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Wehavealreadyshownthatwetanddrymustbothgiverisetoanevaporation：earthquakesareanecessaryconsequenceofthisfact。Theearthisessentiallydry，butrainfillsitwithmoisture。Thenthesunanditsownfirewarmitandgiverisetoaquantityofwindbothoutsideandinsideit。Thiswindsometimesflowsoutwardsinasinglebody，sometimesinwards，andsometimesitisdivided。Allthesearenecessarylaws。Nextwemustfindoutwhatbodyhasthegreatestmotiveforce。Thiswillcertainlybethebodythatnaturallymovesfarthestandismostviolent。Nowthatwhichhasthemostrapidmotionisnecessarilythemostviolent;foritsswiftnessgivesitsimpactthegreatestforce。Again，therarestbody，thatwhichcanmostreadilypassthrougheveryotherbody，isthatwhichnaturallymovesfarthest。Windsatisfiestheseconditionsinthehighestdegree（fireonlybecomesflameandmovesrapidlywhenwindaccompaniesit）：sothatnotwaternorearthisthecauseofearthquakesbutwind-thatis，theinrushoftheexternalevaporationintotheearth。
Hence，sincetheevaporationgenerallyfollowsinacontinuousbodyinthedirectioninwhichitfirststarted，andeitherallofitflowsinwardsoralloutwards，mostearthquakesandthegreatestareaccompaniedbycalm。Itistruethatsometakeplacewhenawindisblowing，butthispresentsnodifficulty。Wesometimesfindseveralwindsblowingsimultaneously。Ifoneoftheseenterstheearthwegetanearthquakeattendedbywind。Onlytheseearthquakesarelessseverebecausetheirsourceandcauseisdivided。
Again，mostearthquakesandtheseverestoccuratnightor，ifbyday，aboutnoon，thatbeinggenerallythecalmestpartoftheday。Forwhenthesunexertsitsfullpower（asitdoesaboutnoon）itshutstheevaporationintotheearth。Night，too，iscalmerthanday。Theabsenceofthesunmakestheevaporationreturnintotheearthlikeasortofebbtide，correspondingtotheoutwardflow;especiallytowardsdawn，forthewinds，asarule，begintoblowthen，andiftheirsourcechangesaboutliketheEuripusandflowsinwardsthequantityofwindintheearthisgreaterandamoreviolentearthquakeresults。
Theseverestearthquakestakeplacewheretheseaisfullofcurrentsortheearthspongyandcavernous：sotheyoccurneartheHellespontandinAchaeaandSicily，andthosepartsofEuboeawhichcorrespondtoourdescription-wheretheseaissupposedtoflowinchannelsbelowtheearth。Thehotsprings，too，nearAedepsusareduetoacauseofthiskind。Itistheconfinedcharacteroftheseplacesthatmakesthemsoliabletoearthquakes。Agreatandthereforeviolentwindisdeveloped，whichwouldnaturallyblowawayfromtheearth：buttheonrushoftheseainagreatmassthrustsitbackintotheearth。Thecountriesthatarespongybelowthesurfaceareexposedtoearthquakesbecausetheyhaveroomforsomuchwind。