326 ATMOSPHERIC ELECTRICITY. 



'has shown in this way that the lesult may be very well inter- 

 preted by the hypothesis of atmospheric strata more intensely electri- 

 fied with height, since then the upper strata, more strongly charged 

 ■with positive electricity, exercise on the metallic wire an action of 

 induction more powerful than that produced by the electricity of the 

 lower strata. 



§11. — Of the diurnal and annual variations of atmospheric electricity 



in clear loeaiher. 



Le Monnierf first observed that in the same place positive elec- 

 tricity in clear days was subject to regular variations of intensity. 

 He noticed that it diminished by degrees towards sunset, and finally 

 at last disappeared one or two hours afterwards, and did not re- 

 appear till towards eight or nine o'clock in the morning. He at first 

 attributed this daily variation to the moisture of the night, which, 

 rendered the insulation of his apparatus less perfect ; but after taking 

 suitable precautions he ascertained that there was another cause for 

 this phenomenon. Beccaria,| Gardine,§ Crosse, || and other observers 

 arrived at the same result. De SaussureT[ determined the laws of this 

 daily period by means of observations made with his electrometer, 

 armed with a conductor terminating in a point. He observed that it 

 presented two maxima, which followed some hours after the rising 

 and setting of the sun, and two minima, which preceded the rising and 

 setting of the same luminary. "In winter," he says, "the season 

 during which I have most successfully observed the electricity of clear 

 air, it appears to me that the hours when it is most feeble are those 

 which are comprised between the time when the evening dew has 

 wholly completed its fall and that when the sun has risen, Next, 

 its intensity increases gradually and reaches, almost always, before 

 noon a definite maximum, after passing which it seems to decrease 

 until it is renewed at the fall of the dew, a period at which it is 

 sometimes stronger than during the day. After this it diminishes 

 gradually during the night, without, however, becoming null when 

 the weather is clear." The observations which Schiibler** continued 

 for a whole year, at Stuttgard, lead to the same result. They showed 

 that when the weather is clear atmospheric electricity, which is very 

 feeble before the rising of the sun, afterwards increases at first slowly, 

 then rapidly, until it reaches its first maximum some hours after ; 

 that it also diminishes, at first rapidly, then slowly, until it has reached 

 its minimum, some hours before the setting of the sun ; that it begins 

 again to increase when the sun approaches the horizon, and attains 

 its second maximum some hours after its setting ; to diminish again 

 during the rest of the night. 



* Traitd de Physique Exp^rimentalc et Mathdmatique, torn. II, page 455. 



f M^moires de I'Acadeuiie des Sciences de Paris pour 1752, page 2-il. 



J Lettere dell' Electricismo, page 160. 



§ De influxu Electricitatis Atmosph., §§ 50 and 51. 



|| Biblioth^que r.ritannique, Sciences et Arts, lorn. LVI, page 524, 1814. 



^ Voyages dans les Alpes, torn. II, § 802, page 221. 



*'" Journal de Schweigger, torn. Ill, page 123, and torn. VIII, page 21. 



