. 



Electricity during fogs. 



Mean monthly force. 



+ 16.0 



+ 



25.0 

 20.5 

 18.0 

 18.1 

 32.7 

 84.1 

 32.2 

 21.0 

 15.5 

 14.0 



+ 22.5 



Monthly maximum. 



+ 20.0 



+ 



30.0 

 25.0 

 30.0 

 28.0 

 36.0 

 44.0 

 55.0 

 21.0 

 17.0 

 14.0 



Fogs sometimes exercise a negative action on the instruments ; but 

 fogs which produce this effect are less common. In speaking of the 

 formation of electrical clouds, we shall see on what principle the fre- 

 quency of positive fogs is explained. 



• Philosoph. Transactions, vol. LXII, p. 140. 1772. 

 I Nouv. Mem. de I'Acad. de Berlin for 1780, p. 17, 

 j Dictionn. de Gehler, torn. VI, p. 484. 

 § Voyages dans les Alpes, torn. II, ^ 8ul, p. 221. 

 Journal de Schweigger, torn. VIII, pp. 27, 31. 



