ATM 



intenfity is <reneially dimiiiillied by ftiong winds, wliich 

 blend the difllrent ftrata of the atmolphere, caufe them to 

 fubfide towards the ground, and thus dillribute the eledtr'i- 

 city uniformly between the earth and the air. M.SaulTure 

 has obftrved a ilrono- ekaricity, with a ftrong north wind. 

 In foggy weather, the tleclricity is the llrongeft, unlefs the 

 fog is about to be diflblved into rain. The various moditl- 

 cations of cleftricity in the atmofphcre are obfervcd with 

 the greatell advantage in fcrene weather. M. SaulTure 

 found, in winter and in fuch weather, that the cleclricity 

 was generally weakell in the evening, when the dew had 

 fallen, andfo continued till fun-rife ; afterwards its intenfity 

 augmented by degrees, fomctimcs fooner and fomttimes 

 later ; b't nfually before noon it attained a certain maximum, 

 from which it again declined till the fall of the dew, when 

 it would be fometimes llronger than it had been during the 

 whole day ; after which it' would again gradually decreafe 

 during the whole night ; but it was never quite deftroyed 

 in weather perfeftly ferene. Hence it may be inferred, 

 that atmofphcrical eleftricity, like the water of the ocean, 

 is fiibjea to a flux and reflux, which produce an increafe 

 and diminution twice in twenty-four hours. The momaits 

 of its greatell force are fome hours after the riiing and let- 

 ting of the fun ; and thofe in which it is weakefl: precede 

 its rifing and fetting. Of this periodical flux, M. SauiTure 

 has given a remarkable inllance, deduced from his obferya- 

 tions in an extraordinary- degree of cold, and at an elevation 

 of fixty feet above the level of the lake of Geneva. From 

 the refult of eighteen of thefe obfervations, made during 

 three fucceffive days, when the flcy was quite ferene, we 

 learn, that the eleftricity was pretty ftrong at nine in the 

 morning ; that from this time it gradually decrcafed till 

 about fix in the evening, which was its firll minimum ; after 

 which it increafed again till eight, its fecond maximum ; 

 it then gradually declined till iix in the morning, -which 

 was the period of its fecond minimum ; after which,^ it 

 again increafed tilL ten in the morning, which was the firft 

 maximum of the following day : but as this day was cloudy, 

 its periods were lefs regular. The eleftricity of ferene 

 weather is lefs eafily obferved in fumnier than in winter. 

 In fummer, if tlie ground has been dry for fome days, and 

 the air is alfo dry, the eleftricity increafes from the rifing 

 of the fun, till three or four in the afternoon, when it is 

 ftrono-eil : it then decreafes till th« dew begins to fall, 

 when it again increafes ; but after this it declines and is 

 almoft reduced to nothing during the night. However, 

 the ferene days that fucceed rainy weather in fumnier gene- 

 rally exhibit the fame diurnal periods or ftates of eleftricity, 

 with thofe that are obfervable in winter. The eleftricity 

 of the air is invariably pofitivc in ferene weather, both in 

 winter and fummer, in the day and in the night, in the fun 

 and in the dew. Hence it fliould feem, that the eleftricity 

 of the air is eflentially pofitive ; and that whenever it ap- 

 pears to be negative, as in particular rains or ftorms, this 

 itate is produced by fome clouds which have been expofed 

 to the preflure of the eleftric fluid contained in the upper 

 part of the atmofphcre, or to more elevated clouds that 

 have difcharged a part of their fluid upon the eavtli, or 

 upon other clouds. M. Saufl'arc, having colltftcd thefc 

 and fimilar phenomena, as the refult of numerous and repeated 

 «bfervations, inftituted a fet of experiments on evaporation, 

 in order to inveftigate and afcertain their caufe. Thefe 

 our limits will not allow us to detail ; but the general refult 

 was, that evaporation, which feeras to be the vehicle that 

 conveys eleftric matter into tiie atmofphcre, from china and 

 iilver always produced negative eleftricity ; and from iron 

 and copper, generally pofitive eleftricity : and hence it may 

 be inferred, that eleftricity is pofitive with thofe bodies thai 



A T M 



arc capable of decompofing water, or of being decompofed 

 themfelves by th?ir contact with the water ; and negative, 

 with all thoie which arc not at all decompofed or altered. 

 As to the producing caufes or fources of atmofphcrical 

 eleftricity, we may obfcrvc in general, that they may be 

 reduced to four, viz. friftion, evaporation, heat and cold, 

 and condenfation and expaniion : and with refpeft to the 

 changes and modifications to which the atmofpheiical 

 eleftricity is continually fubjeft, they may be attributed to 

 the operation of the various caufes that produce it, and to 

 the chemical procefles that are conllantly carried on by 

 means of the various ingredients that compofe the atmo- 

 fphcre. M. Volta (Piiil. Tranf. vol. Ixxii. p. 32.), in re- 

 ference to this fubjeft oblerves, that as the vapours on their 

 condenfing lole part of tlieir latent heat, ou account of their 

 capacity being diniinilhed they part with fome electric 

 fluid. Hence (he fays) originates the pofitive eleftricity 

 which is always more or lefs predominant in the atmofphcre, 

 when the flcy is clear, viz. at that height where the vapours 

 begin to be condenfed. Accordingly the atmofphcrical 

 eleftricity is llronger in fogs, in which cafe the vapours 

 are more condenfed, fo as to be almolt reduced to drops, and 

 is Itill ftrongtr when thick fogs become clouds. In ac- 

 counting for clouds, negatively eleftrified, he fuppofes that 

 when a cloud, pofitively eleftrified, has been once formed, its 

 fphere of aftiun is extended a great way round, fo that if 

 another cloud comes within that fphere, its eleftric fluid, 

 according to the well known laws of eleftric atmofpheres, 

 muft retire to the parts of it which are moll remote from 

 the firft cloud : and from thence the eleftric fluid may be 

 communicated to other clouds, or vapours, or terrellrial 

 prominences : thus, a cloud may be eleftrified negatively, 

 which cloud may, after the fame manner, occafion a pofitive 

 eleftricity in another cloud, &c. This explains not only 

 the negative eleftricity, which is often obtained from the 

 atmofphcre in cloudy weather ; and the frequent changes 

 from pofitive to negative eleftricity, and contrarywife, in. 

 ftormy weather ; but alfo the waving motion obferved in 

 the clouds, and the hanging down of them, fo as nearly to 

 touch the earth. For an account of the inftruments that 

 are ufed for difcovering and elHmating the eleftricity of the 

 atmofphcre, fee Collector, Condenser, Conductors, 

 and Electrometer : and for further obfervations on this 

 fubjeft, fee alio Electricity, Evaporation, Light- 

 ning, Rain, Vapour, Sec. 



Atmosphere, Figure of the. The atmofphcre envelopes 

 all parts of the furface of our globe ; if therefore both the 

 one and the other continued at reil, and were not endowed 

 with a diurnal motion roimd their axis, then the atmofphcre 

 would be cxaftly fpherical, according to all the laws of 

 gravity ; for all the points of the furface of a fluid in a ftate 

 of rell, muft be equally removed from its center. But the 

 earth and the ambient ntmofphere arc invcfted with a diurnal 

 motion, which carries both the one and the other round 

 their axis : and the different parts of both having a centri- 

 f'.igal force, the tendency of which is more confiderable, and 

 that of the centripetal lefs, as the parts are more remote 

 from the axis ; the figure of the atmofphcre muft become 

 an oblate fphcroid, becaufe the parts. that correfpond to the 

 equator are farther removed from the axis, than the parts 

 which correfpond to the poles. 



Befides, the figure of the atmofphcre rnuft reprefent fuch 

 a fpheroid, becaule the lun ftrikts more direftly on the air 

 which encompaflfes the equator, and is comprehended be- 

 tween the two tropics, than on that which pertains to the 

 polar regions. Whence it follows, that the mais of air, or 

 part of the atmofphcre, adjoining to the poles, being lefs 

 heated, cannot expand fo muchj nor reach lo liigh. Never- 



therlcfsj 



