236. If then it be allowed to reason from that Analogy which took Place in most of the Cases already mentioned; the gentler Depression. of Balloons over Water in milder Weather, may be owing to a Cause somewhat similar, tho’ not so evidently an immediate Object of the Senses, viz. an actual tho’ invisible Descent of Air upon the Water.
237. Blanchard in his Passage over the Sea from Dover to Bologne in France, when near the Middle of the Channel, suffered an unexpected Depression, and at the same Time was nearly becalmed.
A calm also took Place on the Irish Sea: which must have prevented Crosbie from landing,—without Wings, or some propulsive Machinery, connected with the Balloon.
238. Lunardi rose from Liverpool when the Wind blew boisterously: yet was becalmed twenty Minutes over the broad Turn of the Mersey near Ince, when above the Level of the Wind: and, descending into the same Stream of Wind, was hurried along towards Beeston-Castle in Cheshire.
CHAPTER XXXXIV.
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.