an autobiography-第61部分

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til　this　day。　　My　method　was　as　follows：　

I　availed　myself　of　every　favourable　opportunity　for　carrying　on　the　investigation。　　I　made　careful　drawings　with　black　and　white　chalk　on　large　sheets　of　grey…tinted　paper；　of　such　selected　portions　of　the　Moon　as　embodied　the　most　characteristic　and　instructive　features　of　her　wonderful　surface。　　I　was　thus　enabled　to　graphically　represent　the　details　with　due　fidelity　as　to　form；　as　well　as　with　regard　to　the　striking　effect　of　the　original　in　its　masses　of　light　and　shade。　I　thus　educated　my　eye　for　the　special　object　by　systematic　and　careful　observation；　and　at　the　same　time　practised　my　hand　in　no　less　careful　delineation　of　all　that　was　so　distinctly　presented　to　me　by　the　telescopeat　the　side　of　which　my　sheet　of　paper　was　handily　fixed。　I　became　in　a　manner　familiar　with　the　vast　variety　of　those　distinct　manifestations　of　volcanic　action；　which　at　some　inconceivably　remote　period　had　produced　these　wonderful　features　and　details　of　the　moon's　surface。　　So　far　as　could　be　observed；　there　was　an　entire　absence　of　any　agency　of　change；　so　that　their　formation　must　have　remained　absolutely　intact　since　the　original　cosmical　heat　of　the　moon　had　passed　rapidly　into　space。　　The　surface；　with　all　its　wondrous　details；　presents　the　same　aspect　as　it　did　probably　millions　of　ages　ago。

This　consideration　vastly　enhances　the　deep　interest　with　which　we　look　upon　the　moon　and　its　volcanic　details。　　It　is　totally　without　an　atmosphere；　or　of　a　vapour　envelope；　such　as　the　earth　possesses；　and　which　must　have　contributed　to　the　conservation　of　the　cosmical　heat　of　the　latter　orb。　　The　moon　is　of　relatively　small　mass；　and　is　consequently　inferior　in　heat…retaining　power。　　It　must　thus　have　parted　with　its　original　stock　of　cosmical　heat　with　such　rapidity　as　to　bring　about　the　final　termination　of　those　surface　changes　which　give　it　so　peculiar　an　aspect。　　In　the　case　of　the　earth　the　internal　heat　still　continues　in　operation；　though　in　a　vastly　reduced　degree　of　activity。　　Again　in　the　case　of　the　moon；　the　total　absence　of　water　as　well　as　atmosphere　has　removed　from　it　all　those　denudative　activities　which；　in　the　earth；　have　acted　so　powerfully　in　effecting　changes　of　its　surfaces　as　well　as　in　the　distribution　of　its　materials。　Hence　the　appearance　of　the　wonderful　details　of　the　moon's　surface　presents　us　with　objects　of　inconceivably　remote　antiquity。

＇Image＇　　General　structure　of　Lunar　craters。

Another　striking　characteristic　of　the　moon's　surface　is　the　enormous　magnitude　of　its　volcanic　crater　formations。　　In　comparison　with　these；　the　greatest　on　the　surface　of　the　earth　are　reduced　to　insignificance。　Paradoxical　as　the　statement　may　at　first　appear；　the　magnitude　of　the　remains　of　the　primitive　volcanic　energy　in　the　moon　is　simply　due　to　the　smallness　of　its　mass。　　Being　only　about　one…eightieth　part　of　the　bulk　of　the　earth；　the　force　of　gravity　on　the　moon's　surface　is　only　about　one…sixth。　　And　as　eruptive　force　is　quite　independent；　as　a　force；　of　the　law　of　gravitation；　and　as　it　acted　with　its　full　energy　on　matter；　which　in　the　moon　is　little　heavier　than　cork；　it　was　dispersed　in　divergent　flight　from　the　vent　of　the　volcanoes；　free　from　any　atmospheric　resistance；　and　thus　secured　an　enormously　wider　dispersion　of　the　ejected　scoriae。　　Hence　the　building　up　of　those　enormous　ring…formed　craters　which　are　seen　in　such　vast　numbers　on　the　moon's　surfacesome　of　them　being　no　less　than　a　hundred　miles　in　diameter；　with　which　those　of　Etna　and　Vesuvius　are　the　merest　molehills　in　comparison。

I　may　mention；　in　passing；　that　the　frequency　of　a　central　cone　within　these　ring…shaped　lunar　craters　supplies　us　with　one　of　the　most　distinct　and　unquestionable　evidences　of　the　true　nature　and　mode　of　the　formation　of　volcanoes。

They　are　the　result　of　the　expiring　energy　of　the　volcanic　discharge；　which；　when　near　its　termination；　not　having　sufficient　energy　to　eject　the　matter　far　from　its　vent；　becomes　deposited　around　it；　and　thus　builds　up　the　central　cone　as　a　sort　of　monument　to　commemorate　its　expiring　efforts。　　In　this　way　it　recalls　the　exact　features　of　our　own　terrestrial　craters；　though　the　latter　are　infinitely　smaller　in　comparison。　　When　we　consider　how　volcanoes　are　formed　by　the　ejection　and　exudation　of　material　from　beneath　the　solid　crust　it　will　be　seen　how　the　lunar　eminences　are　formed；　that　is；　by　the　forcible　projection　of　fluid　molten　matter　through　cracks　or　vents；　through　which　it　makes　its　way　to　the　surface。

＇Image＇　　Pico；　an　isolated　Lunar　Mountain　8000　feet　high。

It　was　in　reference　to　this　very　interesting　subject　that　I　made　a　drawing　of　the　great　isolated　volcanic　mountain　Pico；　about　8000　feet　high。*　　＇footnote。。。　this　illustration　exhibits　a　class　of　volcanic　formations　that　may　be　seen　on　many　portions　of　the　moon's　surface。　　They　are　what　I　would　term　exudative　volcanic　mountains；　the　results　of　a　comparatively　gentle　discharge　of　volcanic　matter；　which　has　resulted　in　heaped　up　eminences；　a　vast　group　of　which　were　displayed　in　the　illustration；　some　of　them　being　upwards　of　20；000　feet　high。　　。。。＇

It　exhibits　a　very　different　appearance　from　that　of　our　mountain　ranges；　which　are　for　the　most　part　the　result　of　a　tangential　action。　In　the　case　of　the　earth；　the　hard　stratified　crust　had　to　adapt　itself　to　the　shrunken　diameter　of　the　once　much　hotter　globe。　This　tangential　action　is　illustrated　in　our　own　persons；　when　age　causes　the　body　to　shrink　in　bulk；　while　the　skin；　which　does　not　shrink　to　the　same　extent；　has　to　accommodate　itself　to　the　shrunken　interior；　and　so　forms　wrinklesthe　wrinkles　of　age。　　This　theory　opens　up　a　chapter　in　geology　and　physiology　well　worthy　of　consideration。　　It　may　alike　be　seen　in　the　structure　of　the　surface　of　the　earth；　in　an　old　apple；　and　in　an　old　hand。*　　＇footnote。。。　The　shrunken　hand　on　the　other　side　is　that　of　Mr。　Nasmyth；　photographed　by　himself。　　According　to　The　Psychonomy　of　the　Hand；　by　R。　　Beamish；　F。R。S。；　author　of　The　Life　of　Sir　M。　I。　Brunel；　it　exhibits　a　thoroughly　mechanical　hand；　as　well　as　the　hand　of　a　delicate　manipulator；　illustrating　that　remarkable　expression　in　the　Book　of　Job；　that　〃in　the　hand　of　all　the　sons　of　men　God　places　marks；　that　all　the　sons　of　men　may　know　their　own　works。〃ED。　　。。。＇

＇Image＇　　Shrunken　Apple　and　Hand。*　　＇footnote。。。　These　illustrations　serve　to　illustrate　one　of　the　most　potent　of　geological　agencies　which　has　given　the　earth's　surface　its　grandest　characteristics。　　I　mean　the　elevation　of　mountain　ranges　through　the　contraction　of　the　globe　as　a　whole。　　By　the　action　of　gravity　the　former　larger　surface　crushes　down；　as　it　were；　the　contracting　interior；　and　the　superfluous　matter；　which　belonged　to　a　bigger　globe；　arranges　itself　by　tangential　displacement；　and　accommodates　itself　to　the　altered　or　decreased　size　of　the　globe。　Hence　our　mountain　ranges；　which　though　apparently　enormous　when　seen　near　at　hand　are　merely　the　wrinkles　on　the　face　of　the　earth。　　。。。＇

While　earnestly　studying　the　details　of　the　moon's　surface；　it　was　a　source　of　great　additional　interest　to　me　to　endeavour　to　realise　in　the　mind's　eye　the　possible　landscape　effect　of　its　marvellous　elevations　and　depressions。　　Here　my　artisic　faculty　came　into　operation。　　I　endeavoured　to　illustrate　the　landscape。　　scenery　of　the　Moon；　in　like　manner　as　we　illustrate　the　landscape　scenery　of　the　Earth。　　The　telescope　revealed　to　me　distinctly　the　volcanic　craters；　the　cracks；　and　the　ranges　of　mountainsby　means　of　the　light　and　shade　on　the　moon's　surface。　　One　of　the　most　prominent　conditions　of　the　awful　grandeur　of　lunar　scenery　is　the　brilliant　light　of　the　sun；　far　transcending　that　which　we　experience　upon　the　earthenhanced　by　the　contrast　with　the　jet…black　background　of　the　lunar　heavens；　the　result　of　the　total　absence　of　atmosphere。　　One　portion　of　the　moon；　on　which　the　sun　is　shining；　is　brilliantly　illuminated；　while　all　in　shade　is　dark。

While　the　disc　of　the　sun　appears　a　vast　electric　light　of　overpowering　rayless　brilliancy；　every　star　and　planet　in　the　black　vault　of　the　lunar　heavens　is　shining　with　steady　brightness　at　all　times；　as；　whether　the　Sun　be　present　or　absent　during　the　long　fourteen　days'　length　of　the　lunar　day　or　night；　no　difference　on　the　absolutely　black　aspect　of　the　lunar　heavens　can　appear。　　That　aspect　must　be　eternal　there。　　No　modification*　　＇footnote。。。　a　small　degree　of　illumination　is；　however；　given　to　some　portions　of　the　Moon's　surface　by　the　Earth…shine；　when　the　earth　is　in　such　a　position　with　regard　to　the　Moon；　as　to　reflect　some　light　on　to　it；　as　the　Moon　does　to　the　earth。　　。。。＇　of　the　darkness　of　shadows　in　the　Moon　can　result　from　the　illuminative　effect；　as　in　our　case　in　the　earth；　from　light　reflected　into　shadows　by　the　blue　sky　of　our　earthly　day　The　intensity　of　the　contrast　between　light　and　shade　must　thus　lend　another　awful　aspect　to　the　scenery　of　the　Moon；　while　deprived　of　all　those　charming　effects　which　artists　term　〃aerial　perspective；〃　by　which　relative　distances　are　rendered　cognisable　with　such　tender　and　exquisite　beauty。　　The　absence　of　atmosphere　on　the　Moon　causes　the　most　distant　objects　to　appear　as　close　as　the　nearest；　while　the　comparatively　rapid　curvature　of　the　moon；　owing　to　its　being　a　globe　only　one…fourth　the　diameter　of　the　earth；　must　necessarily　limit　very　considerably　the　range　of　view。

＇Image＇　　Lunar　Mountains　and　Extinct　Volcanic　Craters

It　is　the　combination　of　all　these　circumstances；　which　we　know　with　absolute　certainty　must　exist　in　the　Moon；　that　gives　to　the　contemplation　of　her　marvellous　surface；　as　revealed　by　the　aid　of　powerful　telescopes；one　of　the　grandest　and　most　deeply　interesting　subjects　that　can　occupy　our　thoughts；　especially　when　we　regard　the　physical　constitution　and　the　peculiar　structure　of　her　surface；　as　that　of　our　nearest　planetary　neighbour；　and　also　as　our　serviceable　attendant　by　night。

Then　there　are　the　Tides；　so　useful　to　man；　preserving　the　sanitary　condition　of　the　river　mouths　and　tide…swept　shores。　We　must　be　grateful　for　the　Moon's　existence　on　that　account　alone。　She　is　the　grand　scavenger　and　practical　sanitary　co