Airopaidia

by Thomas Baldwin

ON CERTAIN APPEARANCES AT DIFFERENT ALTITUDES OF THE BALLOON.

Of the highest visible Clouds which are always white.

Section 213. THE highest visible white Clouds, often seen in detached Streaks, during the finest and also in the worst Weather, (if not intercepted by lower Clouds) and which, when melting away, are known in some Counties by the common Appellation of Horse-Tails; and, suspended over Great-Britain, are frequently marbled or dappled by the Wind; putting on the Appearance of white Waves, like Sea-Sands ruffled and left by a rapid Tide;—had been disturbed, separated, and almost melted down by the Storm the Day preceding the Excursion.

Two of them only were still visible in Streaks, near the Sun’s Place, at the first Ascent. They seemed without Motion, and became afterwards invisible.

Saussure, the celebrated Professor of Philosophy at Geneva, is very exact in his Definition, Description, and Height of these Appearances: and thinks it probable, their Situation may be “at least fifteen English Miles above the Surface of the Earth.”

“Car quand je considere ces fines Pommelures, &c.” “For when I consider these delicate Dapplings, which, in a Series of fair Weather, begin to cover the azure Vault of Heaven with a white and transparent Gauze, and which portend Rain a long Time before it happens; I am led to believe they occupy a very elevated Situation in the Atmosphere” (Essais sur l’Hygrometrie, P. 271.)

It seems however that Crosbie, in his Excursion from Dublin on the 25th of January 1785, pierced throu’ and soared above these fine Webs, at the Height of 16 Inches by the Barometer in a frosty Air.

Of the Chilliness perceived at a certain Height.

214. It has been already noted, that at a certain Height, a Kind of chilliness was perceived, not ascertainable by the Thermometer.

The Sensation was suddenly impressed four Times, in ascending and descending to and from the same Height, viz. about 26 and 27 Inches, equivalent to between 500 and 1000 Yards above the Surface of the Earth at the first Ascent.

From the Uniformity of Effect at the same Height; the Sensation may be ascribed to the same Cause, viz. the Level of the first or lower Tier of Clouds: altho’ the Aironaut did not pass throu’ any visible Cloud or Vapour, during the Excursion. See [Section 93].

Remarkable Appearances of Earth and Clouds.

215. At the same Height likewise, tho’ the Observations have not been set down at large; the Appearances of the Earth and Clouds were very remarkable.

During the Ascent of the Balloon, between the Altitudes of 26 and 27 Inches; the circular Prospects of the subjàcent Earth instantly contracted, and, during the Descent, about the same Height, instantly enlarged themselves to the Eye of the Aironaut.

216. At the same Height mentioned before, the circular Prospects of the Clouds appeared on the same horizontal Plane with the Eye: tho’ at the Distance of a Mile. See [Section 49].

In this Situation, the Observer endeavoured to discover the Thickness of the Stratum of Clouds: but was always baffled by a Deception of Sight worth recording.

The Strata were plainly composed of three or more Heights of Clouds, sailing at great Intervals, one above the other: all which regularly vanished, as he approached their respective Levels: as if instantly thrown into the Circumference of a Circle, whose Radius was a Mile.

During the Ascent, in passing their supposed Level, the Clouds instantly appeared far below him: and during the Descent, as far above.

217. Quere: Is it not from the same Cause, that all Vapour is generally invisible to a certain Height and Distance from the Eye?

It being incontrovertible that more Vapours rise about noon, than at any other Hour, particularly at Sea, while the Sun continues to shine; which, notwithstanding, are wholly invisible, till arrived at a certain Height?

Visibility of Vapours by mere Distance.

And hence the Visibility of Vapours by mere Distance, which contains a sufficient Number of Particles to intercept and refract the Light, without Cold, Condensation, or actual Accumulation: viz. by Refrangibility of those primary Rays of Light, which Air and Vapour united are most apt to reflect or transmit.

Mons. Saussure has proved by his Horse-Hair comparàble Hygrometer, that “the Air shews Signs of greatest Humidity an Hour after Sunrise, and of least Humidity, between three and four in the Afternoon.” But the Air being then also the hottest, will dissolve or evaporate the greatest Quantity of Vapours, and raise them above the Hygrometer (which by its Heat will not retain, but on the contrary repel and dissipate them) to great Heights in the Atmosphere.

See “Essais sur l’Hygrometrie, C. 6, P. 315.”

218. In general then:

Is not the Cause of the above Deceptions, not an Absence, but a Transparency of Vapour to a certain Distance: (just as the Zenith appears cloudless, when the Air is overcast around;) beyond which Distance, the Number and relative Proximity of Particles with Respect to the Eye, is such, as to intercept the Rays of Light: when only, they put on the Colour of Air, and Form of Vapour and Cloud?

And hence the probable Reason, why no circular Horizon of the Earth’s Surface was presented during the Excursion, Section 79: and why it seldom has or can present itself to Aironauts or Mountaineers, at any considerable Height above the Region or Level of Clouds, even tho’ Clouds do not appear in the Air, either to themselves, or to Spectators below.

This Point seems capable of Illustration by Analogy, from the Impossibility of encreasing the Magnitude, and at the same Time, Distinctness of distant Objects, seen throu’ a common Telescope; on Account of the Quantity of Vapours between them and the Eye Which vapours may be magnified till the Object appears confused and obscure; and even at last become substituted in the Place of the Object, under the Form of Opacity and Cloudiness.

219. The greater the Height of the Balloon, the more contracted was the Circle of Vapour below it; and the more limited the Prospect of the Earth’s Surface below the Vapour.

220. It seemed probable that the Sun shone as bright on the Countries around the Observer, as on Objects immediately below him: which Objects coud not have been illuminated by the Sun’s Rays, darting throu’ the apparent and contracted opening under him; as the Rays which shone on the Balloon, fell beyond the Opening, obliquely on Clouds which caught the Shadow of the Balloon.

221. The extreme Rarity or Tenuity of the Vapours was evident from the progressive Course of the Balloon, which was always in the Center of a circular Opening, limiting the lower Prospects; except when the Spectator lost all Sight of the Earth, by dense, watry, intervening Clouds.

Novel Situation peculiar to the Balloon, again described.

This august central Situation, always changing yet still the same, had the most striking Effect on the Senses and Imagination. Yet, however pleasing the Recollection of this glorious appearance; however strongly impressed, accurately described, or richly painted; it must fall infinitely short of the original sensation. Unity and Sameness were there contrasted with perpetual Variety: Beauty of Colouring; Minuteness, and consummate Arrangement;—with Magnificence and Splendor: actual Immensity;—with apparent Limitation:—all which were distinctly conveyed to the Mind, at the same Instant, throu’ the Intervention of the Organs of Sight: and, to complete the Scene, was added the Charm of novelty.