to be fome other Faculties and Parts requifite in the fame; I fay, befides all this, it does yet render one eminent piece of Service to the whole World, and that upon account of being composed of a fluid Matter, denfer than that which is above it; vizi that by the Refraction or breaking the Rays of the Sun in the faid Air, the Twilight of Morning and Evening are produced; whereby a clear and full Day is prevented from being turned oftentimes in a very little time into a Night as dark as Pitch in the Evening, and fo again a dark Night from be ing turned all at once into a bright Day, to the vifible Prejudice and Weakening the Eyes of Men, and all other Creatures; it being fufficiently known to all that have tried it, how troublefome and'inconvenient are fuch great and fudden Changes, from thick Darkness to a ftrong and clear Light.

Tis owing to this Property of the Air, that the Countries which lie near the Poles, during their long and difmal Nights, do participate of the comfortable Light of the Sun many 'Days before it rifes above the Horizon: From hence it proceeds likewife, that thofe Nations which lie far from the Poles, and in which the Sun daily rifes and fets, do difcover fooner, and are deprived later of the welcome Light of Day, which they therefore enjoy much longer than if there had been no fuch thing as Air about this Globe of the about this Earth.

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To give the Reader fome Notion thereof; Suppose NZS,, to be the Globe of the Earth in Tab. XIV. Fig. 3. E WHT, the Air furrounding it, and E Y, the vifible Horizon of thofe People that dwell at F: Now the Sun would be invifible as foon as it was got below this Horizon, if there were not between the Air and the Sun at A, fuch a denfe Subftance as the Air it felf, which the Ray of the Sun A H falls upon; and Mathemati

cians know, that it mult be confidered as if it fell upon the Line BC, which touches the Air at H; this Ray therefore falls obliquely upon the Air, as making with the Line BC the Angle A H C.

Now it has been fhewn above, when we treat ed about the Sight, in Contemplation XIII. That a Ray (Tab. X. Fig. 2.) coming upon a denfer Matter, which is likewife tranfparent, does not run ftreight forwards to D, but is inflected towards the Perpendicular GQ; that is, being bent or refracted at H, is diverted into another Course HF, fo that in Tab. XIV. Fig. 3. this Ray of the Sum A&H by fuch an Inflection, may reach the Eye of one that ftands at F, whereas it would otherwife have passed a great way above him atD.

dtis likewife plain by Optical Experiments, that a Ray, according to the Right Line HF, falling upon the Eye, the Perfon that fees, does always fancy to himself that the Object is in the Ray FH; for which reafon,the Sun A, being really under the Horizon EFY, they that live at F think that they fee the fame in the Line FH produced, that is at R, and above the Horizon? Duniuers vaum

4

Now that this is fo, has been briefly shewn above in Contemplation XII. Tab. X! Fg. 4. Dand from thence it may in fome manier be compa ratively known, how the Rays of the Sim, being refracted in the Moming and Evening Twilights, do enlighten the Earth, and caufe us to fee the Sun before it be really Rifen, and after it is Set.

ede dele

the foregoing Obfert

Now can the unhappy Atheift fancy again, that this Property of the Air, with relpect to Light, is likewife produced accidentally? Whereas he is nevertheless forced to acknowledge, that

it

it is fo great a Benefit to himself, and the reft of the Inhabitants of the World, that in cafe he had the ordering of it himself, he would think that the Advantage which he had acquired from this one Property of the Air, was alone worth the while to encompass the Earth with such a Body.

SECT. XLIII. The Gravity and Elafticity of the Air unknown to the Ancients.

BEFORE I quit this Subject, I cannot forbear faying fomething very remarkable for the Comfort and Confirmation of fuch as have not fo far for gotten GoD, as to deny the Perfections and Attributes of that adorable Being, by whom all things have been produced; let fuch therefore confider, that the Gravity and Elafticity too of the Air, are new Difcoveries, being accordingly fo term'd by the Gentlemen of the Royal French Academy, in their Hiftory for the Year 1702, of the firft Difcoveries made by Modern Philofophy about the Nature of Light, that they were unknown for fo many thousand Years to the most diligent Enqui rers into Nature, and continued a perfect Secret, even to the most learned Philofophers, till the last Age. For they, and all the Ancients, look'd upon the Air to be a light Body, which would afcend of itself, at-leaft, that it was without Gravity or Weight, to fpeak of that Property in the first Place; till in the laft Age, the Invention of Barometers, together with the fubfequent Experiments made by the Air-Pump, Fire, and otherwife, did fur nish us with undeniable Proofs, that the Air is a heavy Body, and that we are able to compute the Weight thereof. Add to this, that the Barometer, (the first Inftrument that has given Men a Notion of this Gravity of the Air) was not discover'd either by the Study or penetrating Judgment of the

Inventer,

Inventer, Torricellius, who had not this in his View by any means; but (to use the Words of Mr. de Stair, Phyfiolog. Expl.XIX. Sect. 41.) was revealed by the Divine Providence in the Year 1643, and as to him, entirely beyond his Expectation.

CONTEMPLATION XVIII. Of METEOR S.

SECT. I. Tranfition to the Meteors.

EFORE we take leave of the Air, it seems requifite to fay fomething concerning Meteors, fuch as the Clouds, Mifts or Fogs, Wind, Rain, Thunder, Lightning, &c.; forafmuch as an infinite Number of Wonders have at all times appeared therein; and the Almighty has thereby, in a particular manner manifefted his Tremendous Power and Greatness many times to those, who as far as in them lay, endeavour to deny it; and forc'd them to own it with Fear and Trembling: Yet forafmuch as the fame are moftly placed out of the reach of fuch Experiments as might ferve either to make a juft Enquiry into all the Causes thereof, or even to try the Certainty of fome probable Opinions concerning them; Humane Knowledge does not extend itself far enough in these Matters to be able to say with fufficient Certainty, how they are produced, and how they operate.

SECT. II. The Air is a Menftruum or Diffolving Fluid.

THIS feems however to be true, that the ambient Air has the fame Power and Effects upVO L. II.

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on

on many Bodies, as that which the Chymifts call a Menftruum or Diffolving Liquor; upon which it operates after the fame manner as Brandy, for inftance, upon Spices put into it, out of which it extracts fome of the Parts, and incorporates them with itself.

SECT. III. The Air is impregnated with great variety of Particles.

THUS we fee, that all the Effluvia or Exhalations of fuch an infinite number of Bodies; that all the Scents, whether of fweet or ftinking Bodies, the Smoak and Steam of things that are burnt or putrified, the Vapours and Fogs arifing from so many Seas, Rivers, Lakes, Ponds, and other Waters, the Particles of Fire from fo many Flames of Nitrous and Sulphureous, of Acid and of Alcaline Bodies, or of both of them fermented together; in a word, whatever they call Volatile, and which being exhaled can afcend, are all mixed with the Air, and collected in the fame, as in a common Magazine or Ware-Houfe. Add to all these the Rays and Light of the Sun, that move with fo unconceivable a Swiftnefs, as we fhall fhow hereafter, and which are reverberated, or do rebound back into the Air in infinite Streams and Numbers: To fay nothing of the Planets and fix'd Stars, which how little Effect foever they may be fuppofed to produce, by reafon of their vaft Distance, yet, fince thefe Heavenly Bodies are feen thro' the Air, and the Rays are tranfmitted from them with a prodigious Velocity quite thro' it down to us, we have reafon enough not to pass them by in filence. To reckon every thing, would be impoffible, and they who are never fo little converfant in the Experiments of Natural Philofophy