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.