a journey in other worlds-第47部分

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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　Almanac's　statement
that　there　is　a　'small　and　variable'　inclination　to　its　orbit；
while　Io　and　Europa　revolve　exactly　in　the　plane　of　Jupiter's
equator。〃
　　　
They　had　about　as　long　a　journey　before　them　as　they　had　already
made　in　going　from　the　earth　to　Jupiter。　　The　great　planet　soon
appeared　as　a　huge　crescent；　since　it　was　between　them　and　the
sun；　its　moons　became　as　fifth…　and　sixth…magnitude　stars；　and　in
the　evening　of　the　next　day　Jupiter's　disk　became　invisible　to
the　unaided　eye。　　Since　there　were　no　way　stations；　in　the　shape
of　planets　or　asteroids；　between　Jupiter　and　Saturn；　they　kept
the　maximum　repulsion　on　Jupiter　as　long　as　possible；　and　moved
at　tremendous　speed。　　Saturn　was　somewhat　in　advance　of　Jupiter
in　its　orbit；　so　that　their　course　from　the　earth　had　been　along
two　sides　of　a　triangle　with　an　obtuse　angle　between。　　During　the
next　four　terrestrial　days　they　sighted　several　small　comets；　but
spent　most　of　their　time　writing　out　their　Jovian　experiences。　
During　the　sixth　day　Saturn's　rings；　although　not　as　much　tilted
as　they　would　be　later　in　the　planet's　season；　presented　a　most
superb　sight；　while　they　spun　in　the　sun's　rays。　　Soon　after　this
the　eight　moons　became　visible；　and；　while　slightly　reducing　the
Callisto's　speed；　they　crossed　the　orbits　of　Iapetus；　Hyperion；
and　Titan；　when　they　knew　they　were　but　seven　hundred　and　fifty
thousand　miles　from　Saturn。
　　　
〃I　am　anxious　to　ascertain；〃　said　Cortlandt；　〃whether　the
composition　of　yonder　rings　is　similar　to　that　of　the　comet
through　which　we　passed。　　I　am　sure　they　shine　with　more　than
reflected　light。〃
　　　
〃We　have　been　in　the　habit；〃　said　Ayrault；　〃of　associating　heat
with　light；　but　it　is　obvious　there　is　something　far　more　subtle
about　cometary　light　and　that　of　Saturn's　rings；　both　of　which
seem　to　have　their　birth　in　the　intense　cold　of　interplanetary
space。〃
　　　
Passing　close　to　Mimas；　Saturn's　nearest　moon；　they　supplemented
its　attraction；　after　swinging　by；　by　their　own　strong　pull；
bringing　their　speed　down　to　dead　slow　as　they　entered　the
outside　ring。　　At　distances　often　of　half　a　mile　they　found
meteoric　masses；　sometimes　lumps　the　size　of　a　house；　often　no
larger　than　apples；　while　small　particles　like　grains　of　sand
moved　between　them。　　There　were　two　motions。　　The　ring　revolved
about　Saturn；　and　the　particles　vibrated　among　themselves；
evidently　kept　apart　by　a　mutual　repulsion；　which　seemed　both　to
increase　and　decrease　faster　than　gravitation；　for　on　approaching
one　another　they　were　more　strongly　repelled　than　attracted；　but
when　they　separated　the　repulsion　decreased　faster　than　the
attraction；　so　that　after　a　time　divergence　ceased；　and　they
remained　at　fixed　distances。
　　　
The　Callisto　soon　became　imbued　with　motion　also；　but　nothing
ever　struck　it。　　When　any　large　mass　came　unusually　near；　both　it
and　their　car　emitted　light；　and　they　rapidly　separated。　　The
sunlight　was　not　as　strong　here　as　it　had　been　when　they　entered
the　comet；　and　as　they　penetrated　farther　they　were　better　able
to　observe　the　omnipresent　luminosity。　　They　were　somewhat
puzzled　by　the　approach　of　certain　light…centres；　which　seemed　to
contain　nothing　but　this　concentrated　brightness。　　Occasionally
one　of　these　centres　would　glow　very　brightly　near　them；　and
simultaneously　recede。　　At　such　times　the　Callisto　also　glowed；
and　itself　recoiled　slightly。　　At　first　the　travellers　could　not
account　for　this；　but　finally　they　concluded　that　the　centres
must　be　meteoric　masses　consisting　entirely　of　gases；　possessing
weight　though　invisible。
　　　
〃We　have　again　to　face；〃　said　Cortlandt；　〃that　singular　law　that
till　recently　we　did　not　suppose　existed　on　earth。　　All　kinds　of
suppositions　have　been　advanced　in　explanation　of　these　rings。　
Some　writers　have　their　thickness；　looked　at　from　the　thin　edge；
as　four　hundred　miles；　some　one　hundred；　and　some　but　forty。　　One
astronomer　of　the　nineteenth　century；　a　man　of　considerable
eminence；　was　convinced　that　they　consisted　of　sheets　of　liquid。　
Now；　it　should　be　obvious　that　no　liquid　could　maintain　itself
here　for　a　minute；　for　it　would　either　fall　upon　the　planet　as　a
crushing　hail；　or；　if　dependent　for　its　shape　on　its　own
tenacity；　it　would　break　if　formed　of　the　toughest　steel；　on
account　of　the　tremendous　weight。　　Any　number　of　theories　have
been　advanced　by　any　number　of　men；　but　in　weight　we　have　the
rub。　　No　one　has　ever　shown　how　these　innumerable　fragments
maintain　themselves　at　a　height　of　but　a　few　thousand　miles　above
Saturn；　withstanding　the　giant's　gravitation…pull。　　Their　rate　of
revolution；　though　rapid；　does　not　seem　fast　enough　to　sustain
them。　　Neither　have　I　ever　seen　it　explained　why　the　small
fragments　do　not　fall　upon　the　large　ones；　though　many
astronomers　have　pictured　the　composition　of　these　rings　as　we
find　they　exist。　　Nor　do　we　know　why　the　molecules　of　a　gas　are
driven　farther　apart　by　heat；　while　their　activity　is　also
increased；　though　if　this　activity　were　revolution　about　one
another　to　dev