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.