CHAPTER LI.
Section 275. IF then this Reasoning be allowed; aërial Travellers will not be subject, when, at a considerable Height, even in Winter, to great Degrees of Cold, supposing that the Air does not actually freeze the Waters below; and the Apogay or Southerly Winds have continued for a few Days.
On the Contrary; Aironauts may expect Cold, encreasing with their Ascent, even in Summer, tho’ warm below; supposing the Eknèfiai or Northerly Winds to have continued but for a Day before the Ascent: they may possibly, indeed by soaring higher, rise into the regular Stratum of the warm Apogay floating above them.
276. From what has been said, there seems a Degree of Probability, that the Air for a Number of Miles, above warm cultivated Plains shoud differ materially in its Temperature, from Air above Mountains, or even on a Level with their Summits.
That the former Air, in moderate Weather, shoud continue warm and rarefied: while the latter is cool and condensed.
For the same Reason the Air over the Sea, on the Hours of Accumulation; i. e. during the Night, in Summer, and frequently in Winter, shoud be found warm and rarefied: especially during a Continuance of the Apogay Winds.
277. It is likewise probable that the Atmosphere will be found respirable at much greater Heights, than is at present imagined: during the Continuance of the Eknèfiai Winds; and also, on Account of the defloguisticated Air,[83] which is drier and less elastic in Proportion to its Rarity.[84]
278. The Height of 10 Miles seems not too great to limit human Respiration, shoud any Attempt be made, to soar with a Balloon in a mild Atmosphere; and particularly between the Tropics.[85]
But an Objection woud be found in the Size of a Balloon sufficiently capacious to contain nearly 6 Times the Bulk to which the Gass woud necessarily expand itself, at the Height of 10 Miles.
First Cause of Limitation, in the Ascent of Balloons.
Second Cause of Limitation in the Ascent of Balloons.
279. It seems most likely that the primary Cause that will affect the Ascent of Balloons is the Difficulty of encreasing the Dimension of the Balloon: the Second, is from the excessive Cold; if the Wind blows from any Points of the North.
Supposing the Construction of the Atmosphere to be as represented by different Authors, (which, by the Way, is scarcely credible) ten Miles will perhaps be the utmost attainable Height.
280. There is a Circumstance relative to the Motion of the Air, which has not been sufficiently attended to: and bears some Analogy with that of a Thorough Air.
This Circumstance may not improperly be called the Reception and Dispersion of Air.
In cold Climates, it is an Object of Dread: in warm ones, a most desirable Piece of Luxury.
A gentle Undulation of the Air is perceived in Peru, and other hot Climates, by Persons sitting in Arbours sheltered from the Sun.
The surrounding Air is instantly contracted by Condensation, during the Absence of the Sun’s Rays, and therefore occupies a less Space: fresh Air is received, and as instantly dispersed by Expansion towards those Parts, which are the warmest, i. e. where there is least Resistance: so that a gentle Breeze is constantly kept up, probably by a Depression from above.[86]
281. Analagous to this, are those Winds which generally rise early and die away at Sunset: the nocturnal Condensation of the Air being sufficient for the reception: as Air suffers some Compression without Tumult.
To demonstrate the Changes owing also to remote and invisible Causes least suspected; Boyle somewhere speaks of an Instrument he made, which was so nicely contrived, that he coud tell, while sitting in his own Apartment, whenever any detached Cloud passed beneath the Sun’s Disk. The Principle on which it acted seems to have been that of a Reception and Dispersion of Air that took Place within the shadow proceeding from the Cloud.
282. An oblique Argument supporting the Doctrine of Depression, asserted to take Place, in fair Weather, is that Wind drys up the Moisture from the Ground more than the Sun: and that March which is the windiest, is also the most drying, tho’ not the hottest Month.
Bacon, in his Enquiry into Motions and Undulations of the Air, uses a Metaphor, which tho’ somewhat facetious, is strictly philosophical.[87] “For when winds lead the dance, it woud be agreeable to know the figure.”[88]
And it is probable, that they really press the Earth with a saltatory progressive undulating Motion, descending in elastic Steps of sudden Compression; and rising with quick alternate ones, of Dilatation and Expansion.
Dicker’s Balloon gave Proof of this.
283. Lastly: the chill of Air which always takes Place over water, and moist Grounds, even in the finest weather, strongly favours the Reception and Dispersion of it, to the surrounding and more heated Lands: (which can only be supplied, as before mentioned, by Torrents of fresh Air gradually descending from the etherial or middle Region of the Atmosphere;) and seems to produce the same Effect, viz. a constant Breeze, with that of the Arbor, Shade, or Shelter from the Sun: also with that of the Shadow from the Cloud passing under his Disk, which affected a complete Thermometer and Hygrometer.
284. On a Change of Weather from Frost to Thaw, the Colour of the upper Air first alters from a clear and deep, to a dull and faint Blue, or to a muddy Haze, not distinguishable into Clouds, but visible above them; a vivid Brightness still remaining, for many Hours, to about 500 Yards above the Surface of the Earth.
Or, soft warm Showers fall gently, without Wind, or any apparent Change in its Direction.
All which seem to favour the Accumulation and Descent of warm Air, by Waves of the Apogay rolling over the Eknèfiai Winds.
