a history of science-2-第3章

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ian　translation；　and　reference　has　more　than　once　been　made　to　the　Arabic　translation　of　Ptolemy's　great　work；　to　which　we　still　apply　its　Arabic　title　of　Almagest。　The　familiar　story　that　when　the　Arabs　invaded　Egypt　they　burned　the　Alexandrian　library　is　now　regarded　as　an　invention　of　later　times。　It　seems　much　more　probable　that　the　library　bad　been　largely　scattered　before　the　coming　of　the　Moslems。　Indeed；　it　has　even　been　suggested　that　the　Christians　of　an　earlier　day　removed　the　records　of　pagan　thought。　Be　that　as　it　may；　the　famous　Alexandrian　library　had　disappeared　long　before　the　revival　of　interest　in　classical　learning。　Meanwhile；　as　we　have　said；　the　Arabs；　far　from　destroying　the　western　literature；　were　its　chief　preservers。　Partly　at　least　because　of　their　regard　for　the　records　of　the　creative　work　of　earlier　generations　of　alien　peoples；　the　Arabs　were　enabled　to　outstrip　their　contemporaries。　For　it　cannot　be　in　doubt　that；　during　that　long　stretch　of　time　when　the　western　world　was　ignoring　science　altogether　or　at　most　contenting　itself　with　the　casual　reading　of　Aristotle　and　Pliny；　the　Arabs　had　the　unique　distinction　of　attempting　original　investigations　in　science。　To　them　were　due　all　important　progressive　steps　which　were　made　in　any　scientific　field　whatever　for　about　a　thousand　years　after　the　time　of　Ptolemy　and　Galen。　The　progress　made　even　by　the　Arabs　during　this　long　period　seems　meagre　enough；　yet　it　has　some　significant　features。　These　will　now　demand　our　attention。

II。　MEDIAEVAL　SCIENCE　AMONG　THE　ARABIANS　The　successors　of　Mohammed　showed　themselves　curiously　receptive　of　the　ideas　of　the　western　people　whom　they　conquered。　They　came　in　contact　with　the　Greeks　in　western　Asia　and　in　Egypt；　and；　as　has　been　said；　became　their　virtual　successors　in　carrying　forward　the　torch　of　learning。　It　must　not　be　inferred；　however；　that　the　Arabian　scholars；　as　a　class；　were　comparable　to　their　predecessors　in　creative　genius。　On　the　contrary；　they　retained　much　of　the　conservative　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　immediat