[35]See“Priestley on Electricity.”[36]Εὔροια.[37]An Account of theBreathbeing visible at Sea, when the Thermometer was at 61.The Breath is said to becomevisibleat Sea or Land at any Temperature of the Thermometer notexceeding60°: tho’, in Latitude 41°, and Westward of the Azores Islands, being in Sight of the Peak ofst. george, (which probably equals, if not exceeds, the Height of Teneriffe) the Observer has seen hisown Breath, andthatof the Sailors on Deck, when the Thermometer in theShadewas at 61: the Air (in January) beingthen remarkablydamp.[38]This Assertion may seem to contradict what was said in Section 44: When—“every Thing, that coud be seen at all, was seendistinct:” but it only proves that the Balloon had attained a greater Altitude during the Re-ascent, and that theshadowsweremuch lengthened, as the Evening advanced.[39]Angelica Kauffman.[40]It consists of a Frame, made by placing two strong Posts, moveable at Pleasure, each nine or ten Feet high, upright in the Ground, at the Distance of two Yards: the Posts being well secured by broad Pedestals, to keep them firm: a strong horizontal Iron Axis goes throu’ the Top of the Posts; and throu’ the Centers of four Arms or Levers at their Junction.Between the four corresponding Ends of each two Arms, (which Arms are also strengthened by Beams from one to the other), are fixed four Seats or Boxes, well secured, each holding three or four Persons, and moving on Iron Pivots, near the Top of the Boxes, so as always to preserve theverticalEquilibrium.[41]Recommended toInvalids.Why not recommend the Use of that Machine to Invalids? who woud find Refreshment in theopenAir: as its Rotation communicates a gentle Motion to the System,[42]without the least Fatigue;ratherencreasing theAnimalSpirits.[42]Particularly the Stomach and Diaphragm. See“Berdoe’s Enquiry.”[43]Talis Aër qualis Spiritus. See “Health’s Improvement,” by Dr. Moffet, Chapter 3, Of Air, Page 79.[44]Or Solid ofleastresistance, seeChambers’s Dictionary, with the Supplement.[45]It will be found, that, on comparing thetwoCalculations in Section 52, NoteNote [18],corrected; the circularDistancefrom the Eye, above the Clouds, was 102 Miles, 1 Quarter, 320 Yards: whilethatabove the Earth, seen from the same elevated Situation, (supposing the Day to have beenclearfor such a View,) was 102 Miles, 1 Quarter, 307 Yards: whose Difference is only 13 Yards: that is, theDistanceabove the Clouds to thenebulousHorizon, wasrathermore extensive, thanthatabove the Earth to theterrestrialHorizon.It may not, to some Readers, be deemed either unentertaining, or foreign to the Subject; if the Distance of theProspectfrom the Balloon at its greatestbarometricAltitude, viz. 2332 Yards, or a Mile and Half within 33 Yards, be compared with the Distance which may be seen from theSummitof the principal Mountains in different Parts of the Globe.1. Cotopàzy, a Mountain in the Province of Quito, in America, and under the equinoctial Line, issaidby Ullòa (Vol. 1. Page 422) to be 3126 Toizes or Fathom, i. e. 6252 Yards, or 3 Miles and a Half and 92 Yards in Height.2. White Mountain, called by the French Mount Blanc, near Geneva, is considered by Sir G. Shuckburgh (Phil. Trans. Vol. 67, Part 2d, Page 598, for the Year 1777) as the highest Land in Europe, Asia, or Africa (known to Europeans) and calculated by him at 5220 Yards, or 3 Miles within 60 Yards above the Level of the Mediterranean Sea.Mons. Bourit just returned from his last Tour, see his “Description de Glacieres” in 1773, makes the White Mountain but 5102 Yards in Height, (which is 30 Yards lower than Teneriffe) including the 410 Yards for the Level of the Lake of Geneva above the Mediterranean.3. The Peak of Teneriffe in the Canary Islands, which,in approaching towards it, Authors agree, may be seen at the Distance of 120 Miles at Sea, if the Weather is clear; (Modern History, Vol. 14th, Page 451;) and, inreturning from it, is discoverable at the Distance of 150 Miles, according toGlas’s History of the Canaries(Page 234);—has been estimated byDr. Heberden in Madeira (Guide to the Lakes, Page 187)at 5132 Yards, or 3 Miles within 148 Yards.Glas remarks farther, that in sailing from Teneriffe, the Peak, at the Distance of 150 Miles is very little darker than theazureSky, on Account of the great Quantity of Vapour intercepted between the Eye and the Mountain: andnotbecause it ceased to be an Object too small for the Sight; or was in Fact, below the Horizon, and only raised by Refraction of the Vapour.With Respect to the Peak of St. George, situated in the Island calledPico, one of the Azòres; the Writer of this Account asserts, from the Mouth of an able and experienced Officer in his em some Weeks off those Islands; that the latter has frequently observed the Peak, at the Distance of 120 Miles, and coud then distinguish athirdPart of its Heightdownthe Mountain. Section 126,Note [37]), see also[46]below.4. Etna is 3877 Yards above the Mediterranean: (according toBrydone’s Tour throu’ Sicily and Malta, Vol. 1.Page 211) or 2 Miles and 357 Yards.5. Blue Ridge, the highest Mountain in the Island of Jamaica, is, according to Dr. Clark, who measured it in November last, 3080 Yards, or 1 Mile and three Quarters, above the Level of the Ocean.Thedistanceto beseenis considered as terminating the Radius of a Circle, whose Center is theeyeof the Observer, oneachMountain.Height of the Mountains.distanceto beseenfrom them in Miles.Cotopàzy3 Miles and a Half and 92 Yards, (for the Process, see Section 52,Note [18]1671⁄2 and 405 Yards.White Mountain3 Miles within 60 Yards.1531⁄4 and 13 Yards.Peak of Teneriffe3 Miles within 148 Yards.152 within 72 Yards.Mount Etna2 Miles and 357 Yards.132 and 127 Yards.Blue Ridge1 Mile and 3 Quarters.1173⁄4 and 30 Yards.Balloon1 Mile and half within 33 Yards.1021⁄4 and 307As it is well known that Objects of thegreatestMagnitude appear but asblue airat even alessDistance than 100 Miles; to which add the Difficulty of Journies, and Ascent to the Summit of these astonishing Mounds of Earth; and all this for the Sake, not of a completedown prospect, subject toa perpetual Variety, but merely animperfect Side-View: thepleasureandeaseof attaining stillmorestupendous Heights atanyPlace and Time, by Means of theballoon, are strikingly in Favor of that Invention. And, notwithstanding the confessed Merit of Dr. Black’s Project with theFarciminàlisof a Calf, and Mr. Cavallo’s Soap Bubbles with inflammable Air; (see his History of Aerostation, Page 34;) if the Emperor had been alive who offered a Reward for the Invention of aNEW PLEASURE; thefirstPrize had been due to the Brothers Montgolfier, and asecondto the Brothers Roberts.[46]As therefore it may be supposed that the Peak of St. George, inrecedingfrom it, woudvanishat the Distance of 150 Miles; its Height mayeasilybe ascertained geometrically thus:See the Figure annexed.Let M be the Summit of the Mountain: and let the Line M T drawn to the Circumference of the Circle at T, be theevanescentDistance of the Mountain in the Horizon, viz. 150 Miles.Join T C, viz. a Line drawn from theTangentto the Center of the Circle, which Line will therefore represent the Semidiameter of theEarth, viz. 3958 Miles, according to Newton.Draw a Line from C to M, which will pass throu’ some Point of the Circumference as H, the Base of the Mountain.Then, in the Triangle M T C, as the Angle at T is a right Angle (Euclid’s Elements, Book 3, Proposition 18;) and the Sides M T, and T C, containing the right Angle, areknown; thethirdSide C M is readily found: (being a Corollary to the 47th Prop. 1st Book Euclid:) viz. having the two Sides of a right Angle Triangle given to find thethird. ThereforeRULE.Multiply the Sides containing the right Angle, each into itself: viz. 150 and 3958: add the Products into one Sum: from which extract thesquare Root;equalto the Length in Miles, of thethirdSide required.From thethirdSide, subtract that Part, viz. C H, which is equal to the Semidiameter T C already found: and the Remainder H M is theHeight of the Mountain.Thus:150Miles.3958Miles in the Semidiameter of1503958the Earth—————7500316641519790———356222250011874Square of the————greatest visible15665764Square of the SemidiameterDistance.add 22500of the Earth.————Extract the sq. Root,15688264(3960.84 Square Root.93958 subtract.———69) 668Rem. 2.84 Answer in Miles.621——786) 478.2471 6———79208) 6664.00continued to 2 Decimals.6336 64————792164) 32736.00ditto.31686 56————104944To find the .84 Part of a Mile; multiply1760Yards in a Mile,Decimal Parts of a Mile to be reduced.84into Yards.——704014080———1760)1478.40(0Subtract1478——282Answer: the Height of the Mountain is 2 Miles 282 Yards.[47]Rays flowing from the Sun seem to bered orangeoryellow, according to the Quantity of Vapours floating in the Atmosphere, which absorbs the most refrangible ones: and the fewer the Vapours the more does the Sun’s Light approach to a perfect and intensewhite, according to the Doctrine of Newton: which seems to receive Confirmation from the Purity of the Solar Light, when seenabove CloudsandVapours, in the Balloon: where the Sun shines not so much with agoldenas with asparklingsilverLight.[48]Sounds immediately underthe Balloon, seemed, as if originatednearthe Ear, andlouderthan they would have been heard, at the Distance of some Yardsonly, when on a Level with themselves: augmenting rather than decreasing, during theAscentof the Balloon, till it arrived to a Height indicated by the Barometer at 27 Inches. Presently afterwards, the Balloon still rising; the Soundsdied away: much sooner indeed than was expected.The like was observed indescendingfrom a State of perfect Tranquillity and Silence:Soundsfrombelow, when about the same Height,suddenly rushingon the Ear.It must be considered that bythisTime, theshadowswere much encreased; tho’ at half past II, they weremorethan double in Length to the Height of each Object.The Trees woud therefore spread ashadeacrossthe Road.Thetopsof theHouseslikewise, being Part of them in the Shade; and eitherthatchedwith Straw, or covered with Slates of aduskyHue; woud prevent theirthrowing offanystrikingColour.Possibly theEncreaseof Shadealone, might give the Face of the Countrybelow, adark-greenCast.It is certain that the Height of the Balloon must have been very great, to prevent the Sight of public andTurnpike-Roads,abovewhich itfrequentlypassed, and which had beenplainlyseenbeforetheRe-ascent.For suppose the Road but 5 Yards wide, which is less than the Truth; if it be allowed that an Object may be distinguished by asharp-sighted Person, when itsDistancefrom the Eye does notexceed5156 Times the Diameter of the Object; i. e. when the Object does not subtend alessAngle at the Eye than30 Secondsof a Circle, (Smith’s Optics, Article 97) which is thesmallestvisible Point, and equal to the 8000th Part of an Inch on theRetina;—by multiplying 5 Yards, viz. the Diameter of the public Road, into 5156 (or, in round Numbers, into 5000) Times its Distance from the Eye in the Balloon; the Product is 25000 Yards: which Product being divided by 1760, the Number of Yards in a Mile, amounts to 14 Miles, and 360 Yards.Supposing farther, that acommonEye canonlysee an Object athalfthat Distance; the Height woudthenbe 7 Miles.TheImprobability, therefore, (on Account of theWarmthof the Air at that Height, viz. 60°;) of havingsoaredto so great an Altitude, seems to point out, that theshadowsmust have contributed aprincipalShare, in preventing a Sight of the public andTurnpikeRoads.[49]Themagnitudeof an Objectdecreases, as thesquaresof its Distance from the Eyeincrease.At whatever Distance, for Example, the Eye can see any Object clearly; as at the Distance of a Foot, or a Yard, if the Object be removed totwicethat Distance; it will appear 4 Times smaller than it did before: 2 multiplied into 2, equals 4, which is the Square of 2: in the same Manner, if the Object be removed to thrice the Distance from the Eye, it will appear 9 Times as small, as at the first Distance: for 3 into 3 gives 9, the Square of 3: and so of any farther Distance.[50]See “Berkeley’s New Theory of Vision, Section 67.”[51]Dr. Smith having Recourse tointervening Objects; the Writer cannot assent to the Validity of his Argument, illustrated by a well-known Figure, to solve the Appearance of thehorizontal Moon. See“Priestley’s History of Light and Colours, Page 712.”[52]Phil. Trans. for 1785, Part 1, Page 287.[53]Cavallo’s Treatise on Air, Page 576. Vitriolic Acid Air, Alkaline Air, and other elastic Fluids, are instantlyabsorbedbyWater; (Page 673.) Inflammable Air, and fixed Air, are likewiseabsorbedbywater. (Page 434).[54]Nam fit, ut interdum tanquam demissâ Columnâ In Mare de Cœlo descendat.—Lucr. L. 6. V. 425.UnaEurus Notusque ruunt, creberque Procellis Africus. AlsoOmnia VentorumconcurrerePrælia vidi.Virgil.[55]Franklin’s Account of Whirlwinds and Waterspouts, in his Miscellaneous Tracts.Lowthorp’s Abridgement of Phil. Trans. Vol. 2.Page 103.Varenius Geogr. Gen. C. 21, Pag. 265. A clear Account of the Effects of adepressionis to be met with in“the History of Jamaica, in 3 vols.vol. 3. Page 800, onTrade and Land Winds.”[56]Mons. Maupertius has found, that the extreme Cold at Tornea, in the northern Regions beyond the Artic Circle, came directly fromabove: see“La Figure de la Terre,”Page 59.Il semble que le vent souffle—de tous Côtés à la Fois; et il lance laNeigeavec une telle Impetuosité, qu’en un Moment tous les Chemins font perdus.“It seems that the Wind blows from all Points of the Compass at once,” &c.[57]The Doctrine of smokey Chimnies distinctly treated of under the Articlesmoke, in the Encyclopædia Britannica, may receive some Improvement, from Circumstances which ascertain the sudden Descent, Elevation, and quick Depression ofColumnsor ratherTorrentsof Air, viz. by widening the Tubes, and covering their Tops.[58]It is thought morecandid, and will tomanybe moresatisfactory; to make occasional References to different Authors who have treated distinctly on a Subject, and leave the Reader to draw his own Conclusions by applying to theirexpressWords;—than, either to insert abundant Quotations; orweavetheir Thoughts into theTextureof the Work: which must encrease its Bulk, without producing any Thing either new or instructive.[59]Once, particularly, in the Month of January, at Lausanne: Farenheit’s Thermometer at 7 only: the Country covered with Snow; and a North Wind beatingviolentlyon the Lake, which continued liquid without Ice: owing, perhaps, inPart, to subterranean Heat, and Exhalations.[60]The Depression and Reverberation of the Wind near Rivers, and its Descent from Mountains,a Point to be discussed, may furnish a Hint and Reason, why Rain falls more in one Place, than in another not far distant: and why in the same Place it falls in different Quantities, at different Heights, irregularly.[61]Cavallo’s Treatise on Air, Page 446.—[62]442.—[63]441.—[64]442.[65]It islightin Consequence of itsWarmth, when compared with thecooler condensed Airabove it.[66]In the same Manner that Curls and Streams of Airdescendedinto the Bason over the risingSteam, and interrupted the Regularity ofitsElevation; in thelargerTowns, during Winter (the Weather being moderate) the Pressure of Air on all Sides, from without, produces a constant Breeze towards the Center of the Town: as may be discovered, not only by the Smoke in its Deviation from the Perpendicular, as it issues from the Chimneys; but by all who are inclined to make the Trial; for, on leaving the Town, they willmeetthe Breeze.In calm Weather, during Summer, the contrary Event happens: but more particularly inhotClimates. For the Country being hotter than the Town; aDepressionof the Atmosphere takes Place, and scatters the Smoke on all Sides round the Town.The Cities in Italy, and other hot Climates, on Account of the Buildings, anddesirableNarrowness of the Streets, formonecontiguousShelter,Arbor, or grandParasol: For which Reason, the Nobility leave the Country, and reside in the Towns during Summer: there finding a Coolness and Refreshment unknown on thescorchingPlains.AReceptionandDispersionof Air takes Place;as will presently be mentioned.The same ocular Proof and Process in the Evaporation of Steam, accounts at once, for a curious Phenomenon constantly observable on all Waters; viz.a narrowsmoothirregular Surface of considerable Length, nearly in the Direction of the Wind, yet unaffected by it: all which is probably nothing more thanrisingVolumes ofelastic invisibleSteam;resistingthetwonearestdescending Wavesofair; and preventing them from approaching theSurfaceof Water, over which the Steam is compressed; andthereproducing atemporarycalm.[67]Phil. Trans. for 1777, Page 470. Thibet in Lat. 31,coldwith Snow and Frost.SeeUllòa’s Voyage to South-America, Book 6, Chapter 7; where he describes the snowy Mountains, under the Equator.As the Weather, near the Equinoctial, is more regular, its Changes closely following those of the Moon; and also the Winds and Hurricanes more violent; the Truth of the foregoing Theory will receive the strongest Confirmation by tracing the Effects ofdepressing torrents of air, in the Island of Jamaica, extracted from the Author already mentioned.“The cool Vapourrushesfrom the Mountains towards the hot dry Air, which hovers over the Savannahs or Vallies.The Rain falls heaviest in the Mountains. Vol. 3, Page 600.TheLand-Windafter Rain, proceeds from that Quarter whence the Rain has fallenheaviest; and seems torushfrom above.In Spain and North-America, the Windrushes down. Page 601.When theLandismostheated, the Sea-Breeze blows almostallNight. Page 602.The Barometer subsides from 1 Inch to 11⁄2atthe full Moon, or justafterit.Wind blows from the Mountains all round the Island: and still a Sea-Breeze over the Mountains: to the Low-Lands, none, 604.(In Jamaica likewise the Wind blows off the Islandevery wayat once, so that no Ship can any where come in by Night, or go out but early in the Morning, before the Sea-Breeze sets in. See Abr. Phil. Tr. Vol. 3, P. 548.)Mountain Air rushes down in a continual Current to every Part of the Coast, the Stream descending incessantly throu’ the Night: while heavy cold Air descends to the Mountain Tops, 604.With aWestWind below there is anEast Scud above, 605.Mountainscloudy,low Landssunny. 606.Inallthe River-Courses of Jamaica, there is a sensible Current of Air. Rain never comes without some Wind: and the Showers almost invariably follow the very Meanders of the larger Rivers, 608.Rain always cools: the Thermometer falling, after a Shower, from 6 to 8 Degrees, 610.(And Iron rusts least in rainy Weather: [the Air being thendriest,] descending from theupperRegions. Abr. Ph. Tr. V. 3, P. 546.)”It is said also that “in Jamaica the Clouds gather, andshapeaccording to the Mountains: so thatold Seamenwill tell you eachIslandtowards Evening, by theShapeof the Cloudoverit.”The Sea-Breeze, being counterpoised byDescentof theetherial Air, produces acalm.The same Author likewise says, that “the Clouds begin to gather about 2 or 3 o’Clock in the Afternoonat the Mountains, and do notembodyfirst in the Air, and after settle there, butsettlefirst andembodythere: the rest of the Sky being clear tillSun-set. So that they do not passnearthe Earth in aBody, and onlystopwhere they meet with Parts of the Earth elevatedabove the rest; butprecipitatefrom a very great Height, and in Particles of anexceeding rarified Nature;so as not to obscure the Air or Sky at all: that great Variety of beautiful Colours in the Canopy of Heaven being raised to a much greater Distance [he means Height] in Jamaica than it is here.”Abr. Ph. Tr. V. 3, P. 557.(Prognostics of Weather, at certain Periods of the Moon, are mentioned by Captain Langford.Lowthorp’s Abr. Phil. Trans. Vol. 2, Page 105.)[68]The Depression of aTorrentof Air in the Form of an hyperbolic Solid,contractingas itdescendsto the Earth, in Proportion as itsDensityencreases; may furnish a Hint towards the Solution of a Difficulty how to account for the Augmentation of vesiculous Vapours into large solid Drops, frequent duringSummer-Showers.[69]Mons. Saussure’s valuable“Essais sur L’Hygrometrie,”throw new Light on the Doctrine of Rarefaction and Condensation not unfavourable to the Hypothesis here advanced. Page 260.[70]Ice, when exposed to marine acid Air, is dissolved by it, as fast as if it touched a red hot Iron. SeeCavallo’s Treatise on Air, Page 727. AlsoPriestley’s Experiments and Observations, Vol. 1, Page 148.[71]“Thewaterremainstransparentor colourless, tho’ saturated with marine acid Air, and by a very gentle Degree of Heat, the Gass may be again expelled from it, as it is expelled from Spirit of Salt.”This Observation is applicable to the Transparency of Vapours, in the Air, tho’ mixed with the marine Acid exhaled from the Sea: for when the acid or Sea Air is mixed with Alkaline or Land Air, they instantlycombine; lose their Elasticity, and form awhitevisible Substance orCloud. Cavallo, Page 728. Priestley’s Exp. and Obs. Vol. 2, Page 293.[72]On the Descent of Air inThunder-Gusts, see“Chalmer’s Account of the Weather in South-Carolina, Vol. 1, Page 1, to 39.”[73]“Historia Ventorum, Pag. 54, Art. 34.”[74]Book V. Chapter 2d.[75]Vol. 1. Page 184.[76]Page 195.[77]History of the Canary Isles, Page 252.[78]As the superior Clouds, during the Balloon Excursion, did not much exceed the Height of 1000 Yards; supposing then the Clouds at an equal Height above the Sea, near Teneriffe; one ought to conclude, either, that the Peak was not so high as Glas represents it; or, that the Level of the Clouds was less than half the Height of the Mountain.[79]See“Royal Astronomer, by R. Heath, Page 321, onTrade Winds and Monsoons.”[80]One Pound of Nitre only, producing by mere Heat, 6 cubic Feet of Air.“Cavallo, Page 332, and 811, Experiments on Gun-Powder.”[81]“See Recherches surles Modifications de l’Atmosphere. No. 715.”Ph. Trans. Part 2, for 1777.Col. Roy’s Experiments, Sect. 2d, Page 689, 744, 753, 764.[82]The different Phenomena of theAurora Borealismay be owing to the Ascent and Motion of the Apogay, in themiddleRegion, over the Stratum of Eknèfiai orGround-Winds.The Effects ofTides in the Air yet to bementioned, must not, however, be wholly excluded.The Aurora Borealis is seen inSpring,Autumn, andWinter: sometimesculminating, sometimes moving inStreamsandWavesin thesuperiorRegions of the Atmosphere: when culminating; as if rising out of Clouds in the North.This Appearance may be owing to warm moist Air perpetually generating between the Tropics, and rolling over the colddryStratum of Eknèfiai Winds, which cut off its Communication with the Earth: till accumulating over the Poles, it enlightens the Atmosphere, converting a sixMonth’s Nightinto Day; and returns to the Surface silently: or in Lightning, whenever it is communicated to the Earth, throu’Vapour descendingby its own specific Gravity; or along withdepressingTorrents of Air, known to be accompanied by frequentflashes.When the Vapour iscondensedin its Descent, by passing throu’ a Stratum of the Eknèfiai Winds; it becomesoverchargedwith the electric Matter,surroundingandadheringto it; and deposits the Overplus in Lightning, on its Approach tootherClouds, or to theEarth.It is visible in the Form of a Vapour, when the Vapour to which it adheres, becomes overcharged with electric Matter, by Descent into acoolEknèfiai Stratum below: there forming a luminous and transparent Atmosphere: the Particles of Light and Vapour being repelled to great Distances from each other at sorarea Height.It culminates above the Vapour, because less heavy than the circumambient Air: and may be subject to the Attraction of other Planets.The Aurora Borealis is also seen to issue in Streams and Waves of Light, with inexpressible Velocity, on its Return to the South, in a lower Stratum, as itpasses throu’Interstices, between the Vesicles of warm Vapour, raised and dispersed by the turbulent Apogay Winds, in the middle Region.During Summer, the middle Region becomes blended with the lower, throu’ Defect of Cold: and the electric Matter is supposed to be communicated to the Earth, silently, and continually; but by Lightning, when a lower and colder Atmosphere condenses and overcharges the Vapour, and cuts off the Communication.It cannot be seen but in escaping from Vesicle to Vesicle: nor, during Summer, after Sunset, on Account of the Twilight.[83]Air is not unfit for Respiration, by having lost itsvitalPrinciple, but because it has imbibedFloguiston, which cannoteasilybe separated from it, but by Agitation in Water.Cavallo, on Air,Pages 479, 670.[84]For if Moisture be one Cause, which keeps the Particles of Air at greater Distances from each other; this Cause decreases atgreatAltitudes.If also theElasticitydecreases in Proportion, not only to the Height, but the Driness; its Particles must, on both Accounts, approach each other, at great Altitudes: tho’, from the Altitude only; they woud separate according to the Rule, viz. that the Rarity of the Air is proportionable to the Relaxation of the Force compressing it.So that at the Height of 8 or 10 Miles, a Quantity of Air taken from the Surface of the Earth, woud occupy 6 Times its former Space: supposing the Air both below and above to be of thesame Kind, as well as of thesame meanTemperature of 55, on the Thermometer. See“Martin’s Philosophical Grammar, Page 178.”
[35]See“Priestley on Electricity.”
[36]Εὔροια.
[37]An Account of theBreathbeing visible at Sea, when the Thermometer was at 61.The Breath is said to becomevisibleat Sea or Land at any Temperature of the Thermometer notexceeding60°: tho’, in Latitude 41°, and Westward of the Azores Islands, being in Sight of the Peak ofst. george, (which probably equals, if not exceeds, the Height of Teneriffe) the Observer has seen hisown Breath, andthatof the Sailors on Deck, when the Thermometer in theShadewas at 61: the Air (in January) beingthen remarkablydamp.
An Account of theBreathbeing visible at Sea, when the Thermometer was at 61.
[38]This Assertion may seem to contradict what was said in Section 44: When—“every Thing, that coud be seen at all, was seendistinct:” but it only proves that the Balloon had attained a greater Altitude during the Re-ascent, and that theshadowsweremuch lengthened, as the Evening advanced.
[39]Angelica Kauffman.
[40]It consists of a Frame, made by placing two strong Posts, moveable at Pleasure, each nine or ten Feet high, upright in the Ground, at the Distance of two Yards: the Posts being well secured by broad Pedestals, to keep them firm: a strong horizontal Iron Axis goes throu’ the Top of the Posts; and throu’ the Centers of four Arms or Levers at their Junction.Between the four corresponding Ends of each two Arms, (which Arms are also strengthened by Beams from one to the other), are fixed four Seats or Boxes, well secured, each holding three or four Persons, and moving on Iron Pivots, near the Top of the Boxes, so as always to preserve theverticalEquilibrium.
Between the four corresponding Ends of each two Arms, (which Arms are also strengthened by Beams from one to the other), are fixed four Seats or Boxes, well secured, each holding three or four Persons, and moving on Iron Pivots, near the Top of the Boxes, so as always to preserve theverticalEquilibrium.
[41]Recommended toInvalids.Why not recommend the Use of that Machine to Invalids? who woud find Refreshment in theopenAir: as its Rotation communicates a gentle Motion to the System,[42]without the least Fatigue;ratherencreasing theAnimalSpirits.
Recommended toInvalids.
[42]Particularly the Stomach and Diaphragm. See“Berdoe’s Enquiry.”
[43]Talis Aër qualis Spiritus. See “Health’s Improvement,” by Dr. Moffet, Chapter 3, Of Air, Page 79.
[44]Or Solid ofleastresistance, seeChambers’s Dictionary, with the Supplement.
[45]It will be found, that, on comparing thetwoCalculations in Section 52, NoteNote [18],corrected; the circularDistancefrom the Eye, above the Clouds, was 102 Miles, 1 Quarter, 320 Yards: whilethatabove the Earth, seen from the same elevated Situation, (supposing the Day to have beenclearfor such a View,) was 102 Miles, 1 Quarter, 307 Yards: whose Difference is only 13 Yards: that is, theDistanceabove the Clouds to thenebulousHorizon, wasrathermore extensive, thanthatabove the Earth to theterrestrialHorizon.It may not, to some Readers, be deemed either unentertaining, or foreign to the Subject; if the Distance of theProspectfrom the Balloon at its greatestbarometricAltitude, viz. 2332 Yards, or a Mile and Half within 33 Yards, be compared with the Distance which may be seen from theSummitof the principal Mountains in different Parts of the Globe.1. Cotopàzy, a Mountain in the Province of Quito, in America, and under the equinoctial Line, issaidby Ullòa (Vol. 1. Page 422) to be 3126 Toizes or Fathom, i. e. 6252 Yards, or 3 Miles and a Half and 92 Yards in Height.2. White Mountain, called by the French Mount Blanc, near Geneva, is considered by Sir G. Shuckburgh (Phil. Trans. Vol. 67, Part 2d, Page 598, for the Year 1777) as the highest Land in Europe, Asia, or Africa (known to Europeans) and calculated by him at 5220 Yards, or 3 Miles within 60 Yards above the Level of the Mediterranean Sea.Mons. Bourit just returned from his last Tour, see his “Description de Glacieres” in 1773, makes the White Mountain but 5102 Yards in Height, (which is 30 Yards lower than Teneriffe) including the 410 Yards for the Level of the Lake of Geneva above the Mediterranean.3. The Peak of Teneriffe in the Canary Islands, which,in approaching towards it, Authors agree, may be seen at the Distance of 120 Miles at Sea, if the Weather is clear; (Modern History, Vol. 14th, Page 451;) and, inreturning from it, is discoverable at the Distance of 150 Miles, according toGlas’s History of the Canaries(Page 234);—has been estimated byDr. Heberden in Madeira (Guide to the Lakes, Page 187)at 5132 Yards, or 3 Miles within 148 Yards.Glas remarks farther, that in sailing from Teneriffe, the Peak, at the Distance of 150 Miles is very little darker than theazureSky, on Account of the great Quantity of Vapour intercepted between the Eye and the Mountain: andnotbecause it ceased to be an Object too small for the Sight; or was in Fact, below the Horizon, and only raised by Refraction of the Vapour.With Respect to the Peak of St. George, situated in the Island calledPico, one of the Azòres; the Writer of this Account asserts, from the Mouth of an able and experienced Officer in his em some Weeks off those Islands; that the latter has frequently observed the Peak, at the Distance of 120 Miles, and coud then distinguish athirdPart of its Heightdownthe Mountain. Section 126,Note [37]), see also[46]below.4. Etna is 3877 Yards above the Mediterranean: (according toBrydone’s Tour throu’ Sicily and Malta, Vol. 1.Page 211) or 2 Miles and 357 Yards.5. Blue Ridge, the highest Mountain in the Island of Jamaica, is, according to Dr. Clark, who measured it in November last, 3080 Yards, or 1 Mile and three Quarters, above the Level of the Ocean.Thedistanceto beseenis considered as terminating the Radius of a Circle, whose Center is theeyeof the Observer, oneachMountain.Height of the Mountains.distanceto beseenfrom them in Miles.Cotopàzy3 Miles and a Half and 92 Yards, (for the Process, see Section 52,Note [18]1671⁄2 and 405 Yards.White Mountain3 Miles within 60 Yards.1531⁄4 and 13 Yards.Peak of Teneriffe3 Miles within 148 Yards.152 within 72 Yards.Mount Etna2 Miles and 357 Yards.132 and 127 Yards.Blue Ridge1 Mile and 3 Quarters.1173⁄4 and 30 Yards.Balloon1 Mile and half within 33 Yards.1021⁄4 and 307As it is well known that Objects of thegreatestMagnitude appear but asblue airat even alessDistance than 100 Miles; to which add the Difficulty of Journies, and Ascent to the Summit of these astonishing Mounds of Earth; and all this for the Sake, not of a completedown prospect, subject toa perpetual Variety, but merely animperfect Side-View: thepleasureandeaseof attaining stillmorestupendous Heights atanyPlace and Time, by Means of theballoon, are strikingly in Favor of that Invention. And, notwithstanding the confessed Merit of Dr. Black’s Project with theFarciminàlisof a Calf, and Mr. Cavallo’s Soap Bubbles with inflammable Air; (see his History of Aerostation, Page 34;) if the Emperor had been alive who offered a Reward for the Invention of aNEW PLEASURE; thefirstPrize had been due to the Brothers Montgolfier, and asecondto the Brothers Roberts.
It may not, to some Readers, be deemed either unentertaining, or foreign to the Subject; if the Distance of theProspectfrom the Balloon at its greatestbarometricAltitude, viz. 2332 Yards, or a Mile and Half within 33 Yards, be compared with the Distance which may be seen from theSummitof the principal Mountains in different Parts of the Globe.
1. Cotopàzy, a Mountain in the Province of Quito, in America, and under the equinoctial Line, issaidby Ullòa (Vol. 1. Page 422) to be 3126 Toizes or Fathom, i. e. 6252 Yards, or 3 Miles and a Half and 92 Yards in Height.
2. White Mountain, called by the French Mount Blanc, near Geneva, is considered by Sir G. Shuckburgh (Phil. Trans. Vol. 67, Part 2d, Page 598, for the Year 1777) as the highest Land in Europe, Asia, or Africa (known to Europeans) and calculated by him at 5220 Yards, or 3 Miles within 60 Yards above the Level of the Mediterranean Sea.
Mons. Bourit just returned from his last Tour, see his “Description de Glacieres” in 1773, makes the White Mountain but 5102 Yards in Height, (which is 30 Yards lower than Teneriffe) including the 410 Yards for the Level of the Lake of Geneva above the Mediterranean.
3. The Peak of Teneriffe in the Canary Islands, which,in approaching towards it, Authors agree, may be seen at the Distance of 120 Miles at Sea, if the Weather is clear; (Modern History, Vol. 14th, Page 451;) and, inreturning from it, is discoverable at the Distance of 150 Miles, according toGlas’s History of the Canaries(Page 234);—has been estimated byDr. Heberden in Madeira (Guide to the Lakes, Page 187)at 5132 Yards, or 3 Miles within 148 Yards.
Glas remarks farther, that in sailing from Teneriffe, the Peak, at the Distance of 150 Miles is very little darker than theazureSky, on Account of the great Quantity of Vapour intercepted between the Eye and the Mountain: andnotbecause it ceased to be an Object too small for the Sight; or was in Fact, below the Horizon, and only raised by Refraction of the Vapour.
With Respect to the Peak of St. George, situated in the Island calledPico, one of the Azòres; the Writer of this Account asserts, from the Mouth of an able and experienced Officer in his em some Weeks off those Islands; that the latter has frequently observed the Peak, at the Distance of 120 Miles, and coud then distinguish athirdPart of its Heightdownthe Mountain. Section 126,Note [37]), see also[46]below.
4. Etna is 3877 Yards above the Mediterranean: (according toBrydone’s Tour throu’ Sicily and Malta, Vol. 1.Page 211) or 2 Miles and 357 Yards.
5. Blue Ridge, the highest Mountain in the Island of Jamaica, is, according to Dr. Clark, who measured it in November last, 3080 Yards, or 1 Mile and three Quarters, above the Level of the Ocean.
Thedistanceto beseenis considered as terminating the Radius of a Circle, whose Center is theeyeof the Observer, oneachMountain.
As it is well known that Objects of thegreatestMagnitude appear but asblue airat even alessDistance than 100 Miles; to which add the Difficulty of Journies, and Ascent to the Summit of these astonishing Mounds of Earth; and all this for the Sake, not of a completedown prospect, subject toa perpetual Variety, but merely animperfect Side-View: thepleasureandeaseof attaining stillmorestupendous Heights atanyPlace and Time, by Means of theballoon, are strikingly in Favor of that Invention. And, notwithstanding the confessed Merit of Dr. Black’s Project with theFarciminàlisof a Calf, and Mr. Cavallo’s Soap Bubbles with inflammable Air; (see his History of Aerostation, Page 34;) if the Emperor had been alive who offered a Reward for the Invention of aNEW PLEASURE; thefirstPrize had been due to the Brothers Montgolfier, and asecondto the Brothers Roberts.
[46]As therefore it may be supposed that the Peak of St. George, inrecedingfrom it, woudvanishat the Distance of 150 Miles; its Height mayeasilybe ascertained geometrically thus:See the Figure annexed.Let M be the Summit of the Mountain: and let the Line M T drawn to the Circumference of the Circle at T, be theevanescentDistance of the Mountain in the Horizon, viz. 150 Miles.Join T C, viz. a Line drawn from theTangentto the Center of the Circle, which Line will therefore represent the Semidiameter of theEarth, viz. 3958 Miles, according to Newton.Draw a Line from C to M, which will pass throu’ some Point of the Circumference as H, the Base of the Mountain.Then, in the Triangle M T C, as the Angle at T is a right Angle (Euclid’s Elements, Book 3, Proposition 18;) and the Sides M T, and T C, containing the right Angle, areknown; thethirdSide C M is readily found: (being a Corollary to the 47th Prop. 1st Book Euclid:) viz. having the two Sides of a right Angle Triangle given to find thethird. ThereforeRULE.Multiply the Sides containing the right Angle, each into itself: viz. 150 and 3958: add the Products into one Sum: from which extract thesquare Root;equalto the Length in Miles, of thethirdSide required.From thethirdSide, subtract that Part, viz. C H, which is equal to the Semidiameter T C already found: and the Remainder H M is theHeight of the Mountain.Thus:150Miles.3958Miles in the Semidiameter of1503958the Earth—————7500316641519790———356222250011874Square of the————greatest visible15665764Square of the SemidiameterDistance.add 22500of the Earth.————Extract the sq. Root,15688264(3960.84 Square Root.93958 subtract.———69) 668Rem. 2.84 Answer in Miles.621——786) 478.2471 6———79208) 6664.00continued to 2 Decimals.6336 64————792164) 32736.00ditto.31686 56————104944To find the .84 Part of a Mile; multiply1760Yards in a Mile,Decimal Parts of a Mile to be reduced.84into Yards.——704014080———1760)1478.40(0Subtract1478——282Answer: the Height of the Mountain is 2 Miles 282 Yards.
See the Figure annexed.
Let M be the Summit of the Mountain: and let the Line M T drawn to the Circumference of the Circle at T, be theevanescentDistance of the Mountain in the Horizon, viz. 150 Miles.
Join T C, viz. a Line drawn from theTangentto the Center of the Circle, which Line will therefore represent the Semidiameter of theEarth, viz. 3958 Miles, according to Newton.
Draw a Line from C to M, which will pass throu’ some Point of the Circumference as H, the Base of the Mountain.
Then, in the Triangle M T C, as the Angle at T is a right Angle (Euclid’s Elements, Book 3, Proposition 18;) and the Sides M T, and T C, containing the right Angle, areknown; thethirdSide C M is readily found: (being a Corollary to the 47th Prop. 1st Book Euclid:) viz. having the two Sides of a right Angle Triangle given to find thethird. Therefore
RULE.
Multiply the Sides containing the right Angle, each into itself: viz. 150 and 3958: add the Products into one Sum: from which extract thesquare Root;equalto the Length in Miles, of thethirdSide required.
From thethirdSide, subtract that Part, viz. C H, which is equal to the Semidiameter T C already found: and the Remainder H M is theHeight of the Mountain.
150
3958
150
3958
——
———
7500
31664
15
19790
———
35622
22500
11874
————
15665764
add 22500
————
15688264
9
—
69) 668
621
——
786) 478.2
471 6
———
79208) 6664.00
6336 64
————
792164) 32736.00
31686 56
————
104944
To find the .84 Part of a Mile; multiply
1760
.84
——
7040
14080
———
1760)1478.40
Subtract
1478
——
282
Answer: the Height of the Mountain is 2 Miles 282 Yards.
[47]Rays flowing from the Sun seem to bered orangeoryellow, according to the Quantity of Vapours floating in the Atmosphere, which absorbs the most refrangible ones: and the fewer the Vapours the more does the Sun’s Light approach to a perfect and intensewhite, according to the Doctrine of Newton: which seems to receive Confirmation from the Purity of the Solar Light, when seenabove CloudsandVapours, in the Balloon: where the Sun shines not so much with agoldenas with asparklingsilverLight.
[48]Sounds immediately underthe Balloon, seemed, as if originatednearthe Ear, andlouderthan they would have been heard, at the Distance of some Yardsonly, when on a Level with themselves: augmenting rather than decreasing, during theAscentof the Balloon, till it arrived to a Height indicated by the Barometer at 27 Inches. Presently afterwards, the Balloon still rising; the Soundsdied away: much sooner indeed than was expected.The like was observed indescendingfrom a State of perfect Tranquillity and Silence:Soundsfrombelow, when about the same Height,suddenly rushingon the Ear.It must be considered that bythisTime, theshadowswere much encreased; tho’ at half past II, they weremorethan double in Length to the Height of each Object.The Trees woud therefore spread ashadeacrossthe Road.Thetopsof theHouseslikewise, being Part of them in the Shade; and eitherthatchedwith Straw, or covered with Slates of aduskyHue; woud prevent theirthrowing offanystrikingColour.Possibly theEncreaseof Shadealone, might give the Face of the Countrybelow, adark-greenCast.It is certain that the Height of the Balloon must have been very great, to prevent the Sight of public andTurnpike-Roads,abovewhich itfrequentlypassed, and which had beenplainlyseenbeforetheRe-ascent.For suppose the Road but 5 Yards wide, which is less than the Truth; if it be allowed that an Object may be distinguished by asharp-sighted Person, when itsDistancefrom the Eye does notexceed5156 Times the Diameter of the Object; i. e. when the Object does not subtend alessAngle at the Eye than30 Secondsof a Circle, (Smith’s Optics, Article 97) which is thesmallestvisible Point, and equal to the 8000th Part of an Inch on theRetina;—by multiplying 5 Yards, viz. the Diameter of the public Road, into 5156 (or, in round Numbers, into 5000) Times its Distance from the Eye in the Balloon; the Product is 25000 Yards: which Product being divided by 1760, the Number of Yards in a Mile, amounts to 14 Miles, and 360 Yards.Supposing farther, that acommonEye canonlysee an Object athalfthat Distance; the Height woudthenbe 7 Miles.TheImprobability, therefore, (on Account of theWarmthof the Air at that Height, viz. 60°;) of havingsoaredto so great an Altitude, seems to point out, that theshadowsmust have contributed aprincipalShare, in preventing a Sight of the public andTurnpikeRoads.
The like was observed indescendingfrom a State of perfect Tranquillity and Silence:Soundsfrombelow, when about the same Height,suddenly rushingon the Ear.
It must be considered that bythisTime, theshadowswere much encreased; tho’ at half past II, they weremorethan double in Length to the Height of each Object.
The Trees woud therefore spread ashadeacrossthe Road.
Thetopsof theHouseslikewise, being Part of them in the Shade; and eitherthatchedwith Straw, or covered with Slates of aduskyHue; woud prevent theirthrowing offanystrikingColour.
Possibly theEncreaseof Shadealone, might give the Face of the Countrybelow, adark-greenCast.
It is certain that the Height of the Balloon must have been very great, to prevent the Sight of public andTurnpike-Roads,abovewhich itfrequentlypassed, and which had beenplainlyseenbeforetheRe-ascent.
For suppose the Road but 5 Yards wide, which is less than the Truth; if it be allowed that an Object may be distinguished by asharp-sighted Person, when itsDistancefrom the Eye does notexceed5156 Times the Diameter of the Object; i. e. when the Object does not subtend alessAngle at the Eye than30 Secondsof a Circle, (Smith’s Optics, Article 97) which is thesmallestvisible Point, and equal to the 8000th Part of an Inch on theRetina;—by multiplying 5 Yards, viz. the Diameter of the public Road, into 5156 (or, in round Numbers, into 5000) Times its Distance from the Eye in the Balloon; the Product is 25000 Yards: which Product being divided by 1760, the Number of Yards in a Mile, amounts to 14 Miles, and 360 Yards.
Supposing farther, that acommonEye canonlysee an Object athalfthat Distance; the Height woudthenbe 7 Miles.
TheImprobability, therefore, (on Account of theWarmthof the Air at that Height, viz. 60°;) of havingsoaredto so great an Altitude, seems to point out, that theshadowsmust have contributed aprincipalShare, in preventing a Sight of the public andTurnpikeRoads.
[49]Themagnitudeof an Objectdecreases, as thesquaresof its Distance from the Eyeincrease.At whatever Distance, for Example, the Eye can see any Object clearly; as at the Distance of a Foot, or a Yard, if the Object be removed totwicethat Distance; it will appear 4 Times smaller than it did before: 2 multiplied into 2, equals 4, which is the Square of 2: in the same Manner, if the Object be removed to thrice the Distance from the Eye, it will appear 9 Times as small, as at the first Distance: for 3 into 3 gives 9, the Square of 3: and so of any farther Distance.
At whatever Distance, for Example, the Eye can see any Object clearly; as at the Distance of a Foot, or a Yard, if the Object be removed totwicethat Distance; it will appear 4 Times smaller than it did before: 2 multiplied into 2, equals 4, which is the Square of 2: in the same Manner, if the Object be removed to thrice the Distance from the Eye, it will appear 9 Times as small, as at the first Distance: for 3 into 3 gives 9, the Square of 3: and so of any farther Distance.
[50]See “Berkeley’s New Theory of Vision, Section 67.”
[51]Dr. Smith having Recourse tointervening Objects; the Writer cannot assent to the Validity of his Argument, illustrated by a well-known Figure, to solve the Appearance of thehorizontal Moon. See“Priestley’s History of Light and Colours, Page 712.”
[52]Phil. Trans. for 1785, Part 1, Page 287.
[53]Cavallo’s Treatise on Air, Page 576. Vitriolic Acid Air, Alkaline Air, and other elastic Fluids, are instantlyabsorbedbyWater; (Page 673.) Inflammable Air, and fixed Air, are likewiseabsorbedbywater. (Page 434).
[54]Nam fit, ut interdum tanquam demissâ Columnâ In Mare de Cœlo descendat.—Lucr. L. 6. V. 425.UnaEurus Notusque ruunt, creberque Procellis Africus. AlsoOmnia VentorumconcurrerePrælia vidi.Virgil.
[55]Franklin’s Account of Whirlwinds and Waterspouts, in his Miscellaneous Tracts.Lowthorp’s Abridgement of Phil. Trans. Vol. 2.Page 103.Varenius Geogr. Gen. C. 21, Pag. 265. A clear Account of the Effects of adepressionis to be met with in“the History of Jamaica, in 3 vols.vol. 3. Page 800, onTrade and Land Winds.”
[56]Mons. Maupertius has found, that the extreme Cold at Tornea, in the northern Regions beyond the Artic Circle, came directly fromabove: see“La Figure de la Terre,”Page 59.Il semble que le vent souffle—de tous Côtés à la Fois; et il lance laNeigeavec une telle Impetuosité, qu’en un Moment tous les Chemins font perdus.“It seems that the Wind blows from all Points of the Compass at once,” &c.
[57]The Doctrine of smokey Chimnies distinctly treated of under the Articlesmoke, in the Encyclopædia Britannica, may receive some Improvement, from Circumstances which ascertain the sudden Descent, Elevation, and quick Depression ofColumnsor ratherTorrentsof Air, viz. by widening the Tubes, and covering their Tops.
[58]It is thought morecandid, and will tomanybe moresatisfactory; to make occasional References to different Authors who have treated distinctly on a Subject, and leave the Reader to draw his own Conclusions by applying to theirexpressWords;—than, either to insert abundant Quotations; orweavetheir Thoughts into theTextureof the Work: which must encrease its Bulk, without producing any Thing either new or instructive.
[59]Once, particularly, in the Month of January, at Lausanne: Farenheit’s Thermometer at 7 only: the Country covered with Snow; and a North Wind beatingviolentlyon the Lake, which continued liquid without Ice: owing, perhaps, inPart, to subterranean Heat, and Exhalations.
[60]The Depression and Reverberation of the Wind near Rivers, and its Descent from Mountains,a Point to be discussed, may furnish a Hint and Reason, why Rain falls more in one Place, than in another not far distant: and why in the same Place it falls in different Quantities, at different Heights, irregularly.
[61]Cavallo’s Treatise on Air, Page 446.—
[62]442.—
[63]441.—
[64]442.
[65]It islightin Consequence of itsWarmth, when compared with thecooler condensed Airabove it.
[66]In the same Manner that Curls and Streams of Airdescendedinto the Bason over the risingSteam, and interrupted the Regularity ofitsElevation; in thelargerTowns, during Winter (the Weather being moderate) the Pressure of Air on all Sides, from without, produces a constant Breeze towards the Center of the Town: as may be discovered, not only by the Smoke in its Deviation from the Perpendicular, as it issues from the Chimneys; but by all who are inclined to make the Trial; for, on leaving the Town, they willmeetthe Breeze.In calm Weather, during Summer, the contrary Event happens: but more particularly inhotClimates. For the Country being hotter than the Town; aDepressionof the Atmosphere takes Place, and scatters the Smoke on all Sides round the Town.The Cities in Italy, and other hot Climates, on Account of the Buildings, anddesirableNarrowness of the Streets, formonecontiguousShelter,Arbor, or grandParasol: For which Reason, the Nobility leave the Country, and reside in the Towns during Summer: there finding a Coolness and Refreshment unknown on thescorchingPlains.AReceptionandDispersionof Air takes Place;as will presently be mentioned.The same ocular Proof and Process in the Evaporation of Steam, accounts at once, for a curious Phenomenon constantly observable on all Waters; viz.a narrowsmoothirregular Surface of considerable Length, nearly in the Direction of the Wind, yet unaffected by it: all which is probably nothing more thanrisingVolumes ofelastic invisibleSteam;resistingthetwonearestdescending Wavesofair; and preventing them from approaching theSurfaceof Water, over which the Steam is compressed; andthereproducing atemporarycalm.
In calm Weather, during Summer, the contrary Event happens: but more particularly inhotClimates. For the Country being hotter than the Town; aDepressionof the Atmosphere takes Place, and scatters the Smoke on all Sides round the Town.
The Cities in Italy, and other hot Climates, on Account of the Buildings, anddesirableNarrowness of the Streets, formonecontiguousShelter,Arbor, or grandParasol: For which Reason, the Nobility leave the Country, and reside in the Towns during Summer: there finding a Coolness and Refreshment unknown on thescorchingPlains.
AReceptionandDispersionof Air takes Place;as will presently be mentioned.
The same ocular Proof and Process in the Evaporation of Steam, accounts at once, for a curious Phenomenon constantly observable on all Waters; viz.a narrowsmoothirregular Surface of considerable Length, nearly in the Direction of the Wind, yet unaffected by it: all which is probably nothing more thanrisingVolumes ofelastic invisibleSteam;resistingthetwonearestdescending Wavesofair; and preventing them from approaching theSurfaceof Water, over which the Steam is compressed; andthereproducing atemporarycalm.
[67]Phil. Trans. for 1777, Page 470. Thibet in Lat. 31,coldwith Snow and Frost.SeeUllòa’s Voyage to South-America, Book 6, Chapter 7; where he describes the snowy Mountains, under the Equator.As the Weather, near the Equinoctial, is more regular, its Changes closely following those of the Moon; and also the Winds and Hurricanes more violent; the Truth of the foregoing Theory will receive the strongest Confirmation by tracing the Effects ofdepressing torrents of air, in the Island of Jamaica, extracted from the Author already mentioned.“The cool Vapourrushesfrom the Mountains towards the hot dry Air, which hovers over the Savannahs or Vallies.The Rain falls heaviest in the Mountains. Vol. 3, Page 600.TheLand-Windafter Rain, proceeds from that Quarter whence the Rain has fallenheaviest; and seems torushfrom above.In Spain and North-America, the Windrushes down. Page 601.When theLandismostheated, the Sea-Breeze blows almostallNight. Page 602.The Barometer subsides from 1 Inch to 11⁄2atthe full Moon, or justafterit.Wind blows from the Mountains all round the Island: and still a Sea-Breeze over the Mountains: to the Low-Lands, none, 604.(In Jamaica likewise the Wind blows off the Islandevery wayat once, so that no Ship can any where come in by Night, or go out but early in the Morning, before the Sea-Breeze sets in. See Abr. Phil. Tr. Vol. 3, P. 548.)Mountain Air rushes down in a continual Current to every Part of the Coast, the Stream descending incessantly throu’ the Night: while heavy cold Air descends to the Mountain Tops, 604.With aWestWind below there is anEast Scud above, 605.Mountainscloudy,low Landssunny. 606.Inallthe River-Courses of Jamaica, there is a sensible Current of Air. Rain never comes without some Wind: and the Showers almost invariably follow the very Meanders of the larger Rivers, 608.Rain always cools: the Thermometer falling, after a Shower, from 6 to 8 Degrees, 610.(And Iron rusts least in rainy Weather: [the Air being thendriest,] descending from theupperRegions. Abr. Ph. Tr. V. 3, P. 546.)”It is said also that “in Jamaica the Clouds gather, andshapeaccording to the Mountains: so thatold Seamenwill tell you eachIslandtowards Evening, by theShapeof the Cloudoverit.”The Sea-Breeze, being counterpoised byDescentof theetherial Air, produces acalm.The same Author likewise says, that “the Clouds begin to gather about 2 or 3 o’Clock in the Afternoonat the Mountains, and do notembodyfirst in the Air, and after settle there, butsettlefirst andembodythere: the rest of the Sky being clear tillSun-set. So that they do not passnearthe Earth in aBody, and onlystopwhere they meet with Parts of the Earth elevatedabove the rest; butprecipitatefrom a very great Height, and in Particles of anexceeding rarified Nature;so as not to obscure the Air or Sky at all: that great Variety of beautiful Colours in the Canopy of Heaven being raised to a much greater Distance [he means Height] in Jamaica than it is here.”Abr. Ph. Tr. V. 3, P. 557.(Prognostics of Weather, at certain Periods of the Moon, are mentioned by Captain Langford.Lowthorp’s Abr. Phil. Trans. Vol. 2, Page 105.)
SeeUllòa’s Voyage to South-America, Book 6, Chapter 7; where he describes the snowy Mountains, under the Equator.
As the Weather, near the Equinoctial, is more regular, its Changes closely following those of the Moon; and also the Winds and Hurricanes more violent; the Truth of the foregoing Theory will receive the strongest Confirmation by tracing the Effects ofdepressing torrents of air, in the Island of Jamaica, extracted from the Author already mentioned.
“The cool Vapourrushesfrom the Mountains towards the hot dry Air, which hovers over the Savannahs or Vallies.
The Rain falls heaviest in the Mountains. Vol. 3, Page 600.
TheLand-Windafter Rain, proceeds from that Quarter whence the Rain has fallenheaviest; and seems torushfrom above.
In Spain and North-America, the Windrushes down. Page 601.
When theLandismostheated, the Sea-Breeze blows almostallNight. Page 602.
The Barometer subsides from 1 Inch to 11⁄2atthe full Moon, or justafterit.
Wind blows from the Mountains all round the Island: and still a Sea-Breeze over the Mountains: to the Low-Lands, none, 604.
(In Jamaica likewise the Wind blows off the Islandevery wayat once, so that no Ship can any where come in by Night, or go out but early in the Morning, before the Sea-Breeze sets in. See Abr. Phil. Tr. Vol. 3, P. 548.)
Mountain Air rushes down in a continual Current to every Part of the Coast, the Stream descending incessantly throu’ the Night: while heavy cold Air descends to the Mountain Tops, 604.
With aWestWind below there is anEast Scud above, 605.
Mountainscloudy,low Landssunny. 606.
Inallthe River-Courses of Jamaica, there is a sensible Current of Air. Rain never comes without some Wind: and the Showers almost invariably follow the very Meanders of the larger Rivers, 608.
Rain always cools: the Thermometer falling, after a Shower, from 6 to 8 Degrees, 610.
(And Iron rusts least in rainy Weather: [the Air being thendriest,] descending from theupperRegions. Abr. Ph. Tr. V. 3, P. 546.)”
It is said also that “in Jamaica the Clouds gather, andshapeaccording to the Mountains: so thatold Seamenwill tell you eachIslandtowards Evening, by theShapeof the Cloudoverit.”
The Sea-Breeze, being counterpoised byDescentof theetherial Air, produces acalm.
The same Author likewise says, that “the Clouds begin to gather about 2 or 3 o’Clock in the Afternoonat the Mountains, and do notembodyfirst in the Air, and after settle there, butsettlefirst andembodythere: the rest of the Sky being clear tillSun-set. So that they do not passnearthe Earth in aBody, and onlystopwhere they meet with Parts of the Earth elevatedabove the rest; butprecipitatefrom a very great Height, and in Particles of anexceeding rarified Nature;so as not to obscure the Air or Sky at all: that great Variety of beautiful Colours in the Canopy of Heaven being raised to a much greater Distance [he means Height] in Jamaica than it is here.”Abr. Ph. Tr. V. 3, P. 557.
(Prognostics of Weather, at certain Periods of the Moon, are mentioned by Captain Langford.Lowthorp’s Abr. Phil. Trans. Vol. 2, Page 105.)
[68]The Depression of aTorrentof Air in the Form of an hyperbolic Solid,contractingas itdescendsto the Earth, in Proportion as itsDensityencreases; may furnish a Hint towards the Solution of a Difficulty how to account for the Augmentation of vesiculous Vapours into large solid Drops, frequent duringSummer-Showers.
[69]Mons. Saussure’s valuable“Essais sur L’Hygrometrie,”throw new Light on the Doctrine of Rarefaction and Condensation not unfavourable to the Hypothesis here advanced. Page 260.
[70]Ice, when exposed to marine acid Air, is dissolved by it, as fast as if it touched a red hot Iron. SeeCavallo’s Treatise on Air, Page 727. AlsoPriestley’s Experiments and Observations, Vol. 1, Page 148.
[71]“Thewaterremainstransparentor colourless, tho’ saturated with marine acid Air, and by a very gentle Degree of Heat, the Gass may be again expelled from it, as it is expelled from Spirit of Salt.”This Observation is applicable to the Transparency of Vapours, in the Air, tho’ mixed with the marine Acid exhaled from the Sea: for when the acid or Sea Air is mixed with Alkaline or Land Air, they instantlycombine; lose their Elasticity, and form awhitevisible Substance orCloud. Cavallo, Page 728. Priestley’s Exp. and Obs. Vol. 2, Page 293.
This Observation is applicable to the Transparency of Vapours, in the Air, tho’ mixed with the marine Acid exhaled from the Sea: for when the acid or Sea Air is mixed with Alkaline or Land Air, they instantlycombine; lose their Elasticity, and form awhitevisible Substance orCloud. Cavallo, Page 728. Priestley’s Exp. and Obs. Vol. 2, Page 293.
[72]On the Descent of Air inThunder-Gusts, see“Chalmer’s Account of the Weather in South-Carolina, Vol. 1, Page 1, to 39.”
[73]“Historia Ventorum, Pag. 54, Art. 34.”
[74]Book V. Chapter 2d.
[75]Vol. 1. Page 184.
[76]Page 195.
[77]History of the Canary Isles, Page 252.
[78]As the superior Clouds, during the Balloon Excursion, did not much exceed the Height of 1000 Yards; supposing then the Clouds at an equal Height above the Sea, near Teneriffe; one ought to conclude, either, that the Peak was not so high as Glas represents it; or, that the Level of the Clouds was less than half the Height of the Mountain.
[79]See“Royal Astronomer, by R. Heath, Page 321, onTrade Winds and Monsoons.”
[80]One Pound of Nitre only, producing by mere Heat, 6 cubic Feet of Air.“Cavallo, Page 332, and 811, Experiments on Gun-Powder.”
[81]“See Recherches surles Modifications de l’Atmosphere. No. 715.”Ph. Trans. Part 2, for 1777.Col. Roy’s Experiments, Sect. 2d, Page 689, 744, 753, 764.
[82]The different Phenomena of theAurora Borealismay be owing to the Ascent and Motion of the Apogay, in themiddleRegion, over the Stratum of Eknèfiai orGround-Winds.The Effects ofTides in the Air yet to bementioned, must not, however, be wholly excluded.The Aurora Borealis is seen inSpring,Autumn, andWinter: sometimesculminating, sometimes moving inStreamsandWavesin thesuperiorRegions of the Atmosphere: when culminating; as if rising out of Clouds in the North.This Appearance may be owing to warm moist Air perpetually generating between the Tropics, and rolling over the colddryStratum of Eknèfiai Winds, which cut off its Communication with the Earth: till accumulating over the Poles, it enlightens the Atmosphere, converting a sixMonth’s Nightinto Day; and returns to the Surface silently: or in Lightning, whenever it is communicated to the Earth, throu’Vapour descendingby its own specific Gravity; or along withdepressingTorrents of Air, known to be accompanied by frequentflashes.When the Vapour iscondensedin its Descent, by passing throu’ a Stratum of the Eknèfiai Winds; it becomesoverchargedwith the electric Matter,surroundingandadheringto it; and deposits the Overplus in Lightning, on its Approach tootherClouds, or to theEarth.It is visible in the Form of a Vapour, when the Vapour to which it adheres, becomes overcharged with electric Matter, by Descent into acoolEknèfiai Stratum below: there forming a luminous and transparent Atmosphere: the Particles of Light and Vapour being repelled to great Distances from each other at sorarea Height.It culminates above the Vapour, because less heavy than the circumambient Air: and may be subject to the Attraction of other Planets.The Aurora Borealis is also seen to issue in Streams and Waves of Light, with inexpressible Velocity, on its Return to the South, in a lower Stratum, as itpasses throu’Interstices, between the Vesicles of warm Vapour, raised and dispersed by the turbulent Apogay Winds, in the middle Region.During Summer, the middle Region becomes blended with the lower, throu’ Defect of Cold: and the electric Matter is supposed to be communicated to the Earth, silently, and continually; but by Lightning, when a lower and colder Atmosphere condenses and overcharges the Vapour, and cuts off the Communication.It cannot be seen but in escaping from Vesicle to Vesicle: nor, during Summer, after Sunset, on Account of the Twilight.
The Effects ofTides in the Air yet to bementioned, must not, however, be wholly excluded.
The Aurora Borealis is seen inSpring,Autumn, andWinter: sometimesculminating, sometimes moving inStreamsandWavesin thesuperiorRegions of the Atmosphere: when culminating; as if rising out of Clouds in the North.
This Appearance may be owing to warm moist Air perpetually generating between the Tropics, and rolling over the colddryStratum of Eknèfiai Winds, which cut off its Communication with the Earth: till accumulating over the Poles, it enlightens the Atmosphere, converting a sixMonth’s Nightinto Day; and returns to the Surface silently: or in Lightning, whenever it is communicated to the Earth, throu’Vapour descendingby its own specific Gravity; or along withdepressingTorrents of Air, known to be accompanied by frequentflashes.
When the Vapour iscondensedin its Descent, by passing throu’ a Stratum of the Eknèfiai Winds; it becomesoverchargedwith the electric Matter,surroundingandadheringto it; and deposits the Overplus in Lightning, on its Approach tootherClouds, or to theEarth.
It is visible in the Form of a Vapour, when the Vapour to which it adheres, becomes overcharged with electric Matter, by Descent into acoolEknèfiai Stratum below: there forming a luminous and transparent Atmosphere: the Particles of Light and Vapour being repelled to great Distances from each other at sorarea Height.
It culminates above the Vapour, because less heavy than the circumambient Air: and may be subject to the Attraction of other Planets.
The Aurora Borealis is also seen to issue in Streams and Waves of Light, with inexpressible Velocity, on its Return to the South, in a lower Stratum, as itpasses throu’Interstices, between the Vesicles of warm Vapour, raised and dispersed by the turbulent Apogay Winds, in the middle Region.
During Summer, the middle Region becomes blended with the lower, throu’ Defect of Cold: and the electric Matter is supposed to be communicated to the Earth, silently, and continually; but by Lightning, when a lower and colder Atmosphere condenses and overcharges the Vapour, and cuts off the Communication.
It cannot be seen but in escaping from Vesicle to Vesicle: nor, during Summer, after Sunset, on Account of the Twilight.
[83]Air is not unfit for Respiration, by having lost itsvitalPrinciple, but because it has imbibedFloguiston, which cannoteasilybe separated from it, but by Agitation in Water.Cavallo, on Air,Pages 479, 670.
[84]For if Moisture be one Cause, which keeps the Particles of Air at greater Distances from each other; this Cause decreases atgreatAltitudes.If also theElasticitydecreases in Proportion, not only to the Height, but the Driness; its Particles must, on both Accounts, approach each other, at great Altitudes: tho’, from the Altitude only; they woud separate according to the Rule, viz. that the Rarity of the Air is proportionable to the Relaxation of the Force compressing it.So that at the Height of 8 or 10 Miles, a Quantity of Air taken from the Surface of the Earth, woud occupy 6 Times its former Space: supposing the Air both below and above to be of thesame Kind, as well as of thesame meanTemperature of 55, on the Thermometer. See“Martin’s Philosophical Grammar, Page 178.”
If also theElasticitydecreases in Proportion, not only to the Height, but the Driness; its Particles must, on both Accounts, approach each other, at great Altitudes: tho’, from the Altitude only; they woud separate according to the Rule, viz. that the Rarity of the Air is proportionable to the Relaxation of the Force compressing it.
So that at the Height of 8 or 10 Miles, a Quantity of Air taken from the Surface of the Earth, woud occupy 6 Times its former Space: supposing the Air both below and above to be of thesame Kind, as well as of thesame meanTemperature of 55, on the Thermometer. See“Martin’s Philosophical Grammar, Page 178.”