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PROPOSITION XIII.

Why when we look thro' the Air in an horizontal Line it appears thicker and more cloudy than that above, or that in which we breathe.

THE Cause is twofold; the first because the Air near the Horizon is really more cloudy; the other is a deceit in our Sight; for the Eye takes in the Distances of the Parts of an Arch in the Horizon, by very small Angles; as it does the Distances of Pillars in a long Row: and as we judge those that are diftant to be near, fo the diftant Particles of Air are judged to be joined clofe; but the Distance of the Particles of Air that is higher, the Eye fees under great Angles and apprehends them the better.

THE fame is the Caufe why the Air at a Distance appears to be cloudy; but when we approach to it, it does not seem fo cloudy.

PROPOSITION XIV.

Whether the Atmosphere or Air be always of the fame Height in all Places; or if it's Figure be spherical.

THAT it is not of the fame but of very different Heights, appears in that the Sun is only vertical to one Place at once, and fends it's Rays obliquely to other Places; and fo more weakly the more they are remote from the Sun, or the nearer the Poles: and therefore the power of the Sun is different in different Places, and must raise the Vapours differently; they are highest directly under the Sun, and lowest in the oppofite Point, and in a middle Height at the Pole, fo that the Air is of an oval Figure.

YET the contrary, that the Height is the fame in all Places, feems more probable; tho' the Vapours are more elevated in fome Places than others; yet because the Air is fluid and by it's Gravity tends downward, therefore the higher Parts prefs those below; and those again others fideways, Still all the Parts come to be alike high; and thus it's fpherical Figure is proved the fame way as that of the Water is proved by Archimedes, Chap. xiii, for the Suppofitions here are the fame as there; which if falfe the Demonstration fails.

DES Cartes alfo makes it oval, for a particular Reafon; fee Chap. xiv.

PROPOSITION XV.

The Condensation or Rarifaction of the Air doth not alter it's Height.

FOR not the whole, but a part only is condenfed or rarified, fometimes here, fometimes there; which doth not alter the Height in one Place more than another: only there may be a greater Condensation in one Part than in another: which can alter the Height but very little.

PROPOSITION XVI.

The Altitude of the Atmosphere or Air is not only the fame in different Places, but is always the fame both Summer and Winter.

FOR tho' the Heat in our Summer doth attenuate our Air, and raise it more than in Winter, yet because then there is Winter in another Place, the Air there is lefs raifed, and therefore a Part of our Air will flow there: and when our Air is low by the Cold, the Air of another Place that is hotter

SECT. VI. will move to us, 'till the whole Air be equally diftant from the Center.

AND the fame may be faid as to Day and Night; for while at Night it is condenfed with us, and is low, it rarifies more in another Place, and moves to our Air 'till it makes a spherical Figure; and becaufe all things are every where equal, the Height will continue the fame every where ; and tho’ it may rarify and condenfe more in one Place and Time than another, yet the Difference being small will not much alter the Altitude; as we faid in the preceding Propofition.

THE fame may be faid of the Clouds, Rain, or Vapours, in our or another Place, as from thefe a greater or lefs Altitude feems to arife: but I anfwer, there is scarce any time in which it doth not rain, or a Cloud fall, in fome Place or other; and therefore while it rains in one Place the Air becomes no lefs than it was, because it rained before in another Place, and fo it comes all to the fame thing, and the quantity of the Air is neither encreased nor diminished.

PROPOSITION. XVII.

The colder the Air is, the thicker: and therefore it is for the most part colder in Winter than Summer (in any particular Place), and likewife in the Night more than in the Day, and the grofs Exhalations from the Water in the Winter-time, increase that Denfity, especially in the Evening and Morning.

THE Truth of the Propofition is clear from the preceding; nor is it any Objection, that a Part of the hotter Air moves where it is colder, and more low; for it is not that but fome neighbouring Air that moves to the Place, because of the continual Protrufion, or Preffure; or tho' it came

itself,

itself, yet by coming there, it would become cold.

PROPOSITION XVIII.

There are commonly reckoned three Regions of the Air, of which that is in the middle where the Snow, Hail, and Rain are formed; the first is that in which we live reaching to the middle Region; the third is from the middle Region to the utmost Bounds of the Atmosphere, even to the fiery Region, as the Ariftotelians Speak.

THE middle Region is colder than the first and third, which are counted hotter: because the third contains more fubtile, fiery, and fulphureous Exhalations which go up into it above the Place of the Particles of Water, or are thruft there being lighter. The Ariftotelians fay 'tis hotter because nearer to the fiery Sphere, and colder than the firft; becaufe the Rays falling, join with those that are reflected from the Earth, and fo double the Heat. Moreover the Particles of the fubterraneous Fire coming out of the Earth are diffipated there in the lower Region; and the middle Region being without all thefe Advantages must needs be colder.

PROPOSITION XIX.

The nearer a Place is to the Pole, or the more diftant from the Place where the Sun is vertical, the Place of the Air in which Rain, Snow, and Hail is form ed is the nearer the Earth.

THE Cause is, that the Rays fall more oblique ly on the Places about the Poles than on those about the Equator, and therefore being refracted are far removed from the Perpendicular, and thus

the

the Heat becomes lefs, and the watry Vapours contract into lefs Room, and by joining form the watry Meteors.

COROLLARY.

THE Superficies of the first Region is oval, or rather elliptical, or like a Spheriod, bulging out under the Torrid Zone.

PROPOSITION XX.

The nearer a Place is to the Pole, the third Region (in which the more fubtile and fulphureous parts move up and down) begins further from the Earth.

FOR that Part of the Atmosphere which is nearer the Pole contains fewer fubtile and fulphureous Particles; for the Sun brings fewer of them thither from the Earth. And a lefs Number being raised there than in the Temperate Zone, and fewer in the Temperate than in the Torrid Zone, and the utmoft Bounds of the third Region equally diftant from the Earth's Center by Propofition 16; therefore the beginning of that Region under the Frigid Zone, is further from the Earth's Center than it's beginning in the Torrid or Temperate Zone.

COROLLARY

THE Superficies bounding the second Region is as a Spheroid bulging in the Frigid Zone. Thefe are all to be shown to Students by a Diagram.

PROPOSITION XXI.

The Rays of the Sun, Moon, and Stars, do not come directly from the Heavens thro' the Air, to

our

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