[71] “The +water+ remains +transparent+ or colourless, tho’ saturated with marine acid Air, and by a very gentle Degree of Heat, the Gass may be again expelled from it, as it is expelled from Spirit of Salt.” This Observation is applicable to the Transparency of Vapours, in the Air, tho’ mixed with the marine Acid exhaled from the Sea: for when the acid or Sea Air is mixed with Alkaline or Land Air, they instantly _combine_; lose their Elasticity, and form a _white_ visible Substance or _Cloud_. Cavallo, Page 728. Priestley’s Exp. and Obs. Vol. 2, Page 293. [72] On the Descent of Air in _Thunder-Gusts_, see “Chalmer’s Account of the Weather in South-Carolina, Vol. 1, Page 1, to 39.” [73] “Historia Ventorum, Pag. 54, Art. 34.” [74] Book V. Chapter 2d. [75] Vol. 1. Page 184. [76] Page 195. [77] History of the Canary Isles, Page 252. [78] As the superior Clouds, during the Balloon Excursion, did not much exceed the Height of 1000 Yards; supposing then the Clouds at an equal Height above the Sea, near Teneriffe; one ought to conclude, either, that the Peak was not so high as Glas represents it; or, that the Level of the Clouds was less than half the Height of the Mountain. [79] See “Royal Astronomer, by R. Heath, Page 321, on _Trade Winds and Monsoons_.” [80] One Pound of Nitre only, producing by mere Heat, 6 cubic Feet of Air. “Cavallo, Page 332, and 811, Experiments on Gun-Powder.” [81] “See Recherches surles Modifications de l’Atmosphere. No. 715.” Ph. Trans. Part 2, for 1777. Col. Roy’s Experiments, Sect. 2d, Page 689, 744, 753, 764. [82] The different Phenomena of the _Aurora Borealis_ may be owing to the Ascent and Motion of the Apogay, in the _middle_ Region, over the Stratum of Eknèfiai or _Ground_-Winds. The Effects of _Tides in the Air yet to be_ mentioned, must not, however, be wholly excluded. The Aurora Borealis is seen in _Spring_, _Autumn_, and _Winter_: sometimes _culminating_, sometimes moving in _Streams_ and _Waves_ in the _superior_ Regions of the Atmosphere: when culminating; as if rising out of Clouds in the North. This Appearance may be owing to warm moist Air perpetually generating between the Tropics, and rolling over the cold _dry_ Stratum of Eknèfiai Winds, which cut off its Communication with the Earth: till accumulating over the Poles, it enlightens the Atmosphere, converting a six _Month’s Night_ into Day; and returns to the Surface silently: or in Lightning, whenever it is communicated to the Earth, throu’ _Vapour descending_ by its own specific Gravity; or along with _depressing_ Torrents of Air, known to be accompanied by frequent +flashes+. When the Vapour is _condensed_ in its Descent, by passing throu’ a Stratum of the Eknèfiai Winds; it becomes _overcharged_ with the electric Matter, _surrounding_ and _adhering_ to it; and deposits the Overplus in Lightning, on its Approach to _other_ Clouds, or to the _Earth_.