a journey in other worlds-第36节

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a　velocity　of　some　two　hundred　and　sixty　miles　per　hour　greater
than　the　red　spot。　　Denning　obtained　one　hundred　and　sixty…nine
observations　of　this　bright　marking　during　the　years　1880…1883；
and　determined　the　period　as　nine　hours；　fifty　minutes；　eight　and
seven　tenths　seconds　（five　and　a　half　minutes　less　than　that　of
the　red　spot）。　　Although　the　latter　is　now　somewhat　faint；　the
white　spot　gives　promise　of　remaining　visible　for　many　years。　
During　the　year　1886　a　large　number　of　observations　of　Jupiter
were　made　at　the　Dearborn　Observatory；　Chicago；　U。　S。；　by　Prof。
G。　W。　Hough；　using　the　eighteen…and…a…half…inch　refractor　of　the
observatory。　　Inasmuch　as　these　observations　are　not　only　of　high
intrinsic　interest；　but　are　in　conflict；　to　some　extent；　with
previous　records；　a　somewhat　full　abstract　of　them　will　be
useful：　　The　object　of　general　interest　was　the　great　red　spot。　
The　outline；　shape；　and　size　of　this　remarkable　object　has
remained　without　material　change　from　the　year　1879；　when　it　　was
first　observed　here；　until　the　present　time。　　According　to　our
observations；　during　the　whole　of　this　period　it　has　shown　a
sharp　and　well…defined　outline；　and　at　no　time　has　it　coalesced
or　been　joined　to　any　belt　in　its　proximity；　as　has　been　alleged
by　some　observers。　　During　the　year　1885　the　middle　of　the　spot
was　very　much　paler　in　colour　than　the　margins；　causing　it　to
appear　as　an　elliptical　ring。　　The　ring　form　has　continued　up　to
the　present　time。　　While　the　outline　of　the　spot　has　remained
very　constant；　the　colour　has　changed　materially　from　year　to
year。　　During　the　past　three　years　（1884…　'86）　it　has　at　times
been　very　faint；　so　as　barely　to　be　visible。　　The　persistence　of
this　object　for　so　many　years　leads　me　to　infer　that　the　formerly
accepted　theory；　that　the　phenomena　seen　on　the　surface　of　the
planet　are　atmospheric；　is　no　longer　tenable。　　The　statement　so
often　made　in　text…books；　that　in　the　course　of　a　few　days　or
months　the　whole　aspect　of　the　planet　may　be　changed；　is
obviously　erroneous。　　The　oval　white　spots　on　the　southern
hemisphere　of　the　planet；　nine　degrees　south　of　the　equator；　have
been　systematically　observed　at　every　opposition　during　the　past
eight　years。　　They　are　generally　found　in　groups　of　three　or
more；　but　are　rather　difficult　to　observe。　　The　rotation　period
deduced　from　them　is　nearly　the　same　as　from　the　great　red　spot。　
These　spots　usually　have　a　slow　drift　in　longitude　of　about　five
seconds　daily　in　the　direction　of　the　planet's　rotation；　when
referred　to　the　great　red　spot；　corresponding　to　a　rotation
period　of　twenty　seconds　less　than　the　latter。'
　　　
〃This　shows；〃　continued　Bearwarden；　〃that　as　long　ago　as　towards
the　close　of　the　nineteenth　century　the　old　idea　that　we　saw
nothing　but　the　clouds　in　Jupiter's　atmosphere　was　beginning　to
change；　and　also　how　closely　the　two　English　writers　and　Prof。
Hough　were　studying　the　subject；　though　their　views　did　not
entirely　agree。　　A　white　spot　is　merely　a　storm…centre　passing
round　and　round　the　planet；　the　wind　running　a　little　ahead　of
the　surface；　which　accounts　for　its　rapid　rotation　compared　with
the　red　spot；　which　is　a　fixture。　　A　critic　may　say　we　have　no
such　winds　on　earth；　to　which　I　reply；　that　winds　on　a　planet　of
Jupiter's　size；　with　its　rate　of　rotationthough　it　is
480；000；000　miles　from　the　sun　and　the　internal　heat　is　so　near
the　surfaceand　with　land　and　water　arranged　as　they　are；　may
and　indeed　must　be　very　different　from　those　prevailing　on　earth；
the　conditions　producing　and　affecting　them　being　so　changed。　
Though　the　storm…centre　moves　two　hundred　and　sixty　miles　an
hour；　the　wind　need　not　blow　at　that　rate。〃
　　　
Later　they　saw　several　smaller　spots　drifting　eastward；　but
concluded　that　any　seaworthy　ship　might　pass　safely　through　them；
for；　though　they　were　hurricanes　of　great　violence；　the　waves
were　small。
　　　
〃There　would　be　less　danger；〃　said　Bearwarden；　〃of　shipping　seas
here　than　there　is　on　earth；　the　principal　risk　to　travellers
would　be　that　of　being　blown　from　the　deck。　　On　account　of　the
air's　weight　in　connection　with　its　velocity；　this　would
necessitate　some　precaution。〃
　　　
The　next　object　of　interest　was　the　great　red　spot。　　It　proved；
as　Cortlandt　had　predicted；　to　be　a　continent；　with　at　that　time
no　special　colour；　though　they　easily　recognized　it　by　comparing
its　outlines　with　those　of　the　spot　in　the　map。　　Its　length；　as
they　already　knew；　was　twenty…seven　thousand　miles；　and　its
breadth　about　eight　thousand　miles；　so　that　it　contained　more
square　miles　than　the　entire　surface　of　the　earth；　land　and　water
included。
　　　
〃It　is　clear；〃　said　Cortlandt；　〃that　at　some　season　of　Jupiter's
long　year　a　change　takes　place　that　affects　the　colour　of　the
leavessome　drought　or　prolonged　norther；　for　it　is　obvious　that
that　is　the　simplest　explanation。　　In　like　manner　we　may　expect
that　at　some　times　more　white　spots　will　move　across　the　ocean
than　at　others。〃
　　　
〃On　account　of　the　size　of　these　continents　and　oceans；〃　said
Bearwarden；　〃it　is　easy　to　believe　that　many　climatic　conditions
may　prevail　here　that　can　scarcely　exist　on　earth。　　But　what　a
magnificent　world　to　develop；　with　its　great　rivers；　lakes；　and
mountains　showing　at　even　this　distance；　and　what　natural
resources　must　be　lying　there　dormant；　awaiting　our　call！　　This
constantly　recurs　to　my　mind。　　The　subjugation　and　thorough
opening　up　of　this　red　spot　continent　will　probably　supply　more
interesting　problems　than　straightening　the　axis　of　the　earth。〃
　　　
〃At　our　next　visit；〃　replied　Ayrault；　〃when　we　have　established
regular　interplanetary　lines　of　travel；　we　may　have　an
opportunity　to　examine　it　more　closely。〃　　Then　they　again
attracted　the　nearest　moon　beyond　which　they　had　swung；　increased
the　repulsion　on　Jupiter；　and　soared　away　towards　Saturn。
　　　
〃We　have　a　striking　illustration　of　Jupiter's　enormous　mass；〃
said　Cortlandt；　as　the　apparent　diameter　of　the　mighty　planet
rapidly　decreased；　〃in　the　fact　that　notwithstanding　its　numerous
moons；　it　still　rotates　so　rapidly。　　We　know　that　the　earth's
days　were　formerly　but　half　or　a　quarter　as　long　as　now；　having
lasted　but　six　or　eight　hours。　　The　explanation　of　the　elongation
is　simple：　the　earth　rotates　in　about　twenty…four　hours；　while
the　moon　encircles　it　but　once　in　nearly　twenty…　eight　days；　so
that　our　satellite　is　continually　drawing　the　oceans　backward
against　its　motion。　　These　tidal　brakes　acting　through　the
friction　of　the　water　on　the　bottom；　its　unequal　pressure；　and
the　impact　of　the　waves　on　the　shore；　are　continually　retarding
its　rotation；　so　that　the　day　is　a　fraction　of　a　second　longer
now　than　it　was　in　the　time　of　Caesar。　　This　same　action　is　of
course　taking　place　in　Jupiter　and　the　great　planets；　in　this
case　there　being　five　moons　at　work。　　Our　moon；　we　know；　rotates
on　its　axis　but　once　while　it　revolves　about　the　earth；　this
being　no　doubt　due　to　its　own　comparative　smallness　and　the　great
attraction　of　the　earth；　which　must　have　produced　tremendous
tides　before　the　lunar　oceans　disappeared　from　its　surface。〃
　　　
In　crossing　the　orbits　of　the　satellites；　they　passed　near
Ganymede；　Jupiter's　largest　moon。
　　　
〃This；〃　said　Cortlandt；　〃was　discovered　by　Galileo　in
1610。　　It　is　three　thousand　four　hundred　and　eighty　miles
in　diameter；　while　our　moon　is　but　two　thousand　one
hundred　and　sixty；　revolves　at　a　distance　of　six　hundred
and　seventy…eight　thousand　three　hundred　miles　from
Jupiter；　completes　its　revolution　in　seven　days　and　four
hours；　and　has　a　specific　gravity　of　1。87。〃
　　　
In　passing；　they　observed　that　Ganymede　possessed　an　atmosphere；
and　continents　and　oceans　of　large　area。
　　
〃Here；〃　said　Bearwarden；　〃we　have　a　body　with　a　diameter　about
five　hundred　miles　greater　than　the　planet　Mercury。　　Its　size；
light　specific　gravity；　atmosphere；　and　oceans　seem　to　indicate
that　it　is　less　advanced　than　that　planet；　yet　you　think　Jupiter
has　had　a　longer　separate　existence　than　the　planets　nearer　the
sun？〃　　　　

〃Undoubtedly；〃　said　Cortlandt。　　〃Jupiter　was　condensed　while　in
the　solar…system　nebula；　and　began　its　individual　existence　and
its　evolutionary　career　long　before　Mercury　was　formed。　　The
matter　now　in　Ganymede；　however；　doubtless　remained　part　of　the
Jupiter…system　nebula　till　after　Mercury's　creation；　and；　being
part　of　so　great　a　mass；　did　not　cool　very　rapidly。　　I　should　say
that　this　satellite　has　about　the　same　relation　to　Jupiter　that
Jupiter　has　to　the　sun；　and　is　therefore　younger　in　point　of　time
as　well　as　of　development　than　the　most　distant　Callisto；　and
older；　at　all　events　in　years；　than　Europa　and　Io；　both　of　which
are　nearer。　　This　supposition　is　corroborated　by　the　fact　that
Europa；　the　smallest　of　these　four；　is　also　the　densest；　having　a
specific　gravity　of　2。14；　its　smallness　having　enabled　it　to
overtake　Ganymede　in　development；　notwithstanding　the　latter's
start。　　In　the　face　of　the　evidence　before　us　we　must　believe
this；　or　else　that；　perhaps；　as　in　the　case　of　the　asteroid
Hilda；　something　like　a　collision　has　rejuvenated　it。　　This　might
account　for　its　size；　and　for　the　Nautical　A