CHAPTER XXXXV.

[Sidenote: A _gentle_ Depression of Air over _moist_ Places in _fair_ Weather.]

Section 243. Intimations of depressing Columns in moderate Weather, are the _sluggish_ Clouds, which often make their _first_ Appearance, and remain longest, nay almost continually, _over_ and _along_ great Rivers, and Chains of Mountains, both during a Calm, or from whatever Point the Wind blows.

And hence the greater Quantity, Violence, and Continuance of Wind and Rain, which then _descend_:[60] also of the _greater_ Purity of the Air _during_ such Descent.

244. As, therefore, it is plain that atmospheric Air +descends+ _frequently_, both in bad and fine Weather; if a Cause can be assigned so general, as to make it probable, that such +depression+ does almost continually take Place:—tho’ at present the Effect is only evident to the Senses, by actual Experiment in the Passage of Balloons throu’ such Columns;—it will be sufficient to put Balloonists on their Guard against the Effects of such _Depression_.

245. In order to investigate the Theory of Depression; it may not be unacceptable, particularly to those who have not had Leisure to peruse the Experiments on Air, by Dr. Priestley, or the Collection on the same Subject by Cavallo;—just to extract a few short Quotations, on the chemical Affinities of Air and Water.

246. Article 1. “Water, as Rain, imbibes only the pure Air of the upper Regions, leaving the lighter and floguisticated Air to ascend.”[61]

246. 2. Felìcè Fontana says, “Common Air receives an Encrease of Bulk and _Elasticity_ from being shaken in Water.”[62]

246. 3. Air absorbs Water, and Water absorbs Air:[63] and the Absorption of Air by Water is promoted by Agitation: it also absorbs twice as much _defloguisticated_ Air, as common Air:[64] the whole Bulk of the Air absorbed being equal to one-twelfth of the Bulk of the Water: yet the Bulk of the Water seems but _little_ encreased: the Air being contained within the Interstices of the Water.

247. The following is a pretty and an easy Experiment, to shew how the +absorption of water by air+ takes Place, under the immediate Inspection of the Observer.

Admitting the Sun’s Light into a Room, throu’ one Window only; pour a Pint of _boiling_ Water into a large Bason: hold the Bason, which will not be half full, next the Light, in such a Manner, that the Sun may shine on the Water and Bason; yet the Eyes be shaded by the Top of the Window Frame.

Incline the Side of the Bason towards the Light, so that the Water may rise even with the Top.

The Eye being placed just above the upper Side of the Bason, farthest from the Light; look on the Water.

You may then observe the Surface of the Water next the Light, refract the Sun’s Rays, and produce the primary Colours, particularly the +red+ and +green+: which tho’ _transient_, _continue_ to be _seen_ in Succession; as Vapours rise above the Surface of the Water. Their _first_ Ascent is plainly discoverable: remaining above its Surface, in the Form of _small Dust_, gently agitated, not _separately_ but as a _whole_. Nor do they seem to rise into Steam, till assisted by the Action, and Contact of _dry Air_, which like _dry_ Spunges, _licks off_ and absorbs the small Dust already accumulated by the Force of the Heat from below, and then becomes visible under the Appearance of Steam, flying off in distinct hollow Vesicles.

The more _still_ the Air of the Room, the more slowly will the Spunges of Air come in Contact with the Body of small Dust.—Besides the small Dust already mentioned; the Heat will detach solid Globules of Water; which will remain floating on the Surface of the Body of Water: till the dry Air descends and transports them with it; the Air at the same Instant dissolving the solid Globules into hollow Vesicles.

But the most extraordinary Phenomenon, and which cannot be mistaken, is, that as soon as a Spunge of Air has dipped into the Surface of Water, and received its Lading; the Vesicles continue to accumulate, till another fresh Spunge descends in a similar Form, which may be traced upon the Surface of the Water, and seen in its Shadow, or rather in Beams of Light at the Bottom of the Bason, at the Instant it has flown off with its Burden: for that Part of the Surface of the Water transmits new Rays of Light, on Removal of the Vapour carried away by the Dip and Play of Air.

248. The Removal of the Vapour, likewise exhibits a curious Appearance on the Surface of the Water: which seems as if divided into irregular Parcels detached from each other; like the reticular Daplings visible on the under Side of Clouds elevated to the highest Stratum of the Atmosphere, and there evaporating or dissolving.

249. So powerful is the Attraction between Air and Water; that, while the Steam is rising above and round the Sides of the Bason; _Waves of fresh Air_, by Intervals, press the exterior Parts of the Steam _inwards_, in order to get at the Surface by descending into the Bason.

This Operation is best discovered, when the Bason is held _even_. And the whole Process may be observed more distinctly, if the Bason is raised and fixed on a Frame, near the Height of the Eye of the Observer, standing upright: who will then be able to trace minutely the exact Form of the Steam, and Insinuation of the Waves of Air into the Center of each Curl, or rising Curvature: an Appearance, similar to which, may be seen in _Water_ flowing from a small Orifice in a close Vessel; the fresh Air forcibly entering in an opposite Direction; forming a visible Cavity and Curvature in the Center of the Stream. See Halley’s Experiments on Evaporation in the open Air, and in a close Room, in Lowthorp’s Abridgement of the Phil. Trans. Vol. 2, P. 108.

Having once remarked the foregoing Process at Leisure; the same may be seen over any open Vessel of Water just warm enough to emit visible Steam: but the Air shoud be as _still_ and _calm_ as possible: the Steam never rising from all Parts of the Surface at once; but a depressing Spunge of Air always descends to the Surface, the Instant a Lamina of Vapour has been detached.

Such is the regular and invariable Process of Evaporation.

The same Process may be distinctly traced over the Surface of a Piece of Water or River, the Air being perfectly calm, in a gentle Frost, at Sunrise, particularly in Autumn, while the Water retains a Warmth superior to that of the Air.

250. Hence it follows that _as much light_[65] and _warm_ Air as is raised with the Steam by Evaporation from the Surface of any Water; _so much heavy_ and _cool_ Air is +instantaneously+, constantly, and forcibly +depressed+ upon its Surface, in order to supply the Vacancy, restore the Equilibrium, and continue the Evaporation.[66]

251. Now, besides the mutual Affinity that Water has to almost all Kinds of Air, and to Floguiston; added to its Power of Absorption; and as the +sea+, particularly in Summer, also +rivers+ and _damp_ +meadows+ are generally _cooler_ than the Lands and Countries bordering on them; Currents of _damp cool_ Air press forwards to supply the Defect or Vacancy caused by Heat, Rarefaction and Elevation of _dry warm_ Air, which is necessarily, and almost constantly rising into the Atmosphere, from heated Lands, Plains, and gentle Eminences _long shone_ on by the Sun.

252. Consequently the pure, cool, defloguisticated Atmosphere, is almost continually descending from above; sometimes imperceptibly, often forcibly, on the Surface of the Sea, the Channels of Rivers, Meadows, and all wet Land. Which Depression acts, in Proportion to its Strength, on the Balloon; and always with a sensible Effect: for, being in Equilibrio with the Air at all stationary Heights; the _least_ Depression of the Atmosphere makes the Balloon descend, considerably.

253. This Reasoning is, in many Cases, applicable to the Air, and consequently the Weather and Cold of Mountains.

Nor can it otherways be accounted for, why the Snow is perpetual, and the Cold so intense, on Mountains under the Equinoctial, and between the Tropics: but which admits an easy Solution on the above Hypothesis.[67]