a history of science-2-第11章

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t；　of　course；　we　have　no　present　concern；　but　it　is　worth　our　while　to　inquire　at　some　length　as　to　the　famous　painter's　accomplishments　as　a　scientist。　From　a　passage　in　the　works　of　Leonardo；　first　brought　to　light　by　Venturi；＇1＇　it　would　seem　that　the　great　painter　anticipated　Copernicus　in　determining　the　movement　of　the　earth。　He　made　mathematical　calculations　to　prove　this；　and　appears　to　have　reached　the　definite　conclusion　that　the　earth　does　moveor　what　amounts　to　the　same　thing；　that　the　sun　does　not　move。　Muntz　is　authority　for　the　statement　that　in　one　of　his　writings　he　declares；　〃Il　sole　non　si　mouve〃the　sun　does　not　move。＇2＇　Among　his　inventions　is　a　dynamometer　for　determining　the　traction　power　of　machines　and　animals；　and　his　experiments　with　steam　have　led　some　of　his　enthusiastic　partisans　to　claim　for　him　priority　to　Watt　in　the　invention　of　the　steam…engine。　In　these　experiments；　however；　Leonardo　seems　to　have　advanced　little　beyond　Hero　of　Alexandria　and　his　steam　toy。　Hero's　steam…engine　did　nothing　but　rotate　itself　by　virtue　of　escaping　jets　of　steam　forced　from　the　bent　tubes；　while　Leonardo's　〃steam…engine〃　〃drove　a　ball　weighing　one　talent　over　a　distance　of　six　stadia。〃　In　a　manuscript　now　in　the　library　of　the　Institut　de　France；　Da　Vinci　describes　this　engine　minutely。　The　action　of　this　machine　was　due　to　the　sudden　conversion　of　small　quantities　of　water　into　steam　（〃smoke；〃　as　he　called　it）　by　coming　suddenly　in　contact　with　a　heated　surface　in　a　proper　receptacle；　the　rapidly　formed　steam　acting　as　a　propulsive　force　after　the　manner　of　an　explosive。　It　is　really　a　steam…gun；　rather　than　a　steam…engine；　and　it　is　not　unlikely　that　the　study　of　the　action　of　gunpowder　may　have　suggested　it　to　Leonardo。　It　is　believed　that　Leonardo　is　the　true　discoverer　of　the　camera…obscura；　although　the　Neapolitan　philosopher；　Giambattista　Porta；　who　was　not　born　until　some　twenty　years　after　the　death　of　Leonardo；　is　usually　credited　with　first　describing　this　device。　There　is　little　doubt；　however；　that　Da　Vinci　understood　the　principle　of　this　mechanism；　for　he　describes　how　such　a　camera　can　be　made　by　cutting　a　small；　round　hole　through　the　shutter　of　a　darkened　room；　the　reversed　image　of　objects　outside　being　shown　on　the　opposite　wall。　Like　other　philosophers　in　all　ages；　he　had　observed　a　great　number　of　facts　which　he　was　unable　to　explain　correctly。　But　such　accumulations　of　scientific　observations　are　always　interesting；　as　showing　how　many　centuries　of　observation　frequently　precede　correct　explanation。　He　observed　many　facts　about　sounds；　among　others　that　blows　struck　upon　a　bell　produced　sympathetic　sounds　in　a　bell　of　the　same　kind；　and　that　striking　the　string　of　a　lute　produced　vibration　in　corresponding　strings　of　lutes　strung　to　the　same　pitch。　He　knew；　also；　that　sounds　could　be　heard　at　a　distance　at　sea　by　listening　at　one　end　of　a　tube；　the　other　end　of　which　was　placed　in　the　water；　and　that　the　same　expedient　worked　successfully　on　land；　the　end　of　the　tube　being　placed　against　the　ground。　The　knowledge　of　this　great　number　of　unexplained　facts　is　often　interpreted　by　the　admirers　of　Da　Vinci；　as　showing　an　almost　occult　insight　into　science　many　centuries　in　advance　of　his　time。　Such　interpretations；　however；　are　illusive。　The　observation；　for　example；　that　a　tube　placed　against　the　ground　enables　one　to　hear　movements　on　the　earth　at　a　distance；　is　not　in　itself　evidence　of　anything　more　than　acute　scientific　observation；　as　a　similar　method　is　in　use　among　almost　every　race　of　savages；　notably　the　American　Indians。　On　the　other　hand；　one　is　inclined　to　give　credence　to　almost　any　story　of　the　breadth　of　knowledge　of　the　man　who　came　so　near　anticipating　Hutton；　Lyell；　and　Darwin　in　his　interpretation　of　the　geological　records　as　he　found　them　written　on　the　rocks。　It　is　in　this　field　of　geology　that　Leonardo　is　entitled　to　the　greatest　admiration　by　modern　scientists。　He　had　observed　the　deposit　of　fossil　shells　in　various　strata　of　rocks；　even　on　the　tops　of　mountains；　and　he　rejected　once　for　all　the　theory　that　they　had　been　deposited　there　by　the　Deluge。　He　rightly　interpreted　their　presence　as　evidence　that　they　had　once　been　deposited　at　the　bottom　of　the　sea。　This　process　he　assumed　bad　taken　hundreds　and　thousands　of　centuries；　thus　tacitly　rejecting　the　biblical　tradition　as　to　the　date　of　the　creation。　Notwithstanding　the　obvious　interest　that　attaches　to　the　investigations　of　Leonardo；　it　must　be　admitted　that　his　work　in　science　remained　almost　as　infertile　as　that　of　his　great　precursor；　Bacon。　The　really　stimulative　work　of　this　generation　was　done　by　a　man　of　affairs；　who　knew　little　of　theoretical　science　except　in　one　line；　but　who　pursued　that　one　practical　line　until　he　achieved　a　wonderful　result。　This　man　was　Christopher　Columbus。　It　is　not　necessary　here　to　tell　the　trite　story　of　his　accomplishment。　Suffice　it　that　his　practical　demonstration　of　the　rotundity　of　the　earth　is　regarded　by　most　modern　writers　as　marking　an　epoch　in　history。　With　the　year　of　his　voyage　the　epoch　of　the　Middle　Ages　is　usually　regarded　as　coming　to　an　end。　It　must　not　be　supposed　that　any　very　sudden　change　came　over　the　aspect　of　scholarship　of　the　time；　but　the　preliminaries　of　great　things　had　been　achieved；　and　when　Columbus　made　his　famous　voyage　in　1492；　the　man　was　already　alive　who　was　to　bring　forward　the　first　great　vitalizing　thought　in　the　field　of　pure　science　that　the　Western　world　had　originated　for　more　than　a　thousand　years。　This　man　bore　the　name　of　Kopernik；　or　in　its　familiar　Anglicized　form；　Copernicus。　His　life　work　and　that　of　his　disciples　will　claim　our　attention　in　the　succeeding　chapter。

IV。　THE　NEW　COSMOLOGYCOPERNICUS　TO　KEPLER　AND　GALILEO　We　have　seen　that　the　Ptolemaic　astronomy；　which　was　the　accepted　doctrine　throughout　the　Middle　Ages；　taught　that　the　earth　is　round。　Doubtless　there　was　a　popular　opinion　current　which　regarded　the　earth　as　flat；　but　it　must　be　understood　that　this　opinion　had　no　champions　among　men　of　science　during　the　Middle　Ages。　When；　in　the　year　1492；　Columbus　sailed　out　to　the　west　on　his　memorable　voyage；　his　expectation　of　reaching　India　had　full　scientific　warrant；　however　much　it　may　have　been　scouted　by　certain　ecclesiastics　and　by　the　average　man　of　the　period。　Nevertheless；　we　may　well　suppose　that　the　successful　voyage　of　Columbus；　and　the　still　more　demonstrative　one　made　about　thirty　years　later　by　Magellan；　gave　the　theory　of　the　earth's　rotundity　a　certainty　it　could　never　previously　have　had。　Alexandrian　geographers　had　measured　the　size　of　the　earth；　and　had　not　hesitated　to　assert　that　by　sailing　westward　one　might　reach　India。　But　there　is　a　wide　gap　between　theory　and　practice；　and　it　required　the　voyages　of　Columbus　and　his　successors　to　bridge　that　gap。　After　the　companions　of　Magellan　completed　the　circumnavigation　of　the　globe；　the　general　shape　of　our　earth　would；　obviously；　never　again　be　called　in　question。　But　demonstration　of　the　sphericity　of　the　earth　had；　of　course；　no　direct　bearing　upon　the　question　of　the　earth's　position　in　the　universe。　Therefore　the　voyage　of　Magellan　served　to　fortify；　r