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.