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. CHAPTER LII. [Sidenote: _Proper_ Days in the Month for the Ascent of Balloons.] Section 285. As the _safest Hour_ of the Day has been already pointed out, for the Ascent of those Aironauts, who propose to cross a Channel, or Arm of the Sea, in a Balloon _Air-tight_ or nearly so: it may not be useless to throw out a few Hints on the properest Days in _each Month_, for the Ascent of Balloons.