into exceeding fmall Particles; that it may be fo much fooner enabled, in Conjunction with Fire, to make a compound Body lighter than fo much Air. Thus we fee in all Distillations, that there do not afcend great and entire Drops, but only very fine and fmall Particles. The fame is plain in all Chymical Sublimations; as likewife in the Smoak of Fires made of Coal, Wood, Turf and the like, which being divided into very minute Parts, are carried up into the Air by the adhering Fire: But being collected into a greater Body, when they are turned into Soot, they become fo heavy, that they will not afcend, till they be reduced by other Diftillations, for inftance, to Bodies of a much fmaller fize.

SECT. XVIII. Vapours afcend both by Heat and Cold.

BUT to make an end of this Enquiry; whatever may be the Cause of the Afcent of Watry Vapours, this is certain, that Water being heated, either by the Sun, or by our Common Fires, tho' in itself it is so much heavier than Air, yet it will be carried up into it.

Now, whether we are likewife to fuppofe that there are particular Particles which produce Cold, as Fire does Heat, and which cleaving to the Water, make up a Body lighter than Water itself, and fo cause it to afcend in Vapours, we fhall not here difpute; this is certain, that we fee Vapours afcending from the low Grounds in the coldest Weather, and when the Water is frozen hard, and that even Ice and Snow are lighter than the Water of which they are compofed, and confequently muft evaporate: But of this hereafter.

SECT. XIX. The Laws of Hydroftatics.

To proceed; it is well known to fuch as underftand the Laws of Hydroftatics, that,

1

I. If a Body is to be carried upwards in any Liquor, an equal Bulk of the faid Liquor muft gravitate or weigh more than fuch a Body.

II. That in order to caufe a Body to fink in a Liquor, an equal Bulk of the faid Liquor muft weigh less than the Body.

III. If you would have the Body neither to `rife nor fall, but preferve its Place in any Part of the Liquor, an equal Quantity of the faid Liquor muft weigh equally with the Body, which may be eafily proved by Experiments.

SECT. XX, and XXI. The Vapours in the Air adapt themselves to thefe Hydroftatical Laws ; as appears by feveral Experiments.

Now,

if we fuppofe, that Tab. XV. Fig. 1, reprefents the Globe of the Earth, WPQRS, furrounded by the Air as far as B AD; which being heavy in itself, and thereby capable of being comprefs'd, grows continually finer from below at P, upwards thro' g, and F to B, and confequently lighter; because its Elaftick Faculty dilates it more in proportion as the Preffure of the fuperiour Air is diminifhed, and, as it scatters the Parts of the Air from each other, renders it lighter in an equally large Space. And if we now fuppofe farther, that this fame Air is heavier below, at that Part of the Globe that lies between F and P, and lighter above between F and B, than the Water evaporated or mingled with Fire; fo that about FGH, the faid Air is of equal Gravity with it, it will follow from what has been juft now mention'd,

that

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that the Vapours between F and P will afcend; that being raised to the Bounds of the Equilibrium, FGH, they will float like Clouds at F and IG; and being æquipois'd, will neither rise nor fall; but when raifed higher, to B F, or HD, they will defcend.

This will happen much after the fame manner, as when you pour Quickfilver and Water into a Glafs, and then throw in a piece of Iron, which will fink down into the Water, but float in the Quickfilver, till it arrive at the place between both of 'em, where it can meet with its Equilibrium, and there it will remain between the two Fluids, the uppermost of which, Bulk for Bulk, is lighter, and the lowermoft heavier.

We must not imagine that these Notions of the Air are fupported only by mere Hypothefes: First, Because it has been experimentally proved above, that the Air is of fuch a Property, that when it is preffed by any weight, the Parts of it are fqueez'd closely together, and so taking up a smaller Space, the fame Quantity becomes heavier. So that it having been proved before in Contemplation XVII. §. XX. by a Tube TF, (Tab. XIV. Fig. 1.) filled with Quickfilver, that the fame Air, which without Compreffion is above of the bignefs of F, when fqueez'd clofer by the weight of the Quickfilver, will lie in fo much a fmaller Space below at I, and confequently becomes heavier in proportion to its Bignefs; fo that, for inftance, if we fuppofe that F above, is ten times as large as I below, a Cubical Inch of Air will prefs or weigh ten times as heavy at I as at F; fince, by the Compreffion below, there is ten times the Quantity of Air contained in the fame Space I, as above at F.

And, Secondly, which may ferve for an Experimental Proof, becaufe fuch as have clim'd thofe

high

high Mountains, find it to be true, you may fee, among many others, a remarkable Account thereof in Varenius, Geograph. Gener. Lib. I. cap. 19. §. 41. Where fomebody that climb'd up one of the Carpathian Mountains in Hungary, which are much higher than thofe of the Alps, faw the white Clouds floating in the Air below him, fome of which were however higher than others, according as the Matter whereof they were compofed, being lighter or heavier, determined their Equilibrium higher or lower; for that numerous Particles, and confequently of different Weight, are raifed up into the Air, under the Denomination of Watry-Vapours, or other Exhalations, has been fhewed above in our Dicfourse upon METEOR S. The faid Perfon did likewife obferve the Air in which he was, to be fo Calm and Serene, that it did not produce Wind enough to move the least Hair of his Head; notwithstanding that he had been fenfible of a strong Wind in the lower Parts of that Mountain. But that which feemed to be the clearest Proof of a greater thinnefs of Air, was, that in difcharging a Musket, at the very Top of the Mountain, the Report or Sound of it was no louder than that which is produced by the breaking of a little Stick. Now how much the Rarefaction, or Thinnefs, of the Air contributes to the Diminution of Sound, appears by hanging a little Bell in the Recipient of the Air-Pump, and exhausting the Air from it; of which more largely in Contemplation XVII. §. XXXVI.

SECT. XXII. After what manner Vapours float.

Now, to draw a Conclufion from all this, it is eafie to be understood, how the Waters, by being united to the Sun-Beams or Fire thereof (to fay nothing of the Exhalations in great Frofts) are

railed up into the Air in Vapours, where, according to the Laws of Hydroftaticks, they are driven and remairi pendulous in a lighter Matter, as the Air is in this Cafe, without fubfiding by their own Weight:: But it would be of very little Use to all the Inhabitants of the Earth, both Men and Beafts, in cafe thefe Watry-Vapours should continue always floating in the Air, without ever falling down from thence. Now to form fome Conception, how this floating of the Air may happen; Let us again fuppofe that from the Sea P, in the thick Air F P, (Tab. XV. Fig. 1.) there are fome Vapours raifed up to F; that at the Diftance of FIG, from the Earth, the Air becoming fomething thinner, yet retains fo much Denfity or Thickness, that tho' thefe Warry-Vapours, by reafon of their not being rare or thin enough, cannot rife up higher to B, yet they are hinder'd by a fufficient Weight and Thickness of Air from falling down, and collect themselves there in highflying Mifts, which when feen from the Earth, are called Clouds, as has been already experimentally fhewn; whilft others that are heavier, cannot afcend farther than to K d; becaufe, if they came into a higher Air, which was lighter, they would fall down again.

SECT. XXIII. Experiments to fhew how the Vapours can defcend.

I. I now two Winds blow thefe Mifts or Clouds with any Force, as I G, or F, from oppo fite Quarters, and thereby compel them to run against each other, it is eafie to conclude, that they will be there collected into Drops, and fo becoming heavier than the like Quantity of Air, will fall down; and the rather, because by the Motion of thefe Winds, the Fire that render'd

them