ATMOSPHERIC ELECTRICITY. 



337 



CHAPTER III. 



OP ATMOSPHERIC ELECTRICITY IN OVERCAST, FOGGY, AND CLOUDY WEATHER. 



§ I. — Of atmospheric electricity in clouthj iceatlier and during fogs. 



De Saussure* appears to be the first who observed that in cloudy- 

 weather, but without rain or storm, atinos|)heriG electricity exhib- 

 ited almost the same character that it did in clear weather. The 

 observations made by Schublerf confirm this result. The latter estab- 

 lished the fact that Vv^hen the weather was cloudy, without being at 

 the same time stormy, the electricity was still positive; but he found 

 that its intensity was less than in clear weather^ as Cavalloj and other 

 philosophers had already remarked before him. Further, the indica- 

 tions of the electrometer are stronger in winter than in summer; 

 and although the daily variations may be less regular than in clear 

 weather, they nevertheless present a period in the means of the ob- 

 servations made during the hours which are relative to the maxima 

 and minima of every month. Tliis is shown by the following table 

 which gives the results of the observations that Schiibler made under 

 an overcast sky four times a day: 



Mean niouthly forces of atmospheric electricity. 



June, 1-^11 



July, 1811 



August, 1611.... 

 September, 1811 

 Oclolter, 1811... 

 November, 1811 . 

 December, 1811. 

 January, 1812... 

 February, 181-2. . 



Marcb, 1812 



April, 1^12 



May, 1812 



. 3.40 

 4.00 

 5.00 

 5.50 

 5.20 

 6 00 

 8.93 

 9.76 

 6.60 

 3.00 

 3.00 

 3 50 



to 6 

 H 



"a 



8 



8^ 

 9 

 10 

 10 

 9 

 8| 

 8 

 7 



+ 8. 

 6. 

 8. 



Means -{-5.32 -{-8.46 -f- 6.81 + 10.7 -f 7.81 -f 7.9 



-f .3.83 

 4.00 

 4.66 

 3..W 

 4.83 

 8.50 

 15.31 

 16.86 

 8.50 

 3.83 

 3 50 

 4.50 



+ 7.33 



7.00 



10.00 



9.00 



8.00 



10.66 



19.41 



25.64 



13.10 



7.40 



5.. 50 



5.80 



+ 5.69 

 5.43 

 6.99 

 6.50 

 6.53 

 8.00 

 13.91 

 16.56 

 9.45 

 5.09 

 4.62 

 4.95 



-}-15.8R. 



16.6 



14.3 



11.3 



11.3 



5.1 



1.5 



— 2.4 



+ 3.0 



3.8 



4.3 



11. 1 



The comparison of this table with that given on page 328 shows that 

 the intensity of atraosi)licric electricity indicated by the electrometer 

 is, in fact, less with a cloudy sky than with a clear one. Butychubler§ 

 frequenth^ observed that it was much stronger than ordinary in clear 

 weather, and when, after a cloudy season, the sky suddenly became 

 clear, the hygrometer indicated also considerable moisture in the air. 



If a cloudy sky produced a decrease in atmospheric electricity, 

 without, however, entirely concealing the daily variations, it was not 



■•' Voynges daiie les Alpcs, torn. 11, ^ 201, p. 221. 

 t Journal de Sch weigher, torn. VIII, p. 21. 

 J Traite Coiuplet d'Electricito, p. 293. 



§ Journal de Schweigger, torn. VIII, p. 28. This increase in the intensity of the atmos- 

 pheric electricity when the eky suddenly cleared up had been already noticed by Bcccaria. 



22 



