a history of science-2-第3部分

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oriental　spirit。　They　were　under　the　spell　of　tradition；　and；　in　the　main；　what　they　accepted　from　the　Greeks　they　regarded　as　almost　final　in　its　teaching。　There　were；　however；　a　few　notable　exceptions　among　their　men　of　science；　and　to　these　must　be　ascribed　several　discoveries　of　some　importance。　The　chief　subjects　that　excited　the　interest　and　exercised　the　ingenuity　of　the　Arabian　scholars　were　astronomy；　mathematics；　and　medicine。　The　practical　phases　of　all　these　subjects　were　given　particular　attention。　Thus　it　is　well　known　that　our　so…called　Arabian　numerals　date　from　this　period。　The　revolutionary　effect　of　these　characters；　as　applied　to　practical　mathematics；　can　hardly　be　overestimated；　but　it　is　generally　considered；　and　in　fact　was　admitted　by　the　Arabs　themselves；　that　these　numerals　were　really　borrowed　from　the　Hindoos；　with　whom　the　Arabs　came　in　contact　on　the　east。　Certain　of　the　Hindoo　alphabets；　notably　that　of　the　Battaks　of　Sumatra；　give　us　clews　to　the　originals　of　the　numerals。　It　does　not　seem　certain；　however；　that　the　Hindoos　employed　these　characters　according　to　the　decimal　system；　which　is　the　prime　element　of　their　importance。　Knowledge　is　not　forthcoming　as　to　just　when　or　by　whom　such　application　was　made。　If　this　was　an　Arabic　innovation；　it　was　perhaps　the　most　important　one　with　which　that　nation　is　to　be　credited。　Another　mathematical　improvement　was　the　introduction　into　trigonometry　of　the　sinethe　half…chord　of　the　double　arcinstead　of　the　chord　of　the　arc　itself　which　the　Greek　astronomers　had　employed。　This　improvement　was　due　to　the　famous　Albategnius；　whose　work　in　other　fields　we　shall　examine　in　a　moment。　Another　evidence　of　practicality　was　shown　in　the　Arabian　method　of　attempting　to　advance　upon　Eratosthenes'　measurement　of　the　earth。　Instead　of　trusting　to　the　measurement　of　angles；　the　Arabs　decided　to　measure　directly　a　degree　of　the　earth's　surfaceor　rather　two　degrees。　Selecting　a　level　plain　in　Mesopotamia　for　the　experiment；　one　party　of　the　surveyors　progressed　northward；　another　party　southward；　from　a　given　point　to　the　distance　of　one　degree　of　arc；　as　determined　by　astronomical　observations。　The　result　found　was　fifty…six　miles　for　the　northern　degree；　and　fifty…six　and　two…third　miles　for　the　southern。　Unfortunately；　we　do　not　know　the　precise　length　of　the　mile　in　question；　and　therefore　cannot　be　assured　as　to　the　accuracy　of　the　measurement。　It　is　interesting　to　note；　however；　that　the　two　degrees　were　found　of　unequal　lengths；　suggesting　that　the　earth　is　not　a　perfect　spherea　suggestion　the　validity　of　which　was　not　to　be　put　to　the　test　of　conclusive　measurements　until　about　the　close　of　the　eighteenth　century。　The　Arab　measurement　was　made　in　the　time　of　Caliph　Abdallah　al…Mamun；　the　son　of　the　famous　Harun…al…Rashid。　Both　father　and　son　were　famous　for　their　interest　in　science。　Harun…al…Rashid　was；　it　will　be　recalled；　the　friend　of　Charlemagne。　It　is　said　that　he　sent　that　ruler；　as　a　token　of　friendship；　a　marvellous　clock　which　let　fall　a　metal　ball　to　mark　the　hours。　This　mechanism；　which　is　alleged　to　have　excited　great　wonder　in　the　West；　furnishes　yet　another　instance　of　Arabian　practicality。　Perhaps　the　greatest　of　the　Arabian　astronomers　was　Mohammed　ben　Jabir　Albategnius；　or　El…batani；　who　was　born　at　Batan；　in　Mesopotamia；　about　the　year　850　A。D。；　and　died　in　929。　Albategnius　was　a　student　of　the　Ptolemaic　astronomy；　but　he　was　also　a　practical　observer。　He　made　the　important　discovery　of　the　motion　of　the　solar　apogee。　That　is　to　say；　he　found　that　the　position　of　the　sun　among　the　stars；　at　the　time　of　its　greatest　distance　from　the　earth；　was　not　what　it　had　been　in　the　time　of　Ptolemy。　The　Greek　astronomer　placed　the　sun　in　longitude　65　degrees；　but　Albategnius　found　it　in　longitude　82　degrees；　a　distance　too　great　to　be　accounted　for　by　inaccuracy　of　measurement。　The　modern　inference　from　this　observation　is　that　the　solar　system　is　moving　through　space；　but　of　course　this　inference　could　not　well　be　drawn　while　the　earth　was　regarded　as　the　fixed　centre　of　the　universe。　In　the　eleventh　century　another　Arabian　discoverer；　Arzachel；　observing　the　sun　to　be　less　advanced　than　Albategnius　had　found　it；　inferred　incorrectly　that　the　sun　had　receded　in　the　mean　time。　The　modern　explanation　of　this　observation　is　that　the　measurement　of　Albategnius　was　somewhat　in　error；　since　we　know　that　the　sun's　motion　is　steadily　progressive。　Arzachel；　however；　accepting　the　measurement　of　his　predecessor；　drew　the　false　inference　of　an　oscillatory　motion　of　the　stars；　the　idea　of　the　motion　of　the　solar　system　not　being　permissible。　This　assumed　phenomenon；　which　really　has　no　existence　in　point　of　fact；　was　named　the　〃trepidation　of　the　fixed　stars；〃　and　was　for　centuries　accepted　as　an　actual　phenomenon。　Arzachel　explained　this　supposed　phenomenon　by　assuming　that　the　equinoctial　points；　or　the　points　of　intersection　of　the　equator　and　the　ecliptic；　revolve　in　circles　of　eight　degrees'　radius。　The　first　points　of　Aries　and　Libra　were　supposed　to　describe　the　circumference　of　these　circles　in　about　eight　hundred　years。　All　of　which　illustrates　how　a　difficult　and　false　explanation　may　take　the　place　of　a　simple　and　correct　one。　The　observations　of　later　generations　have　shown　conclusively　that　the　sun's　shift　of　position　is　regularly　progressive；　hence　that　there　is　no　〃trepidation〃　of　the　stars　and　no　revolution　of　the　equinoctial　points。　If　the　Arabs　were　wrong　as　regards　this　supposed　motion　of　the　fixed　stars；　they　made　at　least　one　correct　observation　as　to　the　inequality　of　motion　of　the　moon。　Two　inequalities　of　the　motion　of　this　body　were　already　known。　A　third；　called　the　moon's　variation；　was　discovered　by　an　Arabian　astronomer　who　lived　at　Cairo　and　observed　at　Bagdad　in　975；　and　who　bore　the　formidable　name　of　Mohammed　Aboul　Wefaal…Bouzdjani。　The　inequality　of　motion　in　question；　in　virtue　of　which　the　moon　moves　quickest　when　she　is　at　new　or　full；　and　slowest　at　the　first　and　third　quarter；　was　rediscovered　by　Tycho　Brahe　six　centuries　later；　a　fact　which　in　itself　evidences　the　neglect　of　the　Arabian　astronomer's　discovery　by　his　immediate　successors。　In　the　ninth　and　tenth　centuries　the　Arabian　city　of　Cordova；　in　Spain；　was　another　important　centre　of　scientific　influence。　There　was　a　library　of　several　hundred　thousand　volumes　here；　and　a　college　where　mathematics　and　astronomy　were　taught。　Granada；　Toledo；　and　Salamanca　were　also　important　centres；　to　which　students　flocked　from　western　Europe。　It　was　the　proximity　of　these　Arabian　centres　that　stimulated　the　scientific　interests　of　Alfonso　X。　of　Castile；　at　whose　instance　the　celebrated　Alfonsine　tables　were　constructed。　A　familiar　story　records　that　Alfonso；　pondering　the　complications　of　the　Ptolemaic　cycles　and　epicycles；　was　led　to　remark　that；　had　he　been　consulted　at　the　time　of　creation；　he　could　have　suggested　a　much　better　and　simpler　plan　for　the　universe。　Some　centuries　were　to　elapse　before　Copernicus　was　to　show　that　it　was　not　the　plan　of　the　universe；　but　man's　interpretation　of　it；　that　was　at　fault。　Another　royal　personage　who　came　under　Arabian　influence　was　Frederick　II。　of　Sicilythe　〃Wonder　of　the　World；〃　as　he　was　called　by　his　contemporaries。　The　Almagest　of　Ptolemy　was　translated　into　Latin　at　his　instance；　being　introduced　to　the　Western　world　through　this　curious　channel。　At　this　time　it　became　quite　usual　for　the　Italian　and　Spanish　scholars　to　understand　Arabic　although　they　were　totally　ignorant　of　Greek。　In　the　field　of　physical　science　one　of　the　most　important　of　the　Arabian　scientists　was　Alhazen。　His　work；　published　about　the　year　1100　A。D。；　had　great　celebrity　throughout　the　mediaeval　period。　The　original　investigations　of　Alhazen　had　to　do　largely　with　optics。　He　made　particular　studies　of　the　eye　itself；　and　the　names　given　by　him　to　various　parts　of　the　eye；　as　the　vitreous　humor；　the　cornea；　and　the　retina；　are　still　retained　by　anatomists。　It　is　known　that　Ptolemy　had　studied　the　refraction　of　light；　and　that　he；　in　common　with　his　immediate　predecessors；　was　aware　that　atmospheric　refraction　affects　the　apparent　position　of　stars　near　the　horizon。　Alhazen　carried　forward　these　studies；　and　was　led　through　them　to　make　the　first　recorded　scientific　estimate　of　the　phenomena　of　twilight　and　of　the　height　of　the　atmosphere。　The　persistence　of　a　glow　in　the　atmosphere　after　the　sun　has　disappeared　beneath　the　horizon　is　so　familiar　a　phenomenon　that　the　ancient　philosophers　seem　not　to　have　thought　of　it　as　requiring　an　explanation。　Yet　a　moment's　consideration　makes　it　clear　that；　if　light　travels　in　straight　lines　and　the　rays　of　the　sun　were　in　no　wise　deflected；　the　complete　darkness　of　night　should　instantly　succeed　to　day　when　the　sun　passes　below　the　horizon。　That　this　sudden　change　does　not　occur；　Alhazen　explained　as　due　to　the　reflection　of　light　by　the　earth's　atmosphere。　Alhazen　appears　to　have　conceived　the　atmosphere　as　a　sharply　defined　layer；　and；　assuming　that　twilight　continues　only　so　long　as　rays　of　the　sun　reflected　from　the　outer　surface　of　this　layer　can　reach　the　spectator　at　any　given　point；　he　hit　upon　a　means　of　measurement　that　seem