a history of science-2-第15章

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racter　of　Galileo's　artistic　presentation　may　best　be　judged　from　an　example；　illustrating　the　vigorous　assault　of　Salviati；　the　champion　of　the　new　theory；　and　the　feeble　retorts　of　his　conservative　antagonist：　〃Salviati。　Let　us　then　begin　our　discussion　with　the　consideration　that；　whatever　motion　may　be　attributed　to　the　earth；　yet　we；　as　dwellers　upon　it；　and　hence　as　participators　in　its　motion；　cannot　possibly　perceive　anything　of　it；　presupposing　that　we　are　to　consider　only　earthly　things。　On　the　other　hand；　it　is　just　as　necessary　that　this　same　motion　belong　apparently　to　all　other　bodies　and　visible　objects；　which；　being　separated　from　the　earth；　do　not　take　part　in　its　motion。　The　correct　method　to　discover　whether　one　can　ascribe　motion　to　the　earth；　and　what　kind　of　motion；　is；　therefore；　to　investigate　and　observe　whether　in　bodies　outside　the　earth　a　perceptible　motion　may　be　discovered　which　belongs　to　all　alike。　Because　a　movement　which　is　perceptible　only　in　the　moon；　for　instance；　and　has　nothing　to　do　with　Venus　or　Jupiter　or　other　stars；　cannot　possibly　be　peculiar　to　the　earth；　nor　can　its　seat　be　anywhere　else　than　in　the　moon。　Now　there　is　one　such　universal　movement　which　controls　all　othersnamely；　that　which　the　sun；　moon；　the　other　planets；　the　fixed　starsin　short；　the　whole　universe；　with　the　single　exception　of　the　earthappears　to　execute　from　east　to　west　in　the　space　of　twenty…four　hours。　This　now；　as　it　appears　at　the　first　glance　anyway；　might　just　as　well　be　a　motion　of　the　earth　alone　as　of　all　the　rest　of　the　universe　with　the　exception　of　the　earth；　for　the　same　phenomena　would　result　from　either　hypothesis。　Beginning　with　the　most　general；　I　will　enumerate　the　reasons　which　seem　to　speak　in　favor　of　the　earth's　motion。　When　we　merely　consider　the　immensity　of　the　starry　sphere　in　comparison　with　the　smallness　of　the　terrestrial　ball；　which　is　contained　many　million　times　in　the　former；　and　then　think　of　the　rapidity　of　the　motion　which　completes　a　whole　rotation　in　one　day　and　night；　I　cannot　persuade　myself　how　any　one　can　hold　it　to　be　more　reasonable　and　credible　that　it　is　the　heavenly　sphere　which　rotates；　while　the　earth　stands　still。　〃Simplicio。　I　do　not　well　understand　how　that　powerful　motion　may　be　said　to　as　good　as　not　exist　for　the　sun；　the　moon；　the　other　planets；　and　the　innumerable　host　of　fixed　stars。　Do　you　call　that　nothing　when　the　sun　goes　from　one　meridian　to　another；　rises　up　over　this　horizon　and　sinks　behind　that　one；　brings　now　day；　and　now　night；　when　the　moon　goes　through　similar　changes；　and　the　other　planets　and　fixed　stars　in　the　same　way？　〃Salviati。　All　the　changes　you　mention　are　such　only　in　respect　to　the　earth。　To　convince　yourself　of　it；　only　imagine　the　earth　out　of　existence。　There　would　then　be　no　rising　and　setting　of　the　sun　or　of　the　moon；　no　horizon；　no　meridian；　no　day；　no　nightin　short；　the　said　motion　causes　no　change　of　any　sort　in　the　relation　of　the　sun　to　the　moon　or　to　any　of　the　other　heavenly　bodies；　be　they　planets　or　fixed　stars。　All　changes　are　rather　in　respect　to　the　earth；　they　may　all　be　reduced　to　the　simple　fact　that　the　sun　is　first　visible　in　China；　then　in　Persia；　afterwards　in　Egypt；　Greece；　France；　Spain；　America；　etc。；　and　that　the　same　thing　happens　with　the　moon　and　the　other　heavenly　bodies。　Exactly　the　same　thing　happens　and　in　exactly　the　same　way　if；　instead　of　disturbing　so　large　a　part　of　the　universe；　you　let　the　earth　revolve　about　itself。　The　difficulty　is；　however；　doubled；　inasmuch　as　a　second　very　important　problem　presents　itself。　If；　namely；　that　powerful　motion　is　ascribed　to　the　heavens；　it　is　absolutely　necessary　to　regard　it　as　opposed　to　the　individual　motion　of　all　the　planets；　every　one　of　which　indubitably　has　its　own　very　leisurely　and　moderate　movement　from　west　to　east。　If；　on　the　other　hand；　you　let　the　earth　move　about　itself；　this　opposition　of　motion　disappears。　〃The　improbability　is　tripled　by　the　complete　overthrow　of　that　order　which　rules　all　the　heavenly　bodies　in　which　the　revolving　motion　is　definitely　established。　The　greater　the　sphere　is　in　such　a　case；　so　much　longer　is　the　time　required　for　its　revolution；　the　smaller　the　sphere　the　shorter　the　time。　Saturn；　whose　orbit　surpasses　those　of　all　the　planets　in　size；　traverses　it　in　thirty　years。　Jupiter＇4＇　completes　its　smaller　course　in　twelve　years；　Mars　in　two；　the　moon　performs　its　much　smaller　revolution　within　a　month。　Just　as　clearly　in　the　Medicean　stars；　we　see　that　the　one　nearest　Jupiter　completes　its　revolution　in　a　very　short　timeabout　forty…two　hours；　the　next　in　about　three　and　one…half　days；　the　third　in　seven；　and　the　most　distant　one　in　sixteen　days。　This　rule；　which　is　followed　throughout；　will　still　remain　if　we　ascribe　the　twenty…four…hourly　motion　to　a　rotation　of　the　earth。　If；　however；　the　earth　is　left　motionless；　we　must　go　first　from　the　very　short　rule　of　the　moon　to　ever　greater　onesto　the　two…yearly　rule　of　Mars；　from　that　to　the　twelve…yearly　one　of　Jupiter；　from　here　to　the　thirty…yearly　one　of　Saturn；　and　then　suddenly　to　an　incomparably　greater　sphere；　to　which　also　we　must　ascribe　a　complete　rotation　in　twenty…four　hours。　If；　however；　we　assume　a　motion　of　the　earth；　the　rapidity　of　the　periods　is　very　well　preserved；　from　the　slowest　sphere　of　Saturn　we　come　to　the　wholly　motionless　fixed　stars。　We　also　escape　thereby　a　fourth　difficulty；　which　arises　as　soon　as　we　assume　that　there　is　motion　in　the　sphere　of　the　stars。　I　mean　the　great　unevenness　in　the　movement　of　these　very　stars；　some　of　which　would　have　to　revolve　with　extraordinary　rapidity　in　immense　circles；　while　others　moved　very　slowly　in　small　circles；　since　some　of　them　are　at　a　greater；　others　at　a　less；　distance　from　the　pole。　That　is　likewise　an　inconvenience；　for；　on　the　one　hand；　we　see　all　those　stars；　the　motion　of　which　is　indubitable；　revolve　in　great　circles；　while；　on　the　other　hand；　there　seems　to　be　little　object　in　placing　bodies；　which　are　to　move　in　circles；　at　an　enormous　distance　from　the　centre　and　then　let　them　move　in　very　small　circles。　And　not　only　are　the　size　of　the　different　circles　and　therewith　the　rapidity　of　the　movement　very　different　in　the　different　fixed　stars；　but　the　same　stars　also　change　their　orbits　and　their　rapidity　of　motion。　Therein　consists　the　fifth　inconvenience。　Those　stars；　namely；　which　were　at　the　equator　two　thousand　years　ago；　and　hence　described　great　circles　in　their　revolutions；　must　to…day　move　more　slowly　and　in　smaller　circles；　because　they　are　many　degrees　removed　from　it。　It　will　even　happen；　after　not　so　very　long　a　time；　that　one　of　those　which　have　hitherto　been　continually　in　motion　will　finally　coincide　with　the　pole　and　stand　still；　but　after　a　period　of　repose　will　again　begin　to　move。　The　other　stars　in　the　mean　while；　which　unquestionably　move；　all　have；　as　was　said；　a　great　circle　for　an　orbit　and　keep　this　unchangeably。　〃The　improbability　is　further　increasedthis　may　be　considered　the　sixth　inconvenienceby　the　fact　that　it　is　impossible　to　conceive　what　degree　of　solidity　those　immense　spheres　must　have；　in　the　depths　of　which　so　many　stars　are　fixed　so　enduringly　that　they　are　kept　revolving　evenly　in　spite　of　such　difference　of　motion　without　changing　their　respective　positions。　Or　if；　according　to　the　much　more　probable　theory；　the　heavens　are　fluid；　and　every　star　describes　an　orbit　of　its　own；　according　to　what　law　then；　or　for　what　reason；　are　their　orbits　so　arranged　that；　when　looked　at　from　the　earth；　they　appear　to　be　contained　in　one　single　sphere？　To　attain　this　it　seems　to　me　much　easier　and　more　convenient　to　make　them　motionless　instead　of　moving；　just　as　the　paving…stones　on　the　market…place；　for　instance；　remain　in　order　more　easily　than　the　swarms　of　children　running　about　on　them。　〃Finally；　the　seventh　difficulty：　If　we　attribute　the　daily　rotation　to　the　higher　region　of　the　heavens；　we　should　have　to　endow　it　with　force　and　power　sufficient　to　carry　with　it　the　innumerable　host　of　the　fixed　stars　every　one　a　body　of　very　great　compass　and　much　larger　than　the　earthand　all　the　planets；　although　the　latter；　like　the　earth；　move　naturally　in　an　opposite　direction。　In　the　midst　of　all　this　the　little　earth；　single　and　alone；　would　obstinately　and　wilfully　withstand　such　forcea　supposition　which；　it　appears　to　me；　has　much　against　it。　I　could　also　not　explain　why　the　earth；　a　freely　poised　body；　balancing　itself　about　its　centre；　and　surrounded　on　all　sides　by　a　fluid　medium；　should　not　be　affected　by　the　universal　rotation。　Such　difficulties；　however；　do　not　confront　us　if　we　attribute　motion　to　the　earthsuch　a　small；　insignificant　body　in　comparison　with　the　whole　universe；　and　which　for　that　very　reason　cannot　exercise　any　power　over　the　latter。　〃Simplicio。　You　support　your　arguments　throughout；　it　seems　to　me；　on　the　greater　ease　and　simplicity　with　which　the　said　effects　are　produced。　You　mean　that　as　a　cause　the　motion　of　the　earth　alone　is　just　as　satisfactory　as　the　motion　of　all　the　rest　of　the　universe　with　the　exception　of　the　earth；　you　hold　the　actual　event　to　be　much　easier　in　the　former　case　than　in　the　latter。　For　the　ruler　of　the　universe；　how