a history of science-2-第21章

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etical　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　strike　the　ground　simultaneously　with　a　ball　fired　horizontally　from　the　cannon。　As　to　the　paraboloid　course　pursued　by　the　projectile；　the　resistance　of　the　air　is　a　factor　which　Galileo　could　not　accurately　compute；　and　which　interferes　with　the　practical　realization　of　his　theory。　But　this　is　a　minor　consideration。　The　great　importance　of　his　idea　consists　in　the　recognition　that　such　a　force　as　that　of　gravitation　acts　in　precisely　the　same　way　upon　all　unsupported　bodies；　whether　or　not　such　bodies　be　at　the　same　time　acted　upon　by　a　force　of　translation。　Out　of　these　studies　of　moving　bodies　was　gradually　developed　a　correct　notion　of　several　important　general　laws　of　mechanicslaws　a　knowledge　of　which　was　absolutely　essential　to　the　progress　of　physical　science。　The　belief　in　the　rotation　of　the　earth　made　necessary　a　clear　conception　that　all　bodies　at　the　surface　of　the　earth　partake　of　that　motion　quite　independently　of　their　various　observed　motions　in　relation　to　one　another。　This　idea　was　hard　to　grasp；　as　an　oft…repeated　argument　shows。　It　was　asserted　again　and　again　that；　if　the　earth　rotates；　a　stone　dropped　from　the　top　of　a　tower　could　not　fall　at　the　foot　of　the　tower；　since　the　earth's　motion　would　sweep　the　tower　far　away　from　its　original　position　while　the　stone　is　in　transit。　This　was　one　of　the　stock　arguments　against　the　earth's　motion；　yet　it　was　one　that　could　be　refuted　with　the　greatest　ease　by　reasoning　from　strictly　analogous　experiments。　It　might　readily　be　observed；　for　example；　that　a　stone　dropped　from　a　moving　cart　does　not　strike　the　ground　directly　below　the　point　from　which　it　is　dropped；　but　partakes　of　the　forward　motion　of　the　cart。　If　any　one　doubt　this　he　has　but　to　jump　from　a　moving　cart　to　be　given　a　practical　demonstration　of　the　fact　that　his　entire　body　was　in　some　way　influenced　by　the　motion　of　translation。　Similarly；　the　simple　experiment　of　tossing　a　ball　from　the　deck　of　a　moving　ship　will　convince　any　one　that　the　ball　partakes　of　the　motion　of　the　ship；　so　that　it　can　be　manipulated　precisely　as　if　the　manipulator　were　standing　on　the　earth。　In　short；　every…day　experience　gives　us　illustrations　of　what　might　be　called　compound　motion；　which　makes　it　seem　altogether　plausible　that；　if　the　earth　is　in　motion；　objects　at　its　surface　will　partake　of　that　motion　in　a　way　that　does　not　interfere　with　any　other　movements　to　which　they　may　be　subjected。　As　the　Copernican　doctrine　made　its　way；　this　idea　of　compound　motion　naturally　received　more　and　more　attention；　and　such　experiments　as　those　of　Galileo　prepared　the　way　for　a　new　interpretation　of　the　mechanical　principles　involved。　The　great　difficulty　was　that　the　subject　of　moving　bodies　had　all　along　been　contemplated　from　a　wrong　point　of　view。　Since　force　must　be　applied　to　an　object　to　put　it　in　motion；　it　was　perhaps　not　unnaturally　assumed　that　similar　force　must　continue　to　be　applied　to　keep　the　object　in　motion。　When；　for　example；　a　stone　is　thrown　from　the　hand；　the　direct　force　applied　necessarily　ceases