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.