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.]