Onemorningtheviewwassingularlyclear；thedistantmountainsbeingprojectedwiththesharpestoutlineonaheavybankofdarkblueclouds。
Judgingfromtheappearance，andfromsimilarcasesinEngland，Isupposedthattheairwassaturatedwithmoisture。Thefact，however，turnedoutquitethecontrary。Thehygrometergaveadifferenceof29。6degs。，betweenthetemperatureoftheair，andthepointatwhichdewwasprecipitated。
ThisdifferencewasnearlydoublethatwhichIhadobservedonthepreviousmornings。Thisunusualdegreeofatmosphericdrynesswasaccompaniedbycontinualflashesoflightning。Isitnotanuncommoncase，thustofindaremarkabledegreeofaerialtransparencywithsuchastateofweather？
Generallytheatmosphereishazy；andthisiscausedbythefallingofimpalpablyfinedust，whichwasfoundtohaveslightlyinjuredtheastronomicalinstruments。ThemorningbeforeweanchoredatPortoPraya，Icollectedalittlepacketofthisbrown-colouredfinedust，whichappearedtohavebeenfilteredfromthewindbythegauzeofthevaneatthemasthead。Mr。Lyellhasalsogivenmefourpacketsofdustwhichfellonavesselafewhundredmilesnorthwardoftheseislands。ProfessorEhrenberg[note3]findsthatthisdustconsistsingreatpartofinfusoriawithsiliceousshields，andofthesiliceoustissueofplants。InfivelittlepacketswhichIsenthim，hehasascertainednolessthansixty-sevendifferentorganicforms！Theinfusoria，withtheexceptionoftwomarinespecies，areallinhabitantsoffresh-water。
IhavefoundnolessthanfifteendifferentaccountsofdusthavingfallenonvesselswhenfaroutintheAtlantic。Fromthedirectionofthewindwheneverithasfallen，andfromitshavingalwaysfallenduringthosemonthswhentheharmattanisknowntoraisecloudsofdusthighintotheatmosphere，wemayfeelsurethatitallcomesfromAfrica。Itis，however，averysingularfact，that，althoughProfessorEhrenbergknowsmanyspeciesofinfusoriapeculiartoAfrica，hefindsnoneoftheseinthedustwhichIsenthim。Ontheotherhand，hefindsinittwospecieswhichhithertoheknowsaslivingonlyinSouthAmerica。Thedustfallsinsuchquantitiesastodirtyeverythingonboard，andtohurtpeople’seyes；vesselsevenhaverunonshoreowingtotheobscurityoftheatmosphere。Ithasoftenfallenonshipswhenseveralhundred，andevenmorethanathousandmilesfromthecoastofAfrica，andatpointssixteenhundredmilesdistantinanorthandsouthdirection。Insomedustwhichwascollectedonavesselthreehundredmilesfromtheland，Iwasmuchsurprisedtofindparticlesofstoneabovethethousandthofaninchsquare，mixedwithfinermatter。