CHAPTER XXXXIV.

[Sidenote: Depressing Columns of Air known to the Egyptians.]

Section 239. The Existence of depressing Columns of Air was well known to a People more ancient than either Romans or Greeks.

240. The sultry Climate of Egypt, whose Situation is that of an extensive Meadow watered by a _broad_ River, and enclosed by Mountains to the East and West; consequently not subject to general horizontal Currents of Air, except along the Line of its Meridian,—is _the Country_, wherein Columns of +cool+ Air descending on the Water, woud be soon observed.

And they, in Fact, were almost the only People who applied the Observation to common Life: having, according to Herodotus, as well as later Writers, built lofty Structures +open at the top+. By which Means the cool Air +rushing+ downwards greatly refreshed the Inhabitants.

The ancient Pantheon, at present called All Saints Church, now standing at Rome; built in the lowest Situation of a Street named the Piazza di Navona is on this Construction: and the Hint probably taken from an Egyptian Model.

241. In all inland Countries, whose Lakes are frequently surrounded by Mountains, as Bala-Pool in North-Wales; those of Westmoreland and Cumberland; the Lake of Geneva in Swisserland;—the Air rushes +forcibly+ on the Surface of the Water in descending Torrents: this the Writer has frequently observed.[59]

(In other Languages, the Words applicable to Wind on a Lake, or the Ocean, signify Descent: as, Καταβαινω, and Επικειμαι· also the Northerly or _descending_ Wind corresponded to the Εκνὲφιας while the Southerly or _ascending_ Wind answered to the Απογη.)

All this, which may be allowed to take Place in _bad_ Weather, may perhaps be excepted to, in _fine_, and still more so, in the _finest_ Weather.

As the slightest Change is first observable on the Surface of Water, whether on Lakes or the Ocean, the _Descent of Air_ in the finest Weather is familiar to Mariners by the Appellation of +light airs+, playing in Eddies: and particularly in the _variable_ Latitudes; i.e. between 32 and 42: to these the Writer can also witness: as well as on small and large inland Lakes, by partial _Dimplings_ and _Rufflings_ of the Surface.

OBJECTION TO THE THEORY REMOVED.

242. It may be objected to the above Theory, that the Wind plainly blows in an horizontal Direction, as may be seen from the Motion of Clouds and Trees.

To which it may be answered, that if Clouds are not beside the Question; as it is not asserted that a single Column of Air presses from so great a Height to the Earth; (tho’ it be the Case in Squalls;) yet it is extremely difficult to determine whether Clouds move in a Direction exactly parallel to the Plane of the Horizon: and it is much more probable that they are in a perpetual Change, _encreasing_ or _melting_; rising or falling, according to the _Pressure_ and specific Gravity of the _Medium_ in which they float; its Tendency to Moisture or Driness, Cold or Heat; also the different Combinations and Decompositions, with Respect to which, the Atmosphere is in perpetual Variation.

The Motion of Trees, if carefully attended to, seldom shew Effects of a regular horizontal Current.

And since the more _powerful_ the Wind; the more evident and accurate may be the Observation; it will be found, that the _first_ general Effect is an oblique Depression, succeeded by a Recovery or instant Exaltation: then a momentary Pause, or actual Retreat of the Wind; and in a few Seconds, a Return of the depressing Torrent.

But the strongest, and, at the same Time, an irrefragable Proof, is by _Appeal_ to Men of _Science_ in the Navy, or to skilful Pilots, who are conversant with Winds and Waves; who have weathered Storms off Cape Hatteras in Latitude 36; (where probably the Wind is perpetual;) or have made an East-India Voyage:—whether, if a Gale blew in an horizontal Direction +only+; the Ocean coud produce such an Inequality of Surface: or whether when the Sea runs +mountains+ _high_; the tremendous Surges must not arise from the _violent_ Action of Winds repeated at Intervals, sometimes _descending_ perpendicularly; but oftener in forcible elastic Torrents of oblique +depression+, and instant _Resilition_?