CHAPTER XXXXIX.

[Sidenote: Torrents of Air on _Etna_, and _Teneriffe_.]

Section 265. With Respect to Mountains: on reading what Travellers have written, particularly Ullòa;[74] they seem to answer the Intention of supplying cool Air to the surrounding Plains, or Continents; by Depression and Condensation: and also, if on Islands; to the Sea itself.

266. Brydone, in his Tour throu’ Sicily and Malta, in 1773;[75] giving an Account of his Ascent to the Top of Etna, says, that at the Foot of the Crater, the Snow was frozen hard and solid:[76] and that the Crater was so hot; it was impossible to descend into it.

Further: “that the Smoke rolled down from the Sides, like a Torrent: till of equal Gravity with the Air, when it shot off horizontally; forming a long Track, according to the Direction of the _Wind_: which there rose to a +violent+ Degree: so that it was with Difficulty he coud settle the Barometer for an Observation.”

He also adds “that _Clouds_ began to _gather_ round the _Mountain_; but were _dispelled_ by the Wind.”

Now from the foregoing Theory is it not probable to suppose, that a _Torrent of Air rushed_ continually down from the etherial Regions, not only to supply the Fire of the Crater; but also the Vacuity caused by the perpetual Elevation of Vapours and heated Air from below: the Torrent likewise _depressing_ into the Track with itself, the Volumes of Smoke which were seen to roll directly down the Sides of the Mountain: that this descending Torrent of Air, in its Progress, dispelled the Clouds forming round the Sides of the Mountain, by the Ascent of warm Vapours condensing, as they rose, on their Approach to the cold Mountain: the Smoke shooting _horizontally_, from that Height _only_, at which an _horizontal Current of Air_ began to take Place? For it can hardly be imagined that the Air at the Top of Etna, found to be “_electrical_,” and which must have been replete with a Mixture of Floguiston, inflammable Air, Gasses, and other aërial Fluids highly rarefied, heated, +dry+, (and consequently lighter,) _at the Instant_ of rising out of the glowing Cauldron, became so condensed as to fall like Water, without partaking of the Motion of a _violent_ Wind, supposed to blow in an horizontal Direction.

267. Glas, in his Account of Teneriffe,[77] reports, that the Clouds are generally half as high as the Peak, above the Sea,[78] i.e. according to him, near the Height of a Mile and Half: “_below_ which Clouds, the _North Easterly Winds_ +generally+ prevail: and, at the same Time, _above them_, we find a _fresh Westerly_ +Gale+: which I believe to be the Case _in every Part of the World when the_ +trade wind+ blows.”

In Page 253, he says, that in ascending above the Level of the Clouds, he found the Air sharp, cold and piercing: and the Wind blew strong from South West, and West South West: so that the Wind blew towards the Mountain from three different Points at least, viz. the Trade Wind, from North East below the Clouds; just above them, from South West: and still higher, a fresh Gale, from West.

“The Air on the Top of the Pike was thin, cold, piercing; and of a dry parching Nature, like the South Easterly Winds which I have felt in the great Desert of Africa, or the Levanters in the Mediterranean: or even not unlike those dry easterly Winds which are frequent in the Northern Parts of Europe, in clear Weather, in the Months of March or April,” Page 257.

This dry Wind answers to the Eknèfiai (before mentioned) i.e. _Wind descending_ +from the clouds+.

Glas further observes (Page 250) that the Clouds, in fine Weather, descend gradually towards Evening, and rest on the Woods till Morning: when they re-ascend, and remain suspended above them, till the succeeding Evening.

Here then a nocturnal Depression of the Atmosphere is obvious. But this Appearance will not prove that the Air does not descend below the Level of the Clouds: for, tho’ the Clouds descend with the Air; Vapour-Air, of which they are composed, becomes _transparent_ both by Dissolution, in a warmer Stratum, and Proximity to the Earth, as before mentioned.

[Sidenote: Conclusion drawn from the above, applicable to Balloons.]

268. From the Variety of Winds experienced at different Heights, not only on _Teneriffe_, but in different Places; it is plain, that if Balloons can be made durable and Air-tight; they may be wasted between the Tropics by an East or West Current at Pleasure: and also throu’out the Globe; the Occasion being made, in some Respect, subservient to the Time.[79]