344 COSMOS. 



the aerial ocean rests on a liquid base than where it impends 

 over the laud; and it is very striking to observe how in 

 extensive seas small insular groups affect the condition of the 

 atmosphere, and occasion the formation of storms. In fogs, 

 and in the commencement of falls of snow, I have seen, in a 

 long series of observations, the previously permanent positive 

 electricity rapidly pass into the negative condition, both on the 

 plains of the colder zones, and in the Paramos of the Cordil- 

 leras, at elevations varying from 11,000 to 15,000 feet. The 

 alternate transition was precisely similar to that indicated by 

 the electrometer shortly Before and during a storm.* When 

 the vesicles of vapour have become condensed into clouds, 

 having definite outlines, the electric tension of the external 

 surface will be increased in proportion to the amount of 

 electricity which passes over to it from the separate vesicles 

 of vapour. f Slate-grey clouds are charged according to 

 Peltier's experiments at Paris, with negative, 0,111! white red, 

 and orange-coloured clouds, with positive electricity. Thunder 

 clouds not only envelope the highest summits of the chain of 

 the Andes, (I have myself seen the electric effect of light- 

 ning on one of the rocky pinnacles which project upwards of 

 15,000 feet above the crater of the volcano of Toluca), but 

 they have also been observed at a vertical height of 26,650 

 feet over the low lands in the temperate zone.J Sometimes, 



* Humboldt, Relation hvttorique, t. iii. p. 318. I here only refer to 

 those of my experiments in which the three-foot metallic conductor of 

 Saussure's electrometer was neither moved upwards or downwards, nor, 

 according to Volta's proposal, armed with burning sponge. Those of 

 my readers who are well acquainted with the qucestiones vexaks of 

 atmospheric electricity, will understand the grounds for this limitation. 

 Respecting the formation of storms in the tropics, see my Rel. hist., 

 t. ii. pp. 45 and 202-209. 



t Gay-Lussac, in the Annales de Cliimie et de Physique, t. viii. 

 p. 167. In consequence of the discordant views of Lame, Becquerel, 

 and Peltier, it is difficult to come to a conclusion regarding the cause of 

 the specific distribution of electricity in clouds, some of which have a 

 positive, and others a negative tension. The negative electricity of the 

 air. which near high water-falls is caused by a disintegration of the 

 drops of water a fact originally noticed by Tralles, and confirmed by 

 myself in various latitudes is very remarkable, and is sufficiently 

 Intense lo produce an appreciable effect on a delicate electrometer, at a 

 distance of 300 or 400 feet. 



Arago, in the Annuaire du Bureau des Longitudes pour 1853, 

 p. 246. 



