a history of science-2-第18章

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　is　fifty　thousand　times　greater　than　the　latter。　If；　therefore；　we　consider　the　moving　bodies；　we　must　unquestionably　regard　the　motion　of　the　earth　as　a　much　simpler　process　than　that　of　the　universe；　if；　furthermore；　we　direct　our　attention　to　so　many　other　simplifications　which　may　be　reached　only　by　this　theory；　the　daily　movement　of　the　earth　must　appear　much　more　probable　than　the　motion　of　the　universe　without　the　earth；　for；　according　to　Aristotle's　just　axiom；　'Frustra　fit　per　plura；　quod　potest　fieri　per　p　auciora'　（It　is　vain　to　expend　many　means　where　a　few　are　sufficient）。〃＇2＇

The　work　was　widely　circulated；　and　it　was　received　with　an　interest　which　bespeaks　a　wide…spread　undercurrent　of　belief　in　the　Copernican　doctrine。　Naturally　enough；　it　attracted　immediate　attention　from　the　church　authorities。　Galileo　was　summoned　to　appear　at　Rome　to　defend　his　conduct。　The　philosopher；　who　was　now　in　his　seventieth　year；　pleaded　age　and　infirmity。　He　had　no　desire　for　personal　experience　of　the　tribunal　of　the　Inquisition；　but　the　mandate　was　repeated；　and　Galileo　went　to　Rome。　There；　as　every　one　knows；　he　disavowed　any　intention　to　oppose　the　teachings　of　Scripture；　and　formally　renounced　the　heretical　doctrine　of　the　earth's　motion。　According　to　a　tale　which　so　long　passed　current　that　every　historian　must　still　repeat　it　though　no　one　now　believes　it　authentic；　Galileo　qualified　his　renunciation　by　muttering　to　himself；　〃E　pur　si　muove〃　（It　does　move；　none　the　less）；　as　he　rose　to　his　feet　and　retired　from　the　presence　of　his　persecutors。　The　tale　is　one　of　those　fictions　which　the　dramatic　sense　of　humanity　is　wont　to　impose　upon　history；　but；　like　most　such　fictions；　it　expresses　the　spirit　if　not　the　letter　of　truth；　for　just　as　no　one　believes　that　Galileo's　lips　uttered　the　phrase；　so　no　one　doubts　that　the　rebellious　words　were　in　his　mind。　After　his　formal　renunciation；　Galileo　was　allowed　to　depart；　but　with　the　injunction　that　he　abstain　in　future　from　heretical　teaching。　The　remaining　ten　years　of　his　life　were　devoted　chiefly　to　mechanics；　where　his　experiments　fortunately　opposed　the　Aristotelian　rather　than　the　Hebrew　teachings。　Galileo's　death　occurred　in　1642；　a　hundred　years　after　the　death　of　Copernicus。　Kepler　had　died　thirteen　years　before；　and　there　remained　no　astronomer　in　the　field　who　is　conspicuous　in　the　history　of　science　as　a　champion　of　the　Copernican　doctrine。　But　in　truth　it　might　be　said　that　the　theory　no　longer　needed　a　champion。　The　researches　of　Kepler　and　Galileo　had　produced　a　mass　of　evidence　for　the　Copernican　theory　which　amounted　to　demonstration。　A　generation　or　two　might　be　required　for　this　evidence　to　make　itself　everywhere　known　among　men　of　science；　and　of　course　the　ecclesiastical　authorities　must　be　expected　to　stand　by　their　guns　for　a　somewhat　longer　period。　In　point　of　fact；　the　ecclesiastical　ban　was　not　technically　removed　by　the　striking　of　the　Copernican　books　from　the　list　of　the　Index　Expurgatorius　until　the　year　1822；　almost　two　hundred　years　after　the　date　of　Galileo's　dialogue。　But　this；　of　course；　is　in　no　sense　a　guide　to　the　state　of　general　opinion　regarding　the　theory。　We　shall　gain　a　true　gauge　as　to　this　if　we　assume　that　the　greater　number　of　important　thinkers　had　accepted　the　heliocentric　doctrine　before　the　middle　of　the　seventeenth　century；　and　that　before　the　close　of　that　century　the　old　Ptolemaic　idea　had　been　quite　abandoned。　A　wonderful　revolution　in　man's　estimate　of　the　universe　had　thus　been　effected　within　about　two　centuries　after　the　birth　of　Copernicus。

V。　GALILEO　AND　THE　NEW　PHYSICS　After　Galileo　had　felt　the　strong　hand　of　the　Inquisition；　in　1632；　he　was　careful　to　confine　his　researches；　or　at　least　his　publications；　to　topics　that　seemed　free　from　theological　implications。　In　doing　so　he　reverted　to　the　field　of　his　earliest　studies　namely；　the　field　of　mechanics；　and　the　Dialoghi　delle　Nuove　Scienze；　which　he　finished　in　1636；　and　which　was　printed　two　years　later；　attained　a　celebrity　no　less　than　that　of　the　heretical　dialogue　that　had　preceded　it。　The　later　work　was　free　from　all　apparent　heresies；　yet　perhaps　it　did　more　towards　the　establishment　of　the　Copernican　doctrine；　through　the　teaching　of　correct　mechanical　principles；　than　the　other　work　had　accomplished　by　a　more　direct　method。　Galileo's　astronomical　discoveries　were；　as　we　have　seen；　in　a　sense　accidental；　at　least；　they　received　their　inception　through　the　inventive　genius　of　another。　His　mechanical　discoveries；　on　the　other　hand；　were　the　natural　output　of　his　own　creative　genius。　At　the　very　beginning　of　his　career；　while　yet　a　very　young　man；　though　a　professor　of　mathematics　at　Pisa；　he　had　begun　that　onslaught　upon　the　old　Aristotelian　ideas　which　he　was　to　continue　throughout　his　life。　At　the　famous　leaning　tower　in　Pisa；　the　young　iconoclast　performed；　in　the　year　1590；　one　of　the　most　theatrical　demonstrations　in　the　history　of　science。　Assembling　a　multitude　of　champions　of　the　old　ideas；　he　proposed　to　demonstrate　the　falsity　of　the　Aristotelian　doctrine　that　the　velocity　of　falling　bodies　is　proportionate　to　their　weight。　There　is　perhaps　no　fact　more　strongly　illustrative　of　the　temper　of　the　Middle　Ages　than　the　fact　that　this　doctrine；　as　taught　by　the　Aristotelian　philosopher；　should　so　long　have　gone　unchallenged。　Now；　however；　it　was　put　to　the　test；　Galileo　released　a　half…pound　weight　and　a　hundred…pound　cannon…ball　from　near　the　top　of　the　tower；　and；　needless　to　say；　they　reached　the　ground　together。　Of　course；　the　spectators　were　but　little　pleased　with　what　they　saw。　They　could　not　doubt　the　evidence　of　their　own　senses　as　to　the　particular　experiment　in　question；　they　could　suggest；　however；　that　the　experiment　involved　a　violation　of　the　laws　of　nature　through　the　practice　of　magic。　To　controvert　so　firmly　established　an　idea　savored　of　heresy。　The　young　man　guilty　of　such　iconoclasm　was　naturally　looked　at　askance　by　the　scholarship　of　his　time。　Instead　of　being　applauded；　he　was　hissed；　and　he　found　it　expedient　presently　to　retire　from　Pisa。　Fortunately；　however；　the　new　spirit　of　progress　had　made　itself　felt　more　effectively　in　some　other　portions　of　Italy；　and　so　Galileo　found　a　refuge　and　a　following　in　Padua；　and　afterwards　in　Florence；　and　while；　as　we　have　seen；　he　was　obliged　to　curb　his　enthusiasm　regarding　the　subject　that　was　perhaps　nearest　his　heartthe　promulgation　of　the　Copernican　theoryyet　he　was　permitted　in　the　main　to　carry　on　his　experimental　observations　unrestrained。　These　experiments　gave　him　a　place　of　unquestioned　authority　among　his　contemporaries；　and　they　have　transmitted　his　name　to　posterity　as　that　of　one　of　the　greatest　of　experimenters　and　the　virtual　founder　of　modern　mechanical　science。　The　experiments　in　question　range　over　a　wide　field；　but　for　the　most　part　they　have　to　do　with　moving　bodies　and　with　questions　of　force；　or；　as　we　should　now　say；　of　energy。　The　experiment　at　the　leaning　tower　showed　that　the　velocity　of　falling　bodies　is　independent　of　the　weight　of　the　bodies；　provided　the　weight　is　sufficient　to　overcome　the　resistance　of　the　atmosphere。　Later　experiments　with　falling　bodies　led　to　the　discovery　of　laws　regarding　the　accelerated　velocity　of　fall。　Such　velocities　were　found　to　bear　a　simple　relation　to　the　period　of　time　from　the　beginning　of　the　fall。　Other　experiments；　in　which　balls　were　allowed　to　roll　down　inclined　planes；　corroborated　the　observation　that　the　pull　of　gravitation　gave　a　velocity　proportionate　to　the　length　of　fall；　whether　such　fall　were　direct　or　in　a　slanting　direction。　These　studies　were　associated　with　observations　on　projectiles；　regarding　which　Galileo　was　the　first　to　entertain　correct　notions。　According　to　the　current　idea；　a　projectile　fired；　for　example；　from　a　cannon；　moved　in　a　straight　horizontal　line　until　the　propulsive　force　was　exhausted；　and　then　fell　to　the　ground　in　a　perpendicular　line。　Galileo　taught　that　the　projectile　begins　to　fall　at　once　on　leaving　the　mouth　of　the　cannon　and　traverses　a　parabolic　course。　According　to　his　idea；　which　is　now　familiar　to　every　one；　a　cannon…ball　dropped　from　the　level　of　the　cannon's　muzzle　will　stri