BookI
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WEhavealreadydiscussedthefirstcausesofnature，andallnaturalmotion，alsothestarsorderedinthemotionoftheheavens，andthephysicalelement-enumeratingandspecifyingthemandshowinghowtheychangeintooneanother-andbecomingandperishingingeneral。Thereremainsforconsiderationapartofthisinquirywhichallourpredecessorscalledmeteorology。Itisconcernedwitheventsthatarenatural，thoughtheirorderislessperfectthanthatofthefirstoftheelementsofbodies。Theytakeplaceintheregionnearesttothemotionofthestars。Sucharethemilkyway，andcomets，andthemovementsofmeteors。Itstudiesalsoalltheaffectionswemaycallcommontoairandwater，andthekindsandpartsoftheearthandtheaffectionsofitsparts。Thesethrowlightonthecausesofwindsandearthquakesandalltheconsequencesthemotionsofthesekindsandpartsinvolve。Ofthesethingssomepuzzleus，whileothersadmitofexplanationinsomedegree。Further，theinquiryisconcernedwiththefallingofthunderboltsandwithwhirlwindsandfire-winds，andfurther，therecurrentaffectionsproducedinthesesamebodiesbyconcretion。Whentheinquiryintothesemattersisconcludedletusconsiderwhataccountwecangive，inaccordancewiththemethodwehavefollowed，ofanimalsandplants，bothgenerallyandindetail。Whenthathasbeendonewemaysaythatthewholeofouroriginalundertakingwillhavebeencarriedout。
Afterthisintroductionletusbeginbydiscussingourimmediatesubject。
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Wehavealreadylaiddownthatthereisonephysicalelementwhichmakesupthesystemofthebodiesthatmoveinacircle，andbesidesthisfourbodiesowingtheirexistencetothefourprinciples，themotionoftheselatterbodiesbeingoftwokinds：eitherfromthecentreortothecentre。Thesefourbodiesarefire，air，water，earth。Fireoccupiesthehighestplaceamongthemall，earththelowest，andtwoelementscorrespondtotheseintheirrelationtooneanother，airbeingnearesttofire，watertoearth。Thewholeworldsurroundingtheearth，then，theaffectionsofwhichareoursubject，ismadeupofthesebodies。Thisworldnecessarilyhasacertaincontinuitywiththeuppermotions：consequentlyallitspowerandorderisderivedfromthem。（Fortheoriginatingprincipleofallmotionisthefirstcause。Besides，thatclementiseternalanditsmotionhasnolimitinspace，butisalwayscomplete;whereasalltheseotherbodieshaveseparateregionswhichlimitoneanother。）
Sowemusttreatfireandearthandtheelementslikethemasthematerialcausesoftheeventsinthisworld（meaningbymaterialwhatissubjectandisaffected），butmustassigncausalityinthesenseoftheoriginatingprincipleofmotiontotheinfluenceoftheeternallymovingbodies。
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Letusfirstrecallouroriginalprinciplesandthedistinctionsalreadydrawnandthenexplainthe’milkyway’andcometsandtheotherphenomenaakintothese。
Fire，air，water，earth，weassert，originatefromoneanother，andeachofthemexistspotentiallyineach，asallthingsdothatcanberesolvedintoacommonandultimatesubstrate。
Thefirstdifficultyisraisedbywhatiscalledtheair。Whatarewetotakeitsnaturetobeintheworldsurroundingtheearth？Andwhatisitspositionrelativelytotheotherphysicalelements。（Forthereisnoquestionastotherelationofthebulkoftheearthtothesizeofthebodieswhichexistaroundit，sinceastronomicaldemonstrationshavebythistimeprovedtousthatitisactuallyfarsmallerthansomeindividualstars。Asforthewater，itisnotobservedtoexistcollectivelyandseparately，norcanitdosoapartfromthatvolumeofitwhichhasitsseatabouttheearth：thesea，thatis，andrivers，whichwecansee，andanysubterraneanwaterthatmaybehiddenfromourobservation。）Thequestionisreallyaboutthatwhichliesbetweentheearthandtheneareststars。Arewetoconsiderittobeonekindofbodyormorethanone？Andifmorethanone，howmanyarethereandwhataretheboundsoftheirregions？
Wehavealreadydescribedandcharacterizedthefirstelement，andexplainedthatthewholeworldoftheuppermotionsisfullofthatbody。
Thisisanopinionwearenotaloneinholding：itappearstobeanoldassumptionandonewhichmenhaveheldinthepast，forthewordetherhaslongbeenusedtodenotethatelement。Anaxagoras，itistrue，seemstometothinkthatthewordmeansthesameasfire。
Forhethoughtthattheupperregionswerefulloffire，andthatmenreferredtothoseregionswhentheyspokeofether。Inthelatterpointhewasright，formenseemtohaveassumedthatabodythatwaseternallyinmotionwasalsodivineinnature;and，assuchabodywasdifferentfromanyoftheterrestrialelements，theydeterminedtocallit’ether’。
Fortheumopinionsappearincyclesamongmennotoncenortwice，butinfinitelyoften。
Nowtherearesomewhomaintainthatnotonlythebodiesinmotionbutthatwhichcontainsthemispurefire，andtheintervalbetweentheearthandthestarsair：butiftheyhadconsideredwhatisnowsatisfactorilyestablishedbymathematics，theymighthavegivenupthispuerileopinion。Foritisaltogetherchildishtosupposethatthemovingbodiesareallofthemofasmallsize，becausetheysotous，lookingatthemfromtheearth。
Thisamatterwhichwehavealreadydiscussedinourtreatmentoftheupperregion，butwemayreturntothepointnow。
Iftheintervalswerefulloffireandthebodiesconsistedoffireeveryoneoftheotherelementswouldlongagohavevanished。
However，theycannotsimplybesaidtobefullofaireither;forevenifthereweretwoelementstofillthespacebetweentheearthandtheheavens，theairwouldfarexceedthequantiturequiredtomaintainitsproperproportiontotheotherelements。Forthebulkoftheearth（whichincludesthewholevolumeofwater）isinfinitesimalincomparisonwiththewholeworldthatsurroundsit。
Nowwefindthattheexcessinvolumeisnotproportionatelygreatwherewaterdissolvesintoairorairintofire。Whereastheproportionbetweenanygivensmallquantityofwaterandtheairthatisgeneratedfromitoughttoholdgoodbetweenthetotalamountofairandthetotalamountofwater。Nordoesitmakeanydifferenceifanyonedeniesthattheelementsoriginatefromoneanother，butassertsthattheyareequalinpower。Foronthisviewitiscertainamountsofeachthatareequalinpower，justaswouldbethecaseiftheyactuallyoriginatedfromoneanother。
Soitisclearthatneitherairnorfirealonefillstheintermediatespace。
Itremainstoexplain，afterapreliminarydiscussionofdifficulties，therelationofthetwoelementsairandfiretothepositionofthefirstelement，andthereasonwhythestarsintheupperregionimpartheattotheearthanditsneighbourhood。Letusfirsttreatoftheair，asweproposed，andthengoontothesequestions。
Sincewaterisgeneratedfromair，andairfromwater，whyarecloudsnotformedintheupperair？Theyoughttoformtherethemore，thefurtherfromtheearthandthecolderthatregionis。Foritisneitherappreciablyneartotheheatofthestars，nortotheraysrelectedfromtheearth。Itisthesethatdissolveanyformationbytheirheatandsopreventcloudsfromformingneartheearth。Forcloudsgatheratthepointwherethereflectedraysdisperseintheinfinityofspaceandarelost。Toexplainthiswemustsupposeeitherthatitisnotallairwhichwaterisgenerated，or，ifitisproducedfromallairalike，thatwhatimmediatelysurroundstheearthisnotmereair，butasortofvapour，andthatitsvaporousnatureisthereasonwhyitcondensesbacktowateragain。Butifthewholeofthatvastregionisvapour，theamountofairandofwaterwillbedisproportionatelygreat。Forthespacesleftbytheheavenlybodiesmustbefilledbysomeelement。Thiscannotbefire，forthenalltherestwouldhavebeendriedup。Consequently，whatfillsitmustbeairandthewaterthatsurroundsthewholeearth-vapourbeingwaterdissolved。
Afterthisexpositionofthedifficultiesinvolved，letusgoontolaydownthetruth，withaviewatoncetowhatfollowsandtowhathasalreadybeensaid。Theupperregionasfarasthemoonweaffirmtoconsistofabodydistinctbothfromfireandfromair，butvaryingdegreeofpurityandinkind，especiallytowardsitslimitonthesideoftheair，andoftheworldsurroundingtheearth。Nowthecircularmotionofthefirstelementandofthebodiesitcontainsdissolves，andinflamesbyitsmotion，whateverpartofthelowerworldisnearesttoit，andsogeneratesheat。Fromanotherpointofviewwemaylookatthemotionasfollows。Thebodythatliesbelowthecircularmotionoftheheavensis，inasort，matter，andispotentiallyhot，cold，dry，moist，andpossessedofwhateverotherqualitiesarederivedfromthese。Butitactuallyacquiresorretainsoneoftheseinvirtueofmotionorrest，thecauseandprincipleofwhichhasalreadybeenexplained。Soatthecentreandrounditwegetearthandwater，theheaviestandcoldestelements，bythemselves;roundthemandcontiguouswiththem，airandwhatwecommonlycallfire。Itisnotreallyfire，forfireisanexcessofheatandasortofebullition;butinreality，ofwhatwecallair，thepartsurroundingtheearthismoistandwarm，becauseitcontainsbothvapourandadryexhalationfromtheearth。Butthenextpart，abovethat，iswarmanddry。Forvapourisnaturallymoistandcold，buttheexhalationwarmanddry;andvapourispotentiallylikewater，theexhalationpotentiallylikefire。Sowemusttakethereasonwhycloudsarenotformedintheupperregiontobethis：
thatitisfillednotwithmereairbutratherwithasortoffire。
However，itmaywellbethattheformationofcloudsinthatupperregionisalsopreventedbythecircularmotion。Fortheairroundtheearthisnecessarilyallofitinmotion，exceptthatwhichiscutoffinsidethecircumferencewhichmakestheearthacompletesphere。Inthecaseofwindsitisactuallyobservablethattheyoriginateinmarshydistrictsoftheearth;andtheydonotseemtoblowabovethelevelofthehighestmountains。Itistherevolutionoftheheavenwhichcarriestheairwithitandcausesitscircularmotion，firebeingcontinuouswiththeupperelementandairwithfire。Thusitsmotionisasecondreasonwhythatairisnotcondensedintowater。
Butwheneveraparticleofairgrowsheavy，thewarmthinitissqueezedoutintotheupperregionanditsinks，andotherparticlesinturnarecarrieduptogetherwiththefieryexhalation。Thustheoneregionisalwaysfullofairandtheotheroffire，andeachofthemisperpetuallyinastateofchange。
Somuchtoexplainwhycloudsarenotformedandwhytheairisnotcondensedintowater，andwhataccountmustbegivenofthespacebetweenthestarsandtheearth，andwhatisthebodythatfillsit。
Asfortheheatderivedfromthesun，therightplaceforaspecialandscientificaccountofitisinthetreatiseaboutsense，sinceheatisanaffectionofsense，butwemaynowexplainhowitcanbeproducedbytheheavenlybodieswhicharenotthemselveshot。
Weseethatmotionisabletodissolveandinflametheair;
indeed，movingbodiesareoftenactuallyfoundtomelt。Nowthesun’smotionaloneissufficienttoaccountfortheoriginofterrestrialwarmthandheat。Foramotionthatistohavethiseffectmustberapidandnear，andthatofthestarsisrapidbutdistant，whilethatofthemoonisnearbutslow，whereasthesun’smotioncombinesbothconditionsinasufficientdegree。Thatmostheatshouldbegeneratedwherethesunispresentiseasytounderstandifweconsidertheanalogyofterrestrialphenomena，forhere，too，itistheairthatisnearesttoathinginrapidmotionwhichisheatedmost。Thisisjustwhatweshouldexpect，asitisthenearestairthatismostdissolvedbythemotionofasolidbody。
Thisthenisonereasonwhyheatreachesourworld。Anotheristhatthefiresurroundingtheairisoftenscatteredbythemotionoftheheavensanddrivendownwardsinspiteofitself。
Shooting-starsfurthersuffixtoprovethatthecelestialsphereisnothotorfiery：fortheydonotoccurinthatupperregionbutbelow：yetthemoreandthefasterathingmoves，themoreaptitistotakefire。Besides，thesun，whichmostofallthestarsisconsideredtobehot，isreallywhiteandnotfieryincolour。
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Havingdeterminedtheseprinciplesletusexplainthecauseoftheappearanceintheskyofburningflamesandofshooting-stars，andof’torches’，and’goats’，assomepeoplecallthem。Allthesephenomenaareoneandthesamething，andareduetothesamecause，thedifferencebetweenthembeingoneofdegree。
Theexplanationoftheseandmanyotherphenomenaisthis。Whenthesunwarmstheearththeevaporationwhichtakesplaceisnecessarilyoftwokinds，notofoneonlyassomethink。Onekindisratherofthenatureofvapour，theotherofthenatureofawindyexhalation。Thatwhichrisesfromthemoisturecontainedintheearthandonitssurfaceisvapour，whilethatrisingfromtheearthitself，whichisdry，islikesmoke。Ofthesethewindyexhalation，beingwarm，risesabovethemoistervapour，whichisheavyandsinksbelowtheother。Hencetheworldsurroundingtheearthisorderedasfollows。Firstbelowthecircularmotioncomesthewarmanddryelement，whichwecallfire，forthereisnowordfullyadequatetoeverystateofthefumidevaporation：butwemustusethisterminologysincethiselementisthemostinflammableofallbodies。Belowthiscomesair。Wemustthinkofwhatwejustcalledfireasbeingspreadroundtheterrestrialsphereontheoutsidelikeakindoffuel，sothatalittlemotionoftenmakesitburstintoflamejustassmokedoes：forflameistheebullitionofadryexhalation。Sowheneverthecircularmotionstirsthisstuffupinanyway，itcatchesfireatthepointatwhichitismostinflammable。Theresultdiffersaccordingtothedispositionandquantityofthecombustiblematerial。Ifthisisbroadandlong，weoftenseeaflameburningasinafieldofstubble：
ifitburnslengthwiseonly，weseewhatarecalled’torches’and’goats’andshooting-stars。Nowwhentheinflammablematerialislongerthanitisbroadsometimesitseemstothrowoffsparksasitburns。（Thishappensbecausemattercatchesfireatthesidesinsmallportionsbutcontinuouslywiththemainbody。）Thenitiscalleda’goat’。Whenthisdoesnothappenitisa’torch’。Butifthewholelengthoftheexhalationisscatteredinsmallpartsandinmanydirectionsandinbreadthanddepthalike，wegetwhatarecalledshooting-stars。
Thecauseoftheseshooting-starsissometimesthemotionwhichignitestheexhalation。Atothertimestheairiscondensedbycoldandsqueezesoutandejectsthehotelement;makingtheirmotionlookmorelikethatofathingthrownthanlikearunningfire。Forthequestionmightberaisedwhetherthe’shooting’ofa’star’isthesamethingaswhenyouputanexhalationbelowalampanditlightsthelowerlampfromtheflameabove。Forheretootheflamepasseswonderfullyquicklyandlookslikeathingthrown，andnotasifonethingafteranothercaughtfire。Orisa’star’whenit’shoots’asinglebodythatisthrown？Apparentlybothcasesoccur：sometimesitisliketheflamefromthelampandsometimesbodiesareprojectedbybeingsqueezedout（likefruitstonesfromone’sfingers）
andsoareseentofallintotheseaandonthedryland，bothbynightandbydaywhentheskyisclear。Theyarethrowndownwardsbecausethecondensationwhichpropelstheminclinesdownwards。
Thunderboltsfalldownwardsforthesamereason：theiroriginisnevercombustionbutejectionunderpressure，sincenaturallyallheattendsupwards。
Whenthephenomenonisformedintheupperregionitisduetothecombustionoftheexhalation。Whenittakesplaceatalowerlevelitisduetotheejectionoftheexhalationbythecondensingandcoolingofthemoisterevaporation：forthislatterasitcondensesandinclinesdownwardcontracts，andthrustsoutthehotelementandcausesittobethrowndownwards。Themotionisupwardsordownwardsorsidewaysaccordingtothewayinwhichtheevaporationlies，anditsdispositioninrespectofbreadthanddepth。Inmostcasesthedirectionissidewaysbecausetwomotionsareinvolved，acompulsorymotiondownwardsandanaturalmotionupwards，andunderthesecircumstancesanobjectalwaysmovesobliquely。Hencethemotionof’shooting-stars’isgenerallyoblique。
Sothematerialcauseofallthesephenomenaistheexhalation，theefficientcausesometimestheuppermotion，sometimesthecontractionandcondensationoftheair。Further，allthesethingshappenbelowthemoon。Thisisshownbytheirapparentspeed，whichisequaltothatofthingsthrownbyus;foritisbecausetheyareclosetous，thattheselatterseemfartoexceedinspeedthestars，thesun，andthemoon。
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Sometimesonafinenightweseeavarietyofappearancesthatforminthesky：’chasms’forinstanceand’trenches’andblood-redcolours。These，too，havethesamecause。Forwehaveseenthattheupperaircondensesintoaninflammableconditionandthatthecombustionsometimestakesontheappearanceofaburningflame，sometimesthatofmovingtorchesandstars。Soitisnotsurprisingthatthissameairwhencondensingshouldassumeavarietyofcolours。
Foraweaklightshiningthroughadenseair，andtheairwhenitactsasamirror，willcauseallkindsofcolourstoappear，butespeciallycrimsonandpurple。Forthesecoloursgenerallyappearwhenfire-colourandwhitearecombinedbysuperposition。Thusonahotday，orthroughasmoky，medium，thestarswhentheyriseandsetlookcrimson。Thelightwillalsocreatecoloursbyreflectionwhenthemirrorissuchastoreflectcolouronlyandnotshape。
Theseappearancesdonotpersistlong，becausethecondensationoftheairistransient。
’Chasms’gettheirappearanceofdepthfromlightbreakingoutofadarkblueorblackmassofair。Whentheprocessofcondensationgoesfurtherinsuchacaseweoftenfind’torches’ejected。Whenthe’chasm’contractsitpresentstheappearanceofa’trench’。
Ingeneral，whiteincontrastwithblackcreatesavarietyofcolours;likeflame，forinstance，throughamediumofsmoke。Butbydaythesunobscuresthem，and，withtheexceptionofcrimson，thecoloursarenotseenatnightbecausetheyaredark。
Thesethenmustbetakentobethecausesof’shooting-stars’andthephenomenaofcombustionandalsooftheothertransientappearancesofthiskind。
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Letusgoontoexplainthenatureofcometsandthe’milkyway’，afterapreliminarydiscussionoftheviewsofothers。
AnaxagorasandDemocritusdeclarethatcometsareaconjunctionoftheplanetsapproachingoneanotherandsoappearingtotouchoneanother。
SomeoftheItalianscalledPythagoreanssaythatthecometisoneoftheplanets，butthatitappearsatgreatintervalsoftimeandonlyrisesalittleabovethehorizon。ThisisthecasewithMercurytoo;becauseitonlyrisesalittleabovethehorizonitoftenfailstobeseenandconsequentlyappearsatgreatintervalsoftime。
AviewliketheirswasalsoexpressedbyHippocratesofChiosandhispupilAeschylus。Onlytheysaythatthetaildoesnotbelongtothecometiself，butisoccasionallyassumedbyitonitscourseincertainsituations，whenoursightisreflectedtothesunfromthemoistureattractedbythecomet。Itappearsatgreaterintervalsthantheotherstarsbecauseitisslowesttogetclearofthesunandhasbeenleftbehindbythesuntotheextentofthewholeofitscirclebeforeitreappearsatthesamepoint。Itgetsclearofthesunbothtowardsthenorthandtowardsthesouth。Inthespacebetweenthetropicsitdoesnotdrawwatertoitselfbecausethatregionisdriedupbythesunonitscourse。Whenitmovestowardsthesouthithasnolackofthenecessarymoisture，butbecausethesegmentofitscirclewhichisabovethehorizonissmall，andthatbelowitmanytimesaslarge，itisimpossibleforthesuntobereflectedtooursight，eitherwhenitapproachesthesoutherntropic，oratthesummersolstice。Henceintheseregionsitdoesnotdevelopatailatall。
Butwhenitisvisibleinthenorthitassumesatailbecausethearcabovethehorizonislargeandthatbelowitsmall。Forunderthesecircumstancesthereisnothingtopreventourvisionfrombeingreflectedtothesun。
Theseviewsinvolveimpossibilities，someofwhicharecommontoallofthem，whileothersarepeculiartosomeonly。
Thisisthecase，first，withthosewhosaythatthecometisoneoftheplanets。Foralltheplanetsappearinthecircleofthezodiac，whereasmanycometshavebeenseenoutsidethatcircle。Againmorecometsthanonehaveoftenappearedsimultaneously。Besides，iftheirtailisduetoreflection，asAeschylusandHippocratessay，thisplanetoughtsometimestobevisiblewithoutatailsince，astheyitdoesnotpossessatailineveryplaceinwhichitappears。
But，asamatteroffact，noplanethasbeenobservedbesidesthefive。Andallofthemareoftenvisibleabovethehorizontogetheratthesametime。Further，cometsareoftenfoundtoappear，aswellwhenalltheplanetsarevisibleaswhensomearenot，butareobscuredbytheneighbourhoodofthesun。Moreoverthestatementthatacometonlyappearsinthenorth，withthesunatthesummersolstice，isnottrueeither。ThegreatcometwhichappearedatthetimeoftheearthquakeinAchaeaandthetidalwaveroseduewest;andmanyhavebeenknowntoappearinthesouth。AgaininthearchonshipofEuclees，sonofMolon，atAthensthereappearedacometinthenorthinthemonthGamelion，thesunbeingaboutthewintersolstice。Yettheythemselvesadmitthatreflectionoversogreataspaceisanimpossibility。
Anobjectionthattellsequallyagainstthosewhoholdthistheoryandthosewhosaythatcometsareacoalescenceoftheplanetsis，first，thefactthatsomeofthefixedstarstoogetatail。ForthiswemustnotonlyaccepttheauthorityoftheEgyptianswhoassertit，butwehaveourselvesobservedthefact。ForastarinthethighoftheDoghadatail，thoughafaintone。Ifyoufixedyoursightonititslightwasdim，butifyoujustglancedatit，itappearedbrighter。Besides，allthecometsthathavebeenseeninourdayhavevanishedwithoutsetting，graduallyfadingawayabovethehorizon;andtheyhavenotleftbehindthemeitheroneormorestars。Forinstancethegreatcometwementionedbeforeappearedtothewestinwinterinfrostyweatherwhentheskywasclear，inthearchonshipofAsteius。Onthefirstdayitsetbeforethesunandwasthennotseen。Onthenextdayitwasseen，beingeversolittlebehindthesunandimmediatelysetting。Butitslightextendedoverathirdpartoftheskylikealeap，sothatpeoplecalledita’path’。ThiscometrecededasfarasOrion’sbeltandtheredissolved。
Democritushowever，insistsuponthetruthofhisviewandaffirmsthatcertainstarshavebeenseenwhencometsdissolve。Butonhistheorythisoughtnottooccuroccasionallybutalways。Besides，theEgyptiansaffirmthatconjunctionsoftheplanetswithoneanother，andwiththefixedstars，takeplace，andwehaveourselvesobservedJupitercoincidingwithoneofthestarsintheTwinsandhidingit，andyetnocometwasformed。Further，wecanalsogivearationalproofofourpoint。Itistruethatsomestarsseemtobebiggerthanothers，yeteachonebyitselflooksindivisible。Consequently，justas，iftheyreallyhadbeenindivisible，theirconjunctioncouldnothavecreatedanygreatermagnitude，sonowthattheyarenotinfactindivisiblebutlookasiftheywere，theirconjunctionwillnotmakethemlookanybigger。
Enoughhasbeensaid，withoutfurtherargument，toshowthatthecausesbroughtforwardtoexplaincometsarefalse。
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Weconsiderasatisfactoryexplanationofphenomenainaccessibletoobservationtohavebeengivenwhenouraccountofthemisfreefromimpossibilities。Theobservationsbeforeussuggestthefollowingaccountofthephenomenawearenowconsidering。Weknowthatthedryandwarmexhalationistheoutermostpartoftheterrestrialworldwhichfallsbelowthecircularmotion。It，andagreatpartoftheairthatiscontinuouswithitbelow，iscarriedroundtheearthbythemotionofthecircularrevolution。Inthecourseofthismotionitoftenigniteswhereveritmayhappentobeoftherightconsistency，andthiswemaintaintobethecauseofthe’shooting’ofscattered’stars’。Wemaysay，then，thatacometisformedwhentheuppermotionintroducesintoagatheringofthiskindafieryprinciplenotofsuchexcessivestrengthastoburnupmuchofthematerialquickly，norsoweakassoontobeextinguished，butstrongerandcapableofburningupmuchmaterial，andwhenexhalationoftherightconsistencyrisesfrombelowandmeetsit。Thekindofcometvariesaccordingtotheshapewhichtheexhalationhappenstotake。Ifitisdiffusedequallyoneverysidethestarissaidtobefringed，ifitstretchesoutinonedirectionitiscalledbearded。Wehaveseenthatwhenafieryprincipleofthiskindmovesweseemtohaveashooting-star：similarlywhenitstandsstillweseemtohaveastarstandingstill。Wemaycomparethesephenomenatoaheapormassofchaffintowhichatorchisthrust，orasparkthrown。Thatiswhatashooting-starislike。Thefuelissoinflammablethatthefirerunsthroughitquicklyinaline。Nowifthisfireweretopersistinsteadofrunningthroughthefuelandperishingaway，itscoursethroughthefuelwouldstopatthepointwherethelatterwasdensest，andthenthewholemightbegintomove。Suchisacomet-likeashooting-starthatcontainsitsbeginningandendinitself。
Whenthematterbeginstogatherinthelowerregionindependentlythecometappearsbyitself。Butwhentheexhalationisconstitutedbyoneofthefixedstarsortheplanets，owingtotheirmotion，oneofthembecomesacomet。Thefringeisnotclosetothestarsthemselves。
Justashaloesappeartofollowthesunandthemoonastheymove，andencirclethem，whentheairisdenseenoughforthemtoformalongunderthesun’scourse，sotoothefringe。Itstandsintherelationofahalotothestars，exceptthatthecolourofthehaloisduetoreflection，whereasinthecaseofcometsthecolourissomethingthatappearsactuallyonthem。
Nowwhenthismattergathersinrelationtoastarthecometnecessarilyappearstofollowthesamecourseasthestar。Butwhenthecometisformedindependentlyitfallsbehindthemotionoftheuniverse，liketherestoftheterrestrialworld。Itisthisfact，thatacometoftenformsindependently，indeedoftenerthanroundoneoftheregularstars，thatmakesitimpossibletomaintainthatacometisasortofreflection，notindeed，asHippocratesandhisschoolsay，tothesun，buttotheverystaritisallegedtoaccompany-infact，akindofhalointhepurefueloffire。
Asforthehaloweshallexplainitscauselater。
Thefactthatcometswhenfrequentforeshadowwindanddroughtmustbetakenasanindicationoftheirfieryconstitution。Fortheiroriginisplainlyduetotheplentifulsupplyofthatsecretion。
Hencetheairisnecessarilydrierandthemoistevaporationissodissolvedanddissipatedbythequantityofthehotexhalationasnotreadilytocondenseintowater-Butthisphenomenontooshallbeexplainedmoreclearlylaterwhenthetimecomestospeakofthewinds-Sowhentherearemanycometsandtheyaredense，itisaswesay，andtheyearsareclearlydryandwindy。Whentheyarefewerandfainterthiseffectdoesnotappearinthesamedegree，thoughasaruletheisfoundtobeexcessiveeitherindurationorstrength。
ForinstancewhenthestoneatAegospotamifelloutoftheair-ithadbeencarriedupbyawindandfelldowninthedaytime-thentooacomethappenedtohaveappearedinthewest。Andatthetimeofthegreatcometthewinterwasdryandnorthwindsprevailed，andthewavewasduetoanoppositionofwinds。Forinthegulfanorthwindblewandoutsideitaviolentsouthwind。AgaininthearchonshipofNicomachusacometappearedforafewdaysabouttheequinoctialcircle（thisonehadnotriseninthewest），andsimultaneouslywithittherehappenedthestormatCorinth。
Thattherearefewcometsandthattheyappearrarelyandoutsidethetropiccirclesmorethanwithinthemisduetothemotionofthesunandthestars。Forthismotiondoesnotonlycausethehotprincipletobesecretedbutalsodissolvesitwhenitisgathering。
Butthechiefreasonisthatmostofthisstuffcollectsintheregionofthemilkyway。
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Letusnowexplaintheorigin，cause，andnatureofthemilkyway。
Andheretooletusbeginbydiscussingthestatementsofothersonthesubject。
（1）Oftheso-calledPythagoreanssomesaythatthisisthepathofoneofthestarsthatfellfromheavenatthetimeofPhaethon’sdownfall。Otherssaythatthesunusedoncetomoveinthiscircleandthatthisregionwasscorchedormetwithsomeotheraffectionofthiskind，becauseofthesunanditsmotion。
ButitisabsurdnottoseethatifthiswerethereasonthecircleoftheZodiacoughttobeaffectedinthesameway，andindeedmoresothanthatofthemilkyway，sincenotthesunonlybutalltheplanetsmoveinit。Wecanseethewholeofthiscircle（halfofitbeingvisibleatanytimeofthenight），butitshowsnosignsofanysuchaffectionexceptwhereapartofittouchesthecircleofthemilkyway。
（2）Anaxagoras，Democritus，andtheirschoolssaythatthemilkywayisthelightofcertainstars。For，theysay，whenthesunpassesbelowtheearthsomeofthestarsarehiddenfromit。Nowthelightofthoseonwhichthesunshinesisinvisible，beingobscuredbytheofthesun。Butthemilkywayisthepeculiarlightofthosestarswhichareshadedbytheearthfromthesun’srays。
This，too，isobviouslyimpossible。Themilkywayisalwaysunchangedandamongthesameconstellations（foritisclearlyagreatestcircle），whereas，sincethesundoesnotremaininthesameplace，whatishiddenfromitdiffersatdifferenttimes。Consequentlywiththechangeofthesun’spositionthemilkywayoughttochangeitspositiontoo：butwefindthatthisdoesnothappen。Besides，ifastronomicaldemonstrationsarecorrectandthesizeofthesunisgreaterthanthatoftheearthandthedistanceofthestarsfromtheearthmanytimesgreaterthanthatofthesun（justasthesunisfurtherfromtheearththanthemoon），thentheconemadebytheraysofthesunwouldterminateatnogreatdistancefromtheearth，andtheshadowoftheearth（whatwecallnight）wouldnotreachthestars。Onthecontrary，thesunshinesonallthestarsandtheearthscreensnoneofthem。
（3）Thereisathirdtheoryaboutthemilkyway。Somesaythatitisareflectionofoursighttothesun，justastheysaythatthecometis。
Butthistooisimpossible。Foriftheeyeandthemirrorandthewholeoftheobjectwereseverallyatrest，thenthesamepartoftheimagewouldappearatthesamepointinthemirror。Butifthemirrorandtheobjectmove，keepingthesamedistancefromtheeyewhichisatrest，butatdifferentratesofspeedandsonotalwaysatthesameintervalfromoneanother，thenitisimpossibleforthesameimagealwaystoappearinthesamepartofthemirror。Nowtheconstellationsincludedinthecircleofthemilkywaymove;andsodoesthesun，theobjecttowhichoursightisreflected;butwestandstill。Andthedistanceofthosetwofromusisconstantanduniform，buttheirdistancefromoneanothervaries。FortheDolphinsometimesrisesatmidnight，sometimesinthemorning。Butineachcasethesamepartsofthemilkywayarefoundnearit。Butifitwereareflectionandnotagenuineaffectionofthesethisoughtnottobethecase。
Again，wecanseethemilkywayreflectedatnightinwaterandsimilarmirrors。Butunderthesecircumstancesitisimpossibleforoursighttobereflectedtothesun。
Theseconsiderationsshowthatthemilkywayisnotthepathofoneoftheplanets，northelightofimperceptiblestars，norareflection。Andthosearethechieftheorieshandeddownbyothershitherto。
Letusrecallourfundamentalprincipleandthenexplainourviews。Wehavealreadylaiddownthattheoutermostpartofwhatiscalledtheairispotentiallyfireandthatthereforewhentheairisdissolvedbymotion，thereisseparatedoffakindofmatter-andofthismatterweassertthatcometsconsist。Wemustsupposethatwhathappensisthesameasinthecaseofthecometswhenthematterdoesnotformindependentlybutisformedbyoneofthefixedstarsortheplanets。Thenthesestarsappeartobefringed，becausematterofthiskindfollowstheircourse。Inthesameway，acertainkindofmatterfollowsthesun，andweexplainthehaloasareflectionfromitwhentheairisoftherightconstitution。Nowwemustassumethatwhathappensinthecaseofthestarsseverallyhappensinthecaseofthewholeoftheheavensandalltheuppermotion。Foritisnaturaltosupposethat，ifthemotionofasinglestarexcitesaflame，thatofallthestarsshouldhaveasimilarresult，andespeciallyinthatregioninwhichthestarsarebiggestandmostnumerousandnearesttooneanother。Nowthecircleofthezodiacdissolvesthiskindofmatterbecauseofthemotionofthesunandtheplanets，andforthisreasonmostcometsarefoundoutsidethetropiccircles。Again，nofringeappearsroundthesunormoon：fortheydissolvesuchmattertooquicklytoadmitofitsformation。Butthiscircleinwhichthemilkywayappearstooursightisthegreatestcircle，anditspositionissuchthatitextendsfaroutsidethetropiccircles。Besidestheregionisfullofthebiggestandbrightestconstellationsandalsoofwhatcalled’scattered’stars（youhaveonlytolooktoseethisclearly）。Soforthesereasonsallthismatteriscontinuallyandceaselesslycollectingthere。Aproofofthetheoryisthis：Inthecircleitselfthelightisstrongerinthathalfwherethemilkywayisdivided，andinittheconstellationsaremorenumerousandclosertooneanotherthanintheotherhalf;whichshowsthatthecauseofthelightisthemotionoftheconstellationsandnothingelse。Forifitisfoundinthecircleinwhichtherearemostconstellationsandatthatpointinthecircleatwhichtheyaredensestandcontainthebiggestandthemoststars，itisnaturaltosupposethattheyarethetruecauseoftheaffectioninquestion。Thecircleandtheconstellationsinitmaybeseeninthediagram。Theso-called’scattered’starsitisnotpossibletosetdowninthesamewayonthespherebecausenoneofthemhaveanevidentpermanentposition;
butifyoulookuptotheskythepointisclear。Forinthiscirclealonearetheintervalsfullofthesestars：intheothercirclesthereareobviousgaps。Henceifweacceptthecauseassignedfortheappearanceofcometsasplausiblewemustassumethatthesamekindofthingholdsgoodofthemilkyway。Forthefringewhichintheformercaseisanaffectionofasinglestarhereformsinthesamewayinrelationtoawholecircle。Soifwearetodefinethemilkywaywemaycallit’afringeattachingtothegreatestcircle，andduetothemattersecreted’。This，aswesaidbefore，explainswhytherearefewcometsandwhytheyappearrarely;itisbecauseateachrevolutionoftheheavensthismatterhasalwaysbeenandisalwaysbeingseparatedoffandgatheredintothisregion。
Wehavenowexplainedthephenomenathatoccurinthatpartoftheterrestrialworldwhichiscontinuouswiththemotionsoftheheavens，namely，shooting-starsandtheburningflame，cometsandthemilkyway，thesebeingthechiefaffectionsthatappearinthatregion。
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Letusgoontotreatoftheregionwhichfollowsnextinorderafterthisandwhichimmediatelysurroundstheearth。Itistheregioncommontowaterandair，andtheprocessesattendingtheformationofwaterabovetakeplaceinit。Wemustconsidertheprinciplesandcausesofallthesephenomenatooasbefore。Theefficientandchiefandfirstcauseisthecircleinwhichthesunmoves。Forthesunasitapproachesorrecedes，obviouslycausesdissipationandcondensationandsogivesrisetogenerationanddestruction。Nowtheearthremainsbutthemoisturesurroundingitismadetoevaporatebythesun’sraysandtheotherheatfromabove，andrises。Butwhentheheatwhichwasraisingitleavesit，inpartdispersingtothehigherregion，inpartquenchedthroughrisingsofarintotheupperair，thenthevapourcoolsbecauseitsheatisgoneandbecausetheplaceiscold，andcondensesagainandturnsfromairintowater。
Andafterthewaterhasformeditfallsdownagaintotheearth。
Theexhalationofwaterisvapour：aircondensingintowateriscloud。Mistiswhatisleftoverwhenacloudcondensesintowater，andisthereforeratherasignoffineweatherthanofrain;formistmightbecalledabarrencloud。Sowegetacircularprocessthatfollowsthecourseofthesun。Foraccordingasthesunmovestothissideorthat，themoistureinthisprocessrisesorfalls。Wemustthinkofitasariverflowingupanddowninacircleandmadeuppartlyofair，partlyofwater。Whenthesunisnear，thestreamofvapourflowsupwards;whenitrecedes，thestreamofwaterflowsdown：
andtheorderofsequence，atallevents，inthisprocessalwaysremainsthesame。Soif’Oceanus’hadsomesecretmeaninginearlywriters，perhapstheymayhavemeantthisriverthatflowsinacircleabouttheearth。
Sothemoistureisalwaysraisedbytheheatanddescendstotheearthagainwhenitgetscold。Theseprocessesand，insomecases，theirvarietiesaredistinguishedbyspecialnames。Whenthewaterfallsinsmalldropsitiscalledadrizzle;whenthedropsarelargeritisrain。
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Someofthevapourthatisformedbydaydoesnotrisehighbecausetheratioofthefirethatisraisingittothewaterthatisbeingraisedissmall。Whenthiscoolsanddescendsatnightitiscalleddewandhoar-frost。Whenthevapourisfrozenbeforeithascondensedtowateragainitishoar-frost;andthisappearsinwinterandiscommonerincoldplaces。Itisdewwhenthevapourhascondensedintowaterandtheheatisnotsogreatastodryupthemoisturethathasbeenraisednorthecoldsufficient（owingtothewarmthoftheclimateorseason）forthevapouritselftofreeze。
Fordewismorecommonlyfoundwhentheseasonortheplaceiswarm，whereastheopposite，ashasbeensaid，isthecasewithhoar-frost。
Forobviouslyvapouriswarmerthanwater，havingstillthefirethatraisedit：consequentlymorecoldisneededtofreezeit。
Bothdewandhoar-frostarefoundwhentheskyisclearandthereisnowind。Forthevapourcouldnotberaisedunlesstheskywereclear，andifawindwereblowingitcouldnotcondense。
Thefactthathoar-frostisnotfoundonmountainscontributestoprovethatthesephenomenaoccurbecausethevapourdoesnotrisehigh。Onereasonforthisisthatitrisesfromhollowandwateryplaces，sothattheheatthatisraisingit，bearingasitweretooheavyaburdencannotliftittoagreatheightbutsoonletsitfallagain。Asecondreasonisthatthemotionoftheairismorepronouncedataheight，andthisdissolvesagatheringofthiskind。
Everywhere，exceptinPontus，dewisfoundwithsouthwindsandnotwithnorthwinds。Theretheoppositeisthecaseanditisfoundwithnorthwindsandnotwithsouth。Thereasonisthesameasthatwhichexplainswhydewisfoundinwarmweatherandnotincold。Forthesouthwindbringswarm，andthenorth，wintryweather。Forthenorthwindiscoldandsoquenchestheheatoftheevaporation。ButinPontusthesouthwinddoesnotbringwarmthenoughtocauseevaporation，whereasthecoldnessofthenorthwindconcentratestheheatbyasortofrecoil，sothatthereismoreevaporationandnotless。Thisisathingwhichwecanoftenobserveinotherplacestoo。Wells，forinstance，giveoffmorevapourinanorththaninasouthwind。Onlythenorthwindsquenchtheheatbeforeanyconsiderablequantityofvapourhasgathered，whileinasouthwindtheevaporationisallowedtoaccumulate。
Water，onceformed，doesnotfreezeonthesurfaceoftheearth，inthewaythatitdoesintheregionoftheclouds。
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Fromthelattertherefallthreebodiescondensedbycold，namelyrain，snow，hail。Twoofthesecorrespondtothephenomenaonthelowerlevelandareduetothesamecauses，differingfromthemonlyindegreeandquantity。
Snowandhoar-frostareoneandthesamething，andsoarerainanddew：onlythereisagreatdealoftheformerandlittleofthelatter。Forrainisduetothecoolingofagreatamountofvapour，fortheregionfromwhichandthetimeduringwhichthevapouriscollectedareconsiderable。Butofdewthereislittle：forthevapourcollectsforitinasingledayandfromasmallarea，asitsquickformationandscantyquantityshow。
Therelationofhoar-frostandsnowisthesame：whencloudfreezesthereissnow，whenvapourfreezesthereishoar-frost。
Hencesnowisasignofacoldseasonorcountry。Foragreatdealofheatisstillpresentandunlessthecoldwereoverpoweringitthecloudwouldnotfreeze。Fortherestillsurvivesinitagreatdealoftheheatwhichcausedthemoisturetoriseasvapourfromtheearth。
Hailontheotherhandisfoundintheupperregion，butthecorrespondingphenomenoninthevaporousregionneartheearthislacking。For，aswesaid，tosnowintheupperregioncorrespondshoar-frostinthelower，andtorainintheupperregion，dewinthelower。Butthereisnothingheretocorrespondtohailintheupperregion。Whythisissowillbeclearwhenwehaveexplainedthenatureofhail。
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Butwemustgoontocollectthefactsbearingontheoriginofit，boththosewhichraisenodifficultiesandthosewhichseemparadoxical。
Hailisice，andwaterfreezesinwinter;yethailstormsoccurchieflyinspringandautumnandlessofteninthelatesummer，butrarelyinwinterandthenonlywhenthecoldislessintense。Andingeneralhailstormsoccurinwarmer，andsnowincolderplaces。
Again，thereisadifficultyaboutwaterfreezingintheupperregion。
Itcannothavefrozenbeforebecomingwater：andwatercannotremainsuspendedintheairforanyspaceoftime。Norcanwesaythatthecaseislikethatofparticlesofmoisturewhicharecarriedupowingtotheirsmallsizeandrestontheiar（thewaterswimmingontheairjustassmallparticlesofearthandgoldoftenswimonwater）。Inthatcaselargedropsareformedbytheunionofmanysmall，andsofalldown。Thiscannottakeplaceinthecaseofhail，sincesolidbodiescannotcoalescelikeliquidones。Clearlythendropsofthatsizeweresuspendedintheairorelsetheycouldnothavebeensolargewhenfrozen。
Somethinkthatthecauseandoriginofhailisthis。Thecloudisthrustupintotheupperatmosphere，whichiscolderbecausethereflectionofthesun’sraysfromtheearthceasesthere，anduponitsarrivaltherethewaterfreezes。Theythinkthatthisexplainswhyhailstormsarecommonerinsummerandinwarmcountries;theheatisgreateranditthruststhecloudsfurtherupfromtheearth。Butthefactisthathaildoesnotoccuratallatagreatheight：yetitoughttodoso，ontheirtheory，justasweseethatsnowfallsmostonhighmountains。Againcloudshaveoftenbeenobservedmovingwithagreatnoiseclosetotheearth，terrifyingthosewhoheardandsawthemasportentsofsomecatastrophe。Sometimes，too，whensuchcloudshavebeenseen，withoutanynoise，therefollowsaviolenthailstorm，andthestonesareofincrediblesize，andangularinshape。Thisshowsthattheyhavenotbeenfallingforlongandthattheywerefrozenneartotheearth，andnotasthattheorywouldhaveit。Moreover，wherethehailstonesarelarge，thecauseoftheirfreezingmustbepresentinthehighestdegree：forhailisiceaseveryonecansee。Nowthosehailstonesarelargewhichareangularinshape。Andthisshowsthattheyfrozeclosetotheearth，forthosethatfallfararewornawaybythelengthoftheirfallandbecomeroundandsmallerinsize。
Itclearlyfollowsthatthecongelationdoesnottakeplacebecausethecloudisthrustupintothecoldupperregion。
Nowweseethatwarmandcoldreactupononeanotherbyrecoil。
Henceinwarmweatherthelowerpartsoftheeartharecoldandinafrosttheyarewarm。Thesamething，wemustsuppose，happensintheair，sothatinthewarmerseasonsthecoldisconcentratedbythesurroundingheatandcausesthecloudtogooverintowatersuddenly。（Forthisreasonrain-dropsaremuchlargeronwarmdaysthaninwinter，andshowersmoreviolent。Ashowerissaidtobemoreviolentinproportionasthewatercomesdowninabody，andthishappenswhenthecondensationtakesplacequickly，-thoughthisisjusttheoppositeofwhatAnaxagorassays。Hesaysthatthishappenswhenthecloudhasrisenintothecoldair;whereaswesaythatithappenswhenthecloudhasdescendedintothewarmair，andthatthemorethefurtherthecloudhasdescended）。Butwhenthecoldhasbeenconcentratedwithinstillmorebytheouterheat，itfreezesthewaterithasformedandthereishail。Wegethailwhentheprocessoffreezingisquickerthanthedescentofthewater。Forifthewaterfallsinacertaintimeandthecoldissufficienttofreezeitinless，thereisnodifficultyaboutitshavingfrozenintheair，providedthatthefreezingtakesplaceinashortertimethanitsfall。Thenearertotheearth，andthemoresuddenly，thisprocesstakesplace，themoreviolentistherainthatresultsandthelargertheraindropsandthehailstonesbecauseoftheshortnessoftheirfall。Forthesamereasonlargeraindropsdonotfallthickly。
Hailisrarerinsummerthaninspringandautumn，thoughcommonerthaninwinter，becausetheairisdrierinsummer，whereasinspringitisstillmoist，andinautumnitisbeginningtogrowmoist。
Itisforthesamereasonthathailstormssometimesoccurinthelatesummeraswehavesaid。
Thefactthatthewaterhaspreviouslybeenwarmedcontributestoitsfreezingquickly：forsoitcoolssooner。Hencemanypeople，whentheywanttocoolhotwaterquickly，beginbyputtingitinthesun。SotheinhabitantsofPontuswhentheyencampontheicetofish（theycutaholeintheiceandthenfish）pourwarmwaterroundtheirreedsthatitmayfreezethequicker，fortheyusetheicelikeleadtofixthereeds。Nowitisinhotcountriesandseasonsthatthewaterwhichformssoongrowswarm。
ItisforthesamereasonthatrainfallsinsummerandnotinwinterinArabiaandEthiopiatoo，andthatintorrentsandrepeatedlyonthesameday。Fortheconcentrationorrecoilduetotheextremeheatofthecountrycoolsthecloudsquickly。
Somuchforanaccountofthenatureandcausesofrain，dew，snow，hoar-frost，andhail。
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Letusexplainthenatureofwinds，andallwindyvapours，alsoofriversandofthesea。Buthere，too，wemustfirstdiscussthedifficultiesinvolved：for，asinothermatters，sointhisnotheoryhasbeenhandeddowntousthatthemostordinarymancouldnothavethoughtof。
Somesaythatwhatiscalledair，whenitisinmotionandflows，iswind，andthatthissameairwhenitcondensesagainbecomescloudandwater，implyingthatthenatureofwindandwateristhesame。Sotheydefinewindasamotionoftheair。Hencesome，wishingtosayacleverthing，assertthatallthewindsareonewind，becausetheairthatmovesisinfactallofitoneandthesame;theymaintainthatthewindsappeartodifferowingtotheregionfromwhichtheairmayhappentoflowoneachoccasion，butreallydonotdifferatall。Thisisjustlikethinkingthatallriversareoneandthesameriver，andtheordinaryunscientificviewisbetterthanascientifictheorylikethis。Ifallriversflowfromonesource，andthesameistrueinthecaseofthewinds，theremightbesometruthinthistheory;butifitisnomoretrueintheonecasethanintheother，thisingeniousideaisplainlyfalse。Whatrequiresinvestigationisthis：thenatureofwindandhowitoriginates，itsefficientcauseandwhencetheyderivetheirsource;whetheroneoughttothinkofthewindasissuingfromasortofvesselandflowinguntilthevesselisempty，asifletoutofawineskin，or，aspaintersrepresentthewinds，asdrawingtheirsourcefromthemselves。
Wefindanalogousviewsabouttheoriginofrivers。Itisthoughtthatthewaterisraisedbythesunanddescendsinrainandgathersbelowtheearthandsoflowsfromagreatreservoir，alltheriversfromone，oreachfromadifferentone。Nowateratallisgenerated，butthevolumeoftheriversconsistsofthewaterthatisgatheredintosuchreservoirsinwinter。Henceriversarealwaysfullerinwinterthaninsummer，andsomeareperennial，othersnot。
Riversareperennialwherethereservoirislargeandsoenoughwaterhascollectedinittolastoutandnotbeusedupbeforethewinterrainreturns。Wherethereservoirsaresmallerthereislesswaterintherivers，andtheyaredriedupandtheirvesselemptybeforethefreshraincomeson。
Butifanyonewillpicturetohimselfareservoiradequatetothewaterthatiscontinuouslyflowingdaybyday，andconsidertheamountofthewater，itisobviousthatareceptaclethatistocontainallthewaterthatflowsintheyearwouldbelargerthantheearth，or，atanyrate，notmuchsmaller。
Thoughitisevidentthatmanyreservoirsofthiskinddoexistinmanypartsoftheearth，yetitisunreasonableforanyonetorefusetoadmitthatairbecomeswaterintheearthforthesamereasonasitdoesaboveit。Ifthecoldcausesthevaporousairtocondenseintowaterabovetheearthwemustsupposethecoldintheearthtoproducethissameeffect，andrecognizethattherenotonlyexistsinitandflowsoutofitactuallyformedwater，butthatwateriscontinuallyforminginittoo。
Again，eveninthecaseofthewaterthatisnotbeingformedfromdaytodaybutexistsassuch，wemustnotsupposeassomedothatrivershavetheirsourceindefinitesubterraneanlakes。Onthecontrary，justasabovetheearthsmalldropsformandthesejoinothers，tillfinallythewaterdescendsinabodyasrain，sotoowemustsupposethatintheearththewateratfirsttricklestogetherlittlebylittle，andthatthesourcesoftheriversdrip，asitwere，outoftheearthandthenunite。Thisisprovedbyfacts。Whenmenconstructanaqueducttheycollectthewaterinpipesandtrenches，asiftheearthinthehighergroundweresweatingthewaterout。
Hence，too，thehead-watersofriversarefoundtoflowfrommountains，andfromthegreatestmountainsthereflowthemostnumerousandgreatestrivers。Again，mostspringsareintheneighbourhoodofmountainsandofhighground，whereasifweexceptrivers，waterrarelyappearsintheplains。Formountainsandhighground，suspendedoverthecountrylikeasaturatedsponge，makethewateroozeoutandtrickletogetherinminutequantitiesbutinmanyplaces。Theyreceiveagreatdealofwaterfallingasrain（foritmakesnodifferencewhetheraspongyreceptacleisconcaveandturneduporconvexandturneddown：ineithercaseitwillcontainthesamevolumeofmatter）and，theyalsocoolthevapourthatrisesandcondenseitbackintowater。
Hence，aswesaid，wefindthatthegreatestriversflowfromthegreatestmountains。Thiscanbeseenbylookingatitineraries：whatisrecordedinthemconsistseitherofthingswhichthewriterhasseenhimselforofsuchashehascompiledafterinquiryfromthosewhohaveseenthem。
InAsiawefindthatthemostnumerousandgreatestriversflowfromthemountaincalledParnassus，admittedlythegreatestofallmountainstowardsthesouth-east。Whenyouhavecrossedityouseetheouterocean，thefurtherlimitofwhichisunknowntothedwellersinourworld。BesidesotherriversthereflowfromittheBactrus，theChoaspes，theAraxes：fromthelastabranchseparatesoffandflowsintolakeMaeotisastheTanais。Fromit，too，flowstheIndus，thevolumeofwhosestreamisgreatestofallrivers。FromtheCaucasusflowsthePhasis，andverymanyothergreatriversbesides。NowtheCaucasusisthegreatestofthemountainsthatlietothenortheast，bothasregardsitsextentanditsheight。Aproofofitsheightisthefactthatitcanbeseenfromtheso-called’deeps’andfromtheentrancetothelake。Again，thesunshinesonitspeaksforathirdpartofthenightbeforesunriseandagainaftersunset。Itsextentisprovedbythefactthatthoughtcontainsmanyinhabitableregionswhichareoccupiedbymanynationsandinwhichtherearesaidtobegreatlakes，yettheysaythatalltheseregionsarevisibleuptothelastpeak。FromPyrene（thisisamountaintowardsthewestinCeltice）thereflowtheIstrusandtheTartessus。Thelatterflowsoutsidethepillars，whiletheIstrusflowsthroughallEuropeintotheEuxine。MostoftheremainingriversflownorthwardsfromtheHercynianmountains，whicharethegreatestinheightandextentaboutthatregion。Intheextremenorth，beyondfurthestScythia，arethemountainscalledRhipae。Thestoriesabouttheirsizearealtogethertoofabulous：however，theysaythatthemostand（aftertheIstrus）
thegreatestriversflowfromthem。So，too，inLibyathereflowfromtheAethiopianmountainstheAegonandtheNyses;andfromtheso-calledSilverMountainthetwogreatestofnamedrivers，therivercalledChremetesthatflowsintotheouterocean，andthemainsourceoftheNile。OftheriversintheGreekworld，theAchelousflowsfromPindus，theInachusfromthesamemountain;theStrymon，theNestus，andtheHebrusallthreefromScombrus;manyrivers，too，flowfromRhodope。
Allotherriverswouldbefoundtoflowinthesameway，butwehavementionedtheseasexamples。Evenwhereriversflowfrommarshes，themarshesinalmosteverycasearefoundtoliebelowmountainsorgraduallyrisingground。
Itisclearthenthatwemustnotsupposeriverstooriginatefromdefinitereservoirs：forthewholeearth，wemightalmostsay，wouldnotbesufficient（anymorethantheregionofthecloudswouldbe）ifweweretosupposethattheywerefedbyactuallyexistingwateronlyanditwerenotthecasethatassomewaterpassedoutofexistencesomemorecameintoexistence，butriversalwaysdrewtheirstreamfromanexistingstore。Secondly，thefactthatriversriseatthefootofmountainsprovesthataplacetransmitsthewateritcontainsbygradualpercolationofmanydrops，littlebylittle，andthatthisishowthesourcesofriversoriginate。However，thereisnothingimpossibleabouttheexistenceofsuchplacescontainingaquantityofwaterlikelakes：onlytheycannotbebigenoughtoproducethesupposedeffect。Tothinkthattheyareisjustasabsurdasifoneweretosupposethatriversdrewalltheirwaterfromthesourceswesee（formostriversdoflowfromsprings）。Soitisnomorereasonabletosupposethoselakestocontainthewholevolumeofwaterthanthesesprings。
Thatthereexistsuchchasmsandcavitiesintheearthwearetaughtbytheriversthatareswallowedup。Theyarefoundinmanypartsoftheearth：inthePeloponnesus，forinstance，therearemanysuchriversinArcadia。ThereasonisthatArcadiaismountainousandtherearenochannelsfromitsvalleystothesea。Sotheseplacesgetfullofwater，andthis，havingnooutlet，underthepressureofthewaterthatisaddedabove，findsawayoutforitselfunderground。
InGreecethiskindofthinghappensonquiteasmallscale，butthelakeatthefootoftheCaucasus，whichtheinhabitantsofthesepartscallasea，isconsiderable。ManygreatriversfallintoitandithasnovisibleoutletbutissuesbelowtheearthoffthelandoftheCoraxiabouttheso-called’deepsofPontus’。Thisisaplaceofunfathomabledepthinthesea：atanyratenoonehasyetbeenabletofindbottomtherebysounding。Atthisspot，aboutthreehundredstadiafromland，therecomesupsweetwateroveralargearea，notallofittogetherbutinthreeplaces。AndinLiguriaariverequalinsizetotheRhodanusisswallowedupandappearsagainelsewhere：
theRhodanusbeinganavigableriver。
