CHAPTER XXXVIII.

ON CERTAIN APPEARANCES AT DIFFERENT ALTITUDES OF THE BALLOON.

[Sidenote: 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.

[Sidenote: 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.

[Sidenote: 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?

[Sidenote: 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.

[Sidenote: 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+.