THE SMITHSONIAN INSTITUTION. 227 



absorption now no longer compensates for the condensation, and the 

 excess must be precipitated as rain, hail, or snow. 



12. That the upper stratum should be colder than the lower one, as 

 alleged in the synopsis, is explained: first, b}^ the difference of the alti- 

 tude ; secondl}^ the evaporation going on from the upper one must rob 

 it of heat by a process from which the other stratum is protected by its 

 presence; thirdl5S radiation by the same superiorit}^ of altitude goes on 

 copiously from the upper stratum, while this, by its interposition, checks 

 radiation from the lower stratum. 



13. The formation of clouds within the space occupied by the upper 

 stratum must prove the air associated with those clouds to be saturated 

 with moisture. Hence the clouds formed in any subjacent stratum 

 cannot be absorbed within the interval between the strata, and must, of 

 course, be more likely to accumulate so as to produce rain. 



14. That the upper stratum should be so overcharged with electricity 

 as to give it out in "sheet lightning," undulations, or aurora boreal cor- 

 ruscations, is perfectly consistent with the suggestion which I have ad- 

 vanced respecting the existence of three concentric spaces occupied 

 severally by our globe — the denser non-conducting part of the atmos- 

 phere, and the rarified medium beyond that last mentioned, which is suffi- 

 ciently rare to form a conductor or coating, the terrestrial surface per- 

 forming a similar part within, the atmospheric electric existing inter- 

 mediatel3^ 



15. The frequency of the aurora boreal corruscations and flashes is 

 no doubt the consequence of discharges from one part of the exterior 

 concentric space to another, especially when proceeding from cither the 

 arctic and antarctic regions to those of intermediate latitudes. 



16. It is well known that ice, when very cold and, consequently, 

 dry, performs the part of an electric no less than glass. The friction of 

 globules, as existing in the fog produced by escaping steam, has been in- 

 ferred by Faraday to be a competent source for the torrents of electricity 

 generated by a high steam; and it is not improbable that the friction 

 between wind and the terrestrial surface may induce opposite states in 

 the stratum of air bounded by that surface and the stratum occupied by 

 the earth, and that occupying the space above the region of the clouds. 



17. The discharges of lightning are the means of equalization be- 

 tween these spaces. Observation shows that the higher regions of the 

 atmosphere are equally surcharged with electricity so as to prove dan- 

 gerous to travellers among the Sierras or table lands of the Andes. 

 Hence the upper stratum of clouds is liable to be surcharged with 

 electricity oppositely to the earth. But whatever electrifies the inner 

 terrestrial coating must produce a proportional opposite excitement in 

 the outer one. 



18. When an upper stratum of clouds, such as Mr. Wise describes 

 as the " cloud cap," is highly electrified by corruscations from the outer 

 concentric space, as above supposed, it is consistent that it should at- 

 tract the oppositely electrified air in the vicinity of the terrestrial sur- 

 face. This air must at the same time undergo dilatation like that to 

 which the gas in Wise's balloon was subjected, and must, of course, be 

 repelled by the similarly electrified earth. Calorific expansion may 



