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fcc Power of Air is increafed by the admixture of Humidity therewith; and confequently, that the Air near the Tops of Mountains, being moifter than that belowi becomes thereby more Elaftic, and ratifies in a greater Ratio than natutally and in a drier State it would.— But Dr. Jmin (hews, that the Experi- ments produced to fupport this Syftem, are by no means con- cluhve. Append. adVaren. Geograph.
Be this as it will, the Rarities of the Air at different Heights proving not to bear any conftant Proportion to the Weights wherewith they are prefs'd; Experiments made iwith Barometers at the Feet and Tops of Mountains, carmot give the Height of the Atmofphere; fince our Obfervations ate all made near the Earth; whereas the greatet Part of the Atmofphere is far beyond ; and the farthet from us, the farther does it feem to recede from the Nature and Laws ot ours.— M. de la Hire, therefore, after Kepler, has recourfe to a more limple and fecure Way of afcer- taining the Height of the Ajmofphm j viz. from the confiderati- on of the Crepufcula.
'Tis allowed by Aftronomers, that when the Sun is 1 8" be- low the Horizon, we begin or ceafe to fee the Twilight: Now the Ray whereby we do it, can be no othet than a horizontal Line, or a Tangent to the Earth in the Place where the Obfetver is. But this Ray cannot come directly from the Sun, which is un- der the Horizon; and muft therefore be a Ray refieded to us by the lall inner and concave Surface of the Atmofphere. We are to fuppofe that the Sun when rS° below the Horizon, emits a Ray which is a Tangent to the Earth, and Unites upon this laft Surface of the Atmofphere, and is thence refleded to our Eye, being (till a Tangent, and horizontal.— it there were no Atmof- phere, there would be no Crepufculum ; and confequently if the Atmofphere were not fohigh as it is, the Crepufculum would be- gin and end when the Sun is at a lei's Diftance from the Horizon than 18", And contrarily.— Hence we gather, that the iargenefs of the Arch by which the Sun is deprefs'd when the Oepulculum begins or ends, determines the Height of the At- mofphere. We are to note however, that 32' Minutes muft be fubllraded from the Arch of t8°, for the Refraction, which railcs the Sun fo much higher than he wou'd be; and 16' more for the Height of the upper Limb of the Sun, which is fuppofed to lend the Ray, above his Centre, which is fuppofed to be 18 low. The remaining Arch, therefore, which determines the Height of the Atmofphere, is only 17 12'.
Two Rays, one direct, and the other refl'eaed, but both Tan- gents to the Earth, muft necelTarily meet in the Atmofphere, at the Point of ReBedion, and comprehend an Arch between 'em of 17 12', whereof they are Tangents.— Hence it follows from the Nature of the Circle, that a Line drawn from the Centre of the Earth, and cutting the Arch in two, will go to the Point of Concurrence of thofe two Rays; and as it is eafy finding the excels of this Line over the Semidiameter of the F.atth, which is known, 'tis eafy to find the Height of the Atmofphere, which is only that excels.— On this Principle M. de la Hire difcovers the Height of the Atmofphere to be 37223 Fathoms, or near 17 French Leagues. The fame Method was made ufe of by Kepler, who only rejected it, becaufe it gave the Heiaht of the -Atmof- phere 20 times greater than he otherwife allowed it.
It mull be added, that in this Calculus, the dired and refied- cd Rays are fuppofed to be right Lines ; whereas in fad they are Curves, form'd try the perpetual Refraftion the Rays undergo in prefling through a Seties of different Denfities of Air.— Com- puting, then, upon them as two fimilat Curves; or rather as a lingle Curve, one extreme whereof is a Tangent to the Earth : Irs Vertex equally diftant from both Extremes, detetmines the Height of the Atmofphere; which therefore, will be found fomewhat lower than in the former Cafe ; the Point of Concur- rence of two right Lines, which are here only Tangents to the Curve, the one at one end, and the other at the other ; being higher than the Vertex of the Curve. On this footing, M. de la Hire finds the Atmofphere 35362 Fathoms, 01 16 Leagues. Hift. de 1. Acad. Roy. des Sciem. An. 1713. See the AtticlcS
Refraction, Crepusculum. &c.
Atmospheres of the Heavenly Bodies.
Lunar Atmosphere— Tleat the Moon is furrotindej, like the Earth, with a changeable Atmofphere, fee evinced under the Ar- ticle Moon.
The Reality of the Atmofpheres of the other Planets, fee alfo un- der the Article Planet.
For the Atmospheres of Comets, and the Sun, fee CoMET and Sun.— See alfo Maculae, Tail, &c.
Atmosphere offelid, or conffent Bodies, is a kind of Sphere form'd by the Effluvia, or minute Corpufcles emitted from them. See Sphere and Effluvia.
Mr. Boyle endeavours to fhew that all Bodies, even the hardeft and mod coherent, as Gems, d-c have their Atmofpheres. See Gem.— See alfo Magnet, Magnetism, &c.
ATOM, Atomus, in Philofophy, a Corpufcle; or a Part or Particle of Matter lo minute as to be indivifible. See Cor- puscle.
The Word is Greek, «t.(*» S ; form'd of the privative *, and np, »., I cut, I divide.
Atoms are properly the minima Nature, the lad or ultimate Particles into which Bodies ate divifible; and are conceived as the firil Rudiments, or component Parts of all phyfical Magnitude, or the prc-exiilent and incorruptible Matter whereof Bodies were form'd. See Particle, Body, &c.
The Notion of Atoms arifes hence, that Matter is not divifi- ble, in infinitum. See Divisibility.
And hence many Authors are led to deny the reality of Atoms, together with that of Mathematical Points: An Atom, fay they, either has Parts, or it has none: If it have none, it is a mens Mathematical Point : If it have, then do thefe Parts alfo confift of others, and fo to Infinity. See Continuity, &c.
But this is to recede from the genuine Charader of Atoms, which are not efteemed indivifible, becaufe of their want of Bignefs, or Parts; (for all phyfical Magnitude muft have three Dimenhons, Length, Breadth, and Thickncfs; and all Extenfi- on is divifible) but they are indivifible on acconnt of their Soli- dity, Hardneis, and Impenetrability, which preclude all Divifion, and leave no Vacancy for the Admiffion of any foreign Force to fcparate ot difunite them. See Indivisible, Divisibility, Extension, Matter, &c.
As Atoms are the firft Matter, 'tis neceflarv they fhou'd be in- diffolvible, in order to their being incorruptible.— Sir Ifaac New- ton adds, that 'tis alfo required they be immutable, in order to the World's continuing in the fame Srate, and Bodies being of the fame Nature now as formerly. See Hardness.
Hence the Antients were alfo led to maintain Atoms, eternal; for that what is Immutable muft be Eternal. See Eternity.
They alfo added Gravity, and in Conlequence thereof, Moti- on, to their Atoms : And further, obfervtag that Atoms thus falling perpendicularly, cou'd not join or unite together; they fupcr-added a fortuitous or fide Motion, and furoifli'd them with certain hooked Parts, in order to enable them to catch and hang the better together.— And from a cafual or fortuitous Jumble of thefe hamous Atom:, they fuppofed the whole Univerfe to be formed. See Gravity, Motion, Hamous, fire.
ATOMICAL Philofophy, the Doctrine of Atoms; or a Method of accounting for the Origin and Formation of all things, from the Suppofition of Atoms, endued with Gravity and Motion. See Atom, World, &c.
The Alomical Philofophy was firft broach'd and taught by Moftbus, Leucippus, and Vetmcritm : It was cultivated b°Epicu- rus; whence it became alfo denominated the Epicurean Philofophy. See Epicurean.
It has been fjnee retrieved by Gaffendm, and others; and is now efpoufed and adhered to by a great Patt of the philofophi- cal World, under the Denomination of the Corpufcular Philofo- phy. See Corpuscular. Philofophy. '
ATONEMENT. See Propitiation, Sacrifice, &c.
ATONY", Atonia, in Medicine, &c. a want of Tone or Tcnfion ,■ or a Relaxation of the Solids of a human Body • oc- calioninga lofs of Strength, Faintings, &c. See Tone, Solid, Fibre, Relaxation, &c.
The Word is compounded of the privative*, and nim, tench, I flretch.
ATRABILIS, in the antient Medicine, black or aduft Bile. See Bile and Adust.
Atrabilis was one of the great Humours of the antient Pbyfi- cians; whence arofe the Atrabilary, one of their Temperaments • anfwering to what we call Melamholly. See Humour, Tempe- rament, Melancuolly, &c.
ATROPHY, Atrophia, an Indifpofition or Difcafe, wherein the Body or fome of its Parts, do not receive the ne- ceffary Nutriment, but dwindles ot waites infenfibly. See Nu- trition.
Such are what we commonly call Confimptions, Phthilis, Sec See Consumption, Phthisis, &c
An Atrophy is natural in old Age, call'd Atrophia Senilis. See Old Age, Death, &c.
The Word is compounded of the privative Paiticle « and tpiQm, I feed, nourilh.
ATTACHING, or Attachment, in Law, the taking or apprehending aPerfon or Thing by Commandment or Writ. See Commandment and Writ.
The Word is i form'd of the French Attacher, to fallen, tye; and that from the corrupt Latin Attacliiare, of Attexere, to weave to; or rather from the Celtic Tach, a Nail, and tacha, to nail.
Lambard makes this Difference between an Arrell and an At. tachment; that an Arreft proceeds out of an inferior Court by Precept, and an Attachment out of a higher Court, by Precept or Writ; and that a Precept to arreft hath thefe formal Words, Duci facias, &c. and a Writ of Attachment thefe, Praicipinius tibi quod attachies talem, ojr habeas eum coram nobis. See Arrest.
By this it appears, that he who arrcfts, carries the Party arreil- ed to another higher Perfon, to be difpofed of forthwith; where- as he that attaches keeps the Patty attached, and prefents him in Cout t at the Day affigned in the Attachment.
There is this further Difference, that an Arrell lies only upon the Body of a Man ; and an Attachnent fometimes on his Goods too ; for a Man may be attach 'd by an hundred Sheep.
Attachment, by Writ, differs from a Diftrefs in this, that an
Attachment does not reach Lands, as a Diftrefs does ; and that a
X x Diftrefs
