a history of science-1-第48章

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must　be　said　as　to　his　specific　discoveries　in　this　field。　According　to　his　measurement；　the　tropic　year　consists　of　365　days；　5　hours；　and　49　minutes；　varying　thus　only　12　seconds　from　the　true　year；　as　the　modern　astronomer　estimates　it。　Yet　more　remarkable；　because　of　the　greater　difficulties　involved；　was　Hipparchus's　attempt　to　measure　the　actual　distance　of　the　moon。　Aristarchus　had　made　a　similar　attempt　before　him。　Hipparchus　based　his　computations　on　studies　of　the　moon　in　eclipse；　and　he　reached　the　conclusion　that　the　distance　of　the　moon　is　equal　to　59　radii　of　the　earth　（in　reality　it　is　60。27　radii）。　Here；　then；　was　the　measure　of　the　base…line　of　that　famous　triangle　with　which　Aristarchus　had　measured　the　distance　of　the　sun。　Hipparchus　must　have　known　of　that　measurement；　since　he　quotes　the　work　of　Aristarchus　in　other　fields。　Had　he　now　but　repeated　the　experiment　of　Aristarchus；　with　his　perfected　instruments　and　his　perhaps　greater　observational　skill；　he　was　in　position　to　compute　the　actual　distance　of　the　sun　in　terms　not　merely　of　the　moon's　distance　but　of　the　earth's　radius。　And　now　there　was　the　experiment　of　Eratosthenes　to　give　the　length　of　that　radius　in　precise　terms。　In　other　words；　Hipparchus　might　have　measured　the　distance　of　the　sun　in　stadia。　But　if　he　had　made　the　attemptand；　indeed；　it　is　more　than　likely　that　he　did　sothe　elements　of　error　in　his　measurements　would　still　have　kept　him　wide　of　the　true　figures。　The　chief　studies　of　Hipparchus　were　directed；　as　we　have　seen；　towards　the　sun　and　the　moon；　but　a　phenomenon　that　occurred　in　the　year　134　B。C。　led　him　for　a　time　to　give　more　particular　attention　to　the　fixed　stars。　The　phenomenon　in　question　was　the　sudden　outburst　of　a　new　star；　a　phenomenon　which　has　been　repeated　now　and　again；　but　which　is　sufficiently　rare　and　sufficiently　mysterious　to　have　excited　the　unusual　attention　of　astronomers　in　all　generations。　Modern　science　offers　an　explanation　of　the　phenomenon；　as　we　shall　see　in　due　course。　We　do　not　know　that　Hipparchus　attempted　to　explain　it；　but　he　was　led　to　make　a　chart　of　the　heavens；　probably　with　the　idea　of　guiding　future　observers　in　the　observation　of　new　stars。　Here　again　Hipparchus　was　not　altogether　an　innovator；　since　a　chart　showing　the　brightest　stars　had　been　made　by　Eratosthenes；　but　the　new　charts　were　much　elaborated。　The　studies　of　Hipparchus　led　him　to　observe　the　stars　chiefly　with　reference　to　the　meridian　rather　than　with　reference　to　their　rising；　as　had　hitherto　been　the　custom。　In　making　these　studies　of　the　relative　position　of　the　stars；　Hipparchus　was　led　to　compare　his　observations　with　those　of　the　Babylonians；　which；　it　was　said；　Alexander　had　caused　to　be　transmitted　to　Greece。　He　made　use　also　of　the　observations　of　Aristarchus　and　others　of　his　Greek　precursors。　The　result　of　his　comparisons　proved　that　the　sphere　of　the　fixed　stars　had　apparently　shifted　its　position　in　reference　to　the　plane　of　the　sun's　orbitthat　is　to　say；　the　plane　of　the　ecliptic　no　longer　seemed　to　cut　the　sphere　of　the　fixed　stars　at　precisely　the　point　where　the　two　coincided　in　former　centuries。　The　plane　of　the　ecliptic　must　therefore　be　conceived　as　slowly　revolving　in　such　a　way　as　gradually　to　circumnavigate　the　heavens。　This　important　phenomenon　is　described　as　the　precession　of　the　equinoxes。　It　is　much　in　question　whether　this　phenomenon　was　not　known　to　the　ancient　Egyptian　astronomers；　but　in　any　event；　Hipparchus　is　to　be　credited　with　demonstrating　the　fact　and　making　it　known　to　the　Western　world。　A　further　service　was　rendered　theoretical　astronomy　by　Hipparchus　through　his　invention　of　the　planosphere；　an　instrument　for　the　representation　of　the　mechanism　of　the　heavens。　His　computations　of　the　properties　of　the　spheres　led　him　also　to　what　was　virtually　a　discovery　of　the　method　of　trigonometry；　giving　him；　therefore；　a　high　position　in　the　field　of　mathematics。　All　in　all；　then；　Hipparchus　is　a　most　heroic　figure。　He　may　well　be　considered　the　greatest　star…gazer　of　antiquity；　though　he　cannot；　without　injustice　to　his　great　precursors；　be　allowed　the　title　which　is　sometimes　given　him　of　〃father　of　systematic　astronomy。〃

CTESIBIUS　AND　HERO：　MAGICIANS　OF　ALEXANDRIA　Just　about　the　time　when　Hipparchus　was　working　out　at　Rhodes　his　puzzles　of　celestial　mechanics；　there　was　a　man　in　Alexandria　who　was　exercising　a　strangely　inventive　genius　over　mechanical　problems　of　another　sort；　a　man　who；　following　the　example　set　by　Archimedes　a　century　before；　was　studying　the　problems　of　matter　and　putting　his　studies　to　practical　application　through　the　invention　of　weird　devices。　The　man's　name　was　Ctesibius。　We　know　scarcely　more　of　him　than　that　he　lived　in　Alexandria；　probably　in　the　first　half　of　the　second　century　B。C。　His　antecedents；　the　place　and　exact　time　of　his　birth　and　death；　are　quite　unknown。　Neither　are　we　quite　certain　as　to　the　precise　range　of　his　studies　or　the　exact　number　of　his　discoveries。　It　appears　that　he　had　a　pupil　named　Hero；　whose　personality；　unfortunately；　is　scarcely　less　obscure　than　that　of　his　master；　but　who　wrote　a　book　through　which　the　record　of　the　master's　inventions　was　preserved　to　posterity。　Hero；　indeed；　wrote　several　books；　though　only　one　of　them　has　been　preserved。　The　ones　that　are　lost　bear　the　following　suggestive　titles：　On　the　Construction　of　Slings；　On　the　Construction　of　Missiles；　On　the　Automaton；　On　the　Method　of　Lifting　Heavy　Bodies；　On　the　Dioptric　or　Spying…tube。　The　work　that　remains　is　called　Pneumatics；　and　so　interesting　a　work　it　is　as　to　make　us　doubly　regret　the　loss　of　its　companion　volumes。　Had　these　other　books　been　preserved　we　should　doubtless　have　a　clearer　insight　than　is　now　possible　into　some　at　least　of　the　mechanical　problems　that　exercised　the　minds　of　the　ancient　philosophers。　The　book　that　remains　is　chiefly　concerned；　as　its　name　implies；　with　the　study　of　gases；　or；　rather；　with　the　study　of　a　single　gas；　this　being；　of　course；　the　air。　But　it　tells　us　also　of　certain　studies　in　the　dynamics　of　water　that　are　most　interesting；　and　for　the　historian　of　science　most　important。　Unfortunately；　the　pupil　of　Ctesibius；　whatever　his　ingenuity；　was　a　man　with　a　deficient　sense　of　the　ethics　of　science。　He　tells　us　in　his　preface　that　the　object　of　his　book　is　to　record　some　ingenious　discoveries　of　others；　together　with　additional　discoveries　of　his　own；　but　nowhere　in　the　book　itself　does　he　give　us　the；　slightest　clew　as　to　where　the　line　is　drawn　between　the　old　and　the　new。　Once；　in　discussing　the　weight　of　water；　he　mentions　the　law　of　Archimedes　regarding　a　floating　body；　but　this　is　the　only　case　in　which　a　scientific　principle　is　traced　to　its　source　or　in　which　credit　is　given　to　any　one　for　a　discovery。　This　is　the　more　to　be　regretted　because　Hero　has　discussed　at　some　length　the　theories　involved　in　the　treatment　of　his　subject。　This　reticence　on　the　part　of　Hero；　combined　with　the　fact　that　such　somewhat　later　writers　as　Pliny　and　Vitruvius　do　not　mention　Hero's　name；　while　they　frequently　mention　the　name　of　his　master；　Ctesibius；　has　led　modern　critics　to　a　somewhat　sceptical　attitude　regarding　the　position　of　Hero　as　an　actual　discoverer。　The　man　who　would　coolly　appropriate　some　discoveries　of　others　under　cloak　of　a　mere　prefatorial　reference　was　perhaps　an　expounder　rather　than　an　innovator；　and　had；　it　is　shrewdly　suspected；　not　much　of　his　own　to　offer。　Meanwhile；　it　is　tolerably　certain　that　Ctesibius　was　the　discoverer　of　the　principle　of　the　siphon；　of　the　forcing…pump；　and　of　a　pneumatic　organ。　An　examination　of　Hero's　book　will　show　that　these　are　really　the　chief　principles　involved　in　most　of　the　various　interesting　mechanisms　which　he　describes。　We　are　constrained；　then；　to　believe　that　the　inventive　genius　who　was　really　responsible　for　the　mechanisms　we　are　about　to　describe　was　Ctesibius；　the　master。　Yet　we　owe　a　debt　of　gratitude　to　Hero；　the　pupil；　for　having　given　wider　vogue　to　these　discoveries；　and　in　particular　for　the　discussion　of　the　principles　of　hydrostatics　and　pneumatics　contained　in　the　introduction　to　his　book。　This　discussion　furnishes　us　almost　our　only　knowledge　as　to　the　progress