ATM
3nd, at the altitude of 70 miles, about a million of times rarer ; and at the altitude of 500 miles, if the Atmofphere can reach fo far, the air muft be rarified fo much, that if a globe of what we breathe, of an inch diameter, were as much di- lated, it would occupy a larger fpace than the whole fphere of Saturn.
But it is to beobferved, that thefe computations of the rarity of the Atmofphere, at different heights, are founded on this prin- ciple, that the denfity of the air is every where proportionable to the fuperincumbent weight.
Now this rule holds true only upon the fuppofition, that the heat is uniform, at different diftances from the earth ; for if the air be hotter in one part than in another, the air will be more rarified in the hotter part, than it will be in the cooler, altho' preffed by the fame weight, or at the fame altitude above the earth's furface.
This obfervation will alfo fhew how precarious the common method of meafuring the heights of mountains by the barome- ter is. See Mountain.
It appears from the obfervations of aftronomers, of the dura- tion of twilight, and of the magnitude of the terreftrial fha- dow in lunar eclipfes, that the effect of the Atmofphere to re- flea and intercept the light of the fun, is fenfible to the alti- tude of between 40 and 50 miles. So far then we may be certain that the Atmofphere reaches ; and at that altitude we may collea from what has been already faid, that the air is about 10,000 rarer than at the furface of the earth. How much further the Atmofphere may extend, we are altogether ignorant. Cotes, ibid. p. 123 — 125. The Atmofphere has a refractive power, which is the caufe of many phenomena. Alhazen the Arabian, who lived about A. D. 1100, feems to have been more in- quintive into the nature of refraflions than the preceding writers. So that having made experiments upon this fub- ject, at the common furface betwixt air and water, air and glafs, water and glafs, or cryftal, and being prepoffeffed with the old opinion of cryftalline orbs in the regions above the Atmofphere, he had the boldnefs to fufpeft a refraction there alfo. This, he tells us, may be proved, by taking the diftance of a ftar from the pole of the equator, both when it is very low, and when very high, near the zenith ; and he af- firms, that the former polar diftance will be found lefs than the latter, by reafon of the refraaion of the rays : And this, if we may credit friar Bacon, was taken from Ptolemy's eighth book of afpects. But Dr. Smith obferves, that if it be fo in the book of afpects, which the doflor could never meet with, it muft have been written by Ptolemy, after his almageft, whereby it appears that he had no fufpicion that fuch effeas as that above-mentioned were caufed by a refraaion of the rays of the fun, or ftar. Smith's Optics, rem. 355. However, it is certain that Alhazen deduced feveral proper- ties of this kind of refraaion ; as that it increafes the altitudes of all objeas in the heavens ; that it contraas their diameters, and diftances from each other ; and that it caufes the twink- ling of ftars. But neither Alhazen, nor his follower Vitellio, knew any thing of its juft quantity, which was not known to any tolerable degree of exaanefs, till Tycho Brahe, with in- credible diligence, fettled it.
Neither did Tycho, or Kepler, difcover in what manner the rays of light were refaaed by the Atmofphere. Tycho thought the refraaion was chiefly caufed by denfe vapours, very near the earth's furface. Kepler placed the caufe wholly at the top of the Atmofphere, which he took to be uniformly denfe ; and thence he determined its altitude to be little more than that of the higheft mountains. But the true conftitution of the denfity of the Atmofphere, deduced afterwards from the Torricellian experiment, afforded a jufter idea of thefe re- fraaions, cfpecially after it appeared, by a repetition of Mr. Lowthorp's experiment, that the air's refraaive power is proportionable to its denfity.
By this variation of the air's denfity, a ray of light, in paffing through the Atmofphere, is continually reMed at every point" and thereby defcribes a curve, and not a ftrait line, as it would have done were there no Atmofphere ; or were its denfity uniform.
This refraaive power of the Atmofphere increafes the altitude of the ftars, and contraas their intervals ; it alfo caufes the fun and moon to appear of an oval figure, when near the ho- rizon. But it is to be abferved, that the horizontal moon appears oval but rarely, efpecially in the evenings of warm weather, the refraaions being then fmaller. V. Smith's Op- tics, rem. 371. See alfo the article Refraction, Cyel. and Suppl.
The Afmofphere, or air, has alfo a refleaive power ; and this power is the caufe that enlightens objeas fo uniformly on all fides. The abfence of this power would occafion a ftrange alteration in the appearances of things, their fhadows would be fo very dark, and their fides enlightened by the fun fo very brighr, that, probably, we could fee no more of them than their bright halves ; fo that, for a view of the other halves, we muft turn them half round, or, if immoveable, muft wait sill the fun could come round upon them. Such a pellucid un- Svpsi. Vol. I. •
A T O
refleaive Atmofphere would indeed have been very common dious for afironomical obfervations upon the courfe of the fun and planets among the fixed ftars, vifible by day as well as by night ; but then fuch a fudden tranfition from darknefs to light' and from light to darknefs immediately, upon the rifing°and fctting of the fun, without any twilight, would have been very inconvenient and offenfive to our eyes. Alhazen, and others, attempted to determine the height of the Atmofphere, from the confideration of the twilight, as before mentioned, and in the cyclopaedia. The refult of the com- putation was 50 miles for the height of the Atmofphere, or re- fleaive matter aSove the earth's furface. But this height, ac- cording to Dr. Halley's correaion, in Phil. Tranf. N°. 181. will be reduced to about 44^ miles.
It will follow as a conference from this, that any place is conftantly enlightened in the day-time by rays rcfli-aed from every part of a fegment of the Atmofphere, whofe height is about 44 i miles, and whofe circular bafe is about i200°milcs in diameter. See Smith's Opt. rem. 384. Thus tho' the Atmofphere is greatly affiftant to the illumina- tion of objefls, yet it muft alfo be obferved, that it flops a great deal of light. By Monfieur Bouguer's experiments, it feems that the light of the moon is frequently 2000 times weaker in the horizon, than at the altitude of 66 degrees; and that the proportion of her lights at the altitudes of 66 and 19 degrees is about 3 to 2. The lights of the fun muft bear the fame proportion to eachother at thofeheights ; which Monfieur Bouguer made choice of, as being the meridian heights of the fun, at the fummer and winter folftices, in the latitude of of Croific in France. Botig. Effai dioptr. fur la gradat de la lumiere, p. 12. ap. Smith, Opt. rem. 95. It has been faid that a ray of light paffing through the Atmo- fphere defcribes a curve : To find the nature of this curve is a problem of no fmall difficulty, for which the curious may confult Taylor, Meth. increm. p. 108. feq. This ingenious author computes the refractive power of the air to be to the force of gravity at the furface of the earth, as 320 millions to I.
There have been often feen in the Atmofphere fome very lumi- nous parts, even near the zenith about midnight. It has been thought that thefe luminous parts are nothing elfe but terre- ftrial exhalations floating in the air, at a prodigious altitude, and thereby refleaing the light of the fun, which the? are ex- pofed to, at that great height, to our eyes. But Mr. Cotes juftly obferves, that it will be next to impoffi- ble to give any tolerable account, how thofe exhalations can be denfe enough to reffea fo copious a light at that vafi diftance ; and at the fame time be fupported, by a medium, fo much rarer than the air we breathe in. It feems therefore more probable, that thefe extraordinary lights proceed from fome felf-fhining fubftance, or aerial phofphorus. A furprizing appearance of this kind was feen at Cambridge, on the 20th of March, in the year 1706. It was a femicircle of light, of about two thirds of the ordinary breadth of the milky way, but much brighter. The top of it paffed very near the zenith of that place, inclining about 4 or 5 degrees to the north ; it crofted the horizon at a very fmall diftance from the Weft, towards the South, and again, about as far from the Eaft towards the North. It was molt vivid, and beft defined about the weftern horizon, and molt faint about the zenith, where it firft began to difappear. There was at the fame time an Aurora borealis. The fame appearance was feen in Lincolnfhire, at the diftance of about 70 miles north of Cambridge, and there the femicircle feemed to lie in the plane of the equator. From thefe two obfervations compared together, it may be colkaed, that the matter, from which that light proceeded, was elevated above the earth's furface between 40 and 50 miles. Cotes, Hydroft. Lea. p. 125, 126. F, de Lana thought he had contrived an aeronautic machine for navigating the Atmofphere a : Sturmius, who examined it, declared it not to be impraaicable b : But Dr. Hook was of a different opinion, and deteaed the fallacy of the contrivance c . Roger Bacon long before propofed fomething of the fame kind. The great fecret of this art, is to contrive an engine fo far lighter than air, that it will raife itfelf in the Atmofphere, and, together with itfelf, buoy up and carry men with it. The principle on which it is to be effeaeti, is by exhaufting the air of a very thin and light, yet firm., metalline vcfiel, with an air pump. — [» V. Prodrom. c. 6. which is alfo given in Englifh in Hook's Philof. Collea. N°. 1. p. 18. feq. h Colleg. Curiof. Tent. 10. Morhof Polyhift. Philof. I. 2. p. 2. c. 22. * 2. c Hook, lib. cit. p. 28. feq.] But the hopes of fuccefs in fuch an enterprise will appear very fmall, if it be confidered, that if a globe were to be formed of brafs, of the thicknefs only of T l- inch, that globe muft be above 277 feet in diameter to fwim in the air ; and if, as de Lana fuppofes, the diameter of the globe were but 25 feet, the thicknefs of the metal could not exceed T jj of an inch. V. Herman. Phoronom. p. 158. ATOCION, A7cxto*, in antient natuialifts, denotes an abortive medicine, or a medicament proper to expel the fcetus after conception. Plin. Hift. Nat. 1. 20. c. 4.
3 L ATOL-
