Page:Cyclopaedia, Chambers - Volume 1.djvu/208

 ATM

(i«8)

ATM

The Difference of the Air's Weight, at different times, is meafur'd by the different Height to which the Mercury is rais'd in the Barometer ; and the greateft Variation of the Height of the Mercury being 3 Inches, a Column of Air of any affignable Bafe equal to the Weight of a Cylinder of Mercury of the lame Bafe, and of the Altitude of three Inches, will be taken off from the Preffure upon a Body of an equal Bafe, at fuch times as the Mercury is three Inches lower in the Barometer ; fo that every Inch fquare of the Surface of our Bodies, is preffed upon at one time more than another, by a Weight of Air equal to the Weight of 3 cubical Inches of Mercury. — Now a cubical Foot of Wa- ter being 76 Pound; a cubical Foot of Mercury muft be 1064 Pound =102144 Drams : And as 102144 Drams is to a cubical

192 Foot, or, which is all one, 1728 cubical Inches, fo is 50 — —

Drams, to one cubical Inch. So that a cubical Inch of Mercu- ry being very near —59 Drams ; and there being 144 fquare Inches ui a Foot fquare, therefore a Mafs of Mercury of a Foot fquare, =144 fquare Inches,* and if three Inches high, mull contain 432 cubical Inches of Mercury, which X59 (theNum- ber of Drams in a cubical Inch of Mercury) makes 25488 Drams.— And this Weight does a Foot fquare of the Surfaces of our Bodies, fuftain at one time more than at another.

Suppofe again, the Superficies of a human Body =15 Foot fquare ; then would the Body fuftain at one time more than at

another, a Weight = i<r y 254.8 — ^ ^ 2 ° Drams (= 477

Ounces) =2= 3982 i Pound Troy.

Hence, it is fo far from being a wonder that we fomctimes fuffer in our Health, by a change of Weather ; that it is the greateft, we don't always do fo. — For when we confider that our Bodies are fometimes preffed upon by near a Tun and a half Weight more than at other, and that this Variation is often very fudden ; '[is furprizing that every fuch Change does not intirely break the Frame of our Bodies to pieces.

In effect, the Veffels of our Bodies being fo much ftraitned by an increas'd Preffure, would ftagnate the Blood up to the ve- ry Heart, and the Circulation would quite ceafe ; if Nature had not wifely contriv'd, that when the Refiftance to the circulating Blood is greateft, the Impetus, by which the Heart contracts, mould be fo too. — For, upon an increafe of the Weight of the Air, the Lungs will be more forcibly expanded, and thereby the Blood more intimately broken and divided, fo that it becomes fitter for the more fluid Secretions, fuch as that of the nervous Fluid ; by which the Heart will be more ftrongly contracted. And the Blood's Motion towards the Surface of the Body being obftru&ed, it will pafs in greater quantity to the Brain, where the Preffure of the Air is taken off by the Cranium ,• up- on which fcore alfo, more Spirits will be feparated, and the Heart, on that account too, more enabled to carry on the Circulation, through all paffable Canals, whilft fome others, towards the Sur- face, are obftructed. See Heart, Circulation, &c.

The molt conhderable Alteration made in the Blood, upon the Air's greater or leffer Preflure on the Surface of our Bodies, is its rendring the Blood more or lefs compact, and making ic croud into a lefs, or expand into a greater Space, in the Vef- fels it enters. — For the Air contain'd in the Blood, always keeps it felf in Equilibrio with the external Air that prefles upon our Bodies ; and this it does by a conftant Nifus to unbend it felf, which is always proportionable to the compreffing Weight by which ic was bent : So that if the Compreflion or Weight of the circumambient Air be ever fo little abated, the Air contain'd within the Blood unfolds its Spring, and forces the Blood to take up a larger Space than it did before. See Blood, Heat, Cold,

&>

The Reafon we are not fenuble of this Preffure, is well ex- plained by Borelltti, De mat. not. aGnxv, fac. prop. 29, &c. — After fhewing that Sand perfectly rammed in a hard Vellel is not capable by any means of being penetrated or parted, not even by a Wedge; and likewife that Water contained in a Bladder com- preffed equally on all Sides, cannot yield or give way in any Part: He proceeds, « In like manner, within the Skin of an Animal <* are contained a diverfity of Parts, fome hard, as Bones ; others « Blood, Fat, &c. Now 'tis not poffible the Bones mould be « broke or difplaced in the Body, unlets the Weight lay heavier « on one Part than another, as we fometimes fee in Porters. «« If the Preffure be fubdivided, fo that ic lay equally all around, «* upwards, downwards, and fideways, and no Part of the Skin " fed of folid Fibres, do mutually fuftain each other, and refill « the common Weight. The fame .holds of Blood, and the o- «■ ther Humours; and as Water does not admit any manifeft " Condenfacion, fo the Animal Humours contained in ther Vef- •* fels may fuffer an Attrition from an impulfe made in one or « more particular Places,- but can never be forced out of their « Veffels by an universal Compreffion.— It follows, that as none rt of the Parts undergo either Separation, Luxation, Contufion, or <* any other Change of Situation; 'tis impoffible any Senfe of Pain
 * foft, as Mufcles, Nerves, Membranes, <&c. others fluid, as
 * is exempt therefrom, it is evidently impoffible any Fracture
 * < or Luxation fhould follow. The fame may beobferved of the
 * j Mufcles and Nerves; which, though foft, yet being compo-

« fliOLudenfue,whichcanonlybetheeffea of a Solution of Conti- " nuity." This is confirmed by what we fee in Divers, &c. Sec Diving.

The fame is farther confirmed by Mr. Boyle; who including a young Frog in a Veffel half full of Water, and intruding fo much Air as that the Water muft fuftain eight times the Weight it otherwife would ; yet the Animalcule, notwithstanding the great tendernefs of its Skin, did not feem to be at all affected thereby.

For the EffeBs of the Removal of the Prejfure of the Atmof- phere, fee Am-Pazwp.

For the Caufe of the Variations in the Weight and Preffure of the Atmofphere, fee Barometer.

Height of the ATMOSPHERE.

The Height of the Atmofphere is a Point about which the modern Naturalifts have been very follicitous.— Had not the Air an elaftic Power, but were every where of the fame Denfi- ty, from the Surface of the Earth to the extreme Limit of the Atmofphere, like Water, which is equally Denfe at all Depths; ic being above obferved that the Weight of the Column of Air reaching to the Top of the Atmofphere is equal to the Weight of the Mercury contained in the Barometer;; and the Proporti- on of Weight likewife being known between equal bulks of Air and Mercury ; it were eafy to find the Height of fuch Column, and confequently of the Atmofphere it felf. — For a Column of Air one Inch high, being to an equal Column of Mercury, as 1 to 10800; 'tis evident that 10800 fuch Columns of Air, that is, a, Column 900 Foot high, is equal in Weight to one Inch of Mer- cury : And confequently the 30 Inches of Mercury fuftained in the Barometer, require a Column of Air 17000 Foot high, on which footing the Height of the Atmofphere would only be 27000 Foot, or little more than hvcEngltJb Miles high. See Torricellian.

But the Air, by its elaftic Property, being liable to expand and contract; and it being found by repeated Experiments in Eng- land, France, and Italy, that the Spaces it takes up, when com- preffed by different Weights, are reciprocally proportional to thofe Weights themfelves ; or, that the Air takes up the lefs Space, the more it is preffed : Ic follows, that the Air in the up- per Regions of the Atmofphere where the Weight is fo much lefs, muft be much rarer than nearer the Surface of the Earth ; and confequently that the Height of the Atmofphere muft be much higher than is above aflign'd.

If we fuppofe the Height of the whole Atmofphere divided in- to innumerable equal Parts; the Denfity of the Air in each of the laid Parts being as its Quantity, and the Weight of the At- mofphere being alfo as the Quantity of the whole incumbent Air; 'tis evident the Weight of the incumbent Air is every where as theQuantity of Air contained in the fubjacent Part, which makes a Difference between each two contiguous Parts of incumbent: Air.— Now, we have a Theorem in Geometry, that where the Differences of Magnitudes are geometrically proportionable to the Magnitudes themfelves, thofe Magnitudes are in continual Arithmetical Proportion: Wherefore if, according to the Sup- pofition, the Altitude of the Air by the continual Addition of the new Parts into which ic is divided, do increafe in a continu- al Arithmetical Proportion; its Denfity will be dirninifhed, or which amounts to the fame, its Gravity ine'reafed, in continual Geometrical Proportion.

From fuch a Series it is eafy by making two or three Barome- trical Obfervations of the Rarity of the Atr at two or three different Heights; to find its Rarity ac any other Height, or the Height corresponding to any Rarity; and confequently the Altirude of the whole Atmofphere:, fuppoling the utmoit Degree of Rarity known beyond which the Air cannot go. See the Articles Barometer, Series, Progression, &c. See alio Greg.Aflron. PhyfGeom. Log. <$.Prop. 3. and Halley in Philofoph. Tranfatt. N°. 181.

It muft not be here omitted, that fome Obfervations made by Coffin and his Affociaces, feem to render this Method preca- rious— In continuing the Meridian Line of the Obfervatory at Paris, they meafured the Altitudes of feveral Mountains with great accuracy ; noting the Height of the Barometer at the Top of each ; and found, that the Rarities of the Air as you afcend from the Level of the Earth, are much greater than they ought to be, according to this Proportion.

Sufpecting, therefore, the juftnefs of the Experiments, the Royal Academy made divers others, under great Dilatations of Air, far exceeding the Rarities found on the Tops of the Moun- tains; the Refulc whereof was, that they all exactly anfwered the Proportion of the incumbent Weights. Whence it fhould fol- low, that the higher Air about the Tops of Mountains, is of a different Nature, and obferves a different Lav/ from that near the Earth.

The Reafon hereof may be owing to the great Quantity of grofs Vapours and Exhalations here, more than there; which Va- pours being lefs Elaftic and not capable of fo much Rarefaction as the pure Air above : The Rarefactions of the pure Air increafe in a greacer Ratio than the Weights diminifh.— M. Fontenellc, how- ever, from fome Experiments of M. de la Hire, accounts for the Phenomenon in a different Manner; alledging, that the ela- ftic