a history of science-1-第49章

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　introduction　to　his　book。　This　discussion　furnishes　us　almost　our　only　knowledge　as　to　the　progress　of　Greek　philosophers　in　the　field　of　mechanics　since　the　time　of　Archimedes。　The　main　purpose　of　Hero　in　his　preliminary　thesis　has　to　do　with　the　nature　of　matter；　and　recalls；　therefore；　the　studies　of　Anaxagoras　and　Democritus。　Hero；　however；　approaches　his　subject　from　a　purely　material　or　practical　stand…point。　He　is　an　explicit　champion　of　what　we　nowadays　call　the　molecular　theory　of　matter。　〃Every　body；〃　he　tells　us；　〃is　composed　of　minute　particles；　between　which　are　empty　spaces　less　than　these　particles　of　the　body。　It　is；　therefore；　erroneous　to　say　that　there　is　no　vacuum　except　by　the　application　of　force；　and　that　every　space　is　full　either　of　air　or　water　or　some　other　substance。　But　in　proportion　as　any　one　of　these　particles　recedes；　some　other　follows　it　and　fills　the　vacant　space；　therefore　there　is　no　continuous　vacuum；　except　by　the　application　of　some　force　＇like　suction＇that　is　to　say；　an　absolute　vacuum　is　never　found；　except　as　it　is　produced　artificially。〃　Hero　brings　forward　some　thoroughly　convincing　proofs　of　the　thesis　he　is　maintaining。　〃If　there　were　no　void　places　between　the　particles　of　water；〃　he　says；　〃the　rays　of　light　could　not　penetrate　the　water；　moreover；　another　liquid；　such　as　wine；　could　not　spread　itself　through　the　water；　as　it　is　observed　to　do；　were　the　particles　of　water　absolutely　continuous。〃　The　latter　illustration　is　one　the　validity　of　which　appeals　as　forcibly　to　the　physicists　of　to…day　as　it　did　to　Hero。　The　same　is　true　of　the　argument　drawn　from　the　compressibility　of　gases。　Hero　has　evidently　made　a　careful　study　of　this　subject。　He　knows　that　an　inverted　tube　full　of　air　may　be　immersed　in　water　without　becoming　wet　on　the　inside；　proving　that　air　is　a　physical　substance；　but　he　knows　also　that　this　same　air　may　be　caused　to　expand　to　a　much　greater　bulk　by　the　application　of　heat；　or　may；　on　the　other　hand；　be　condensed　by　pressure；　in　which　case；　as　he　is　well　aware；　the　air　exerts　force　in　the　attempt　to　regain　its　normal　bulk。　But；　he　argues；　surely　we　are　not　to　believe　that　the　particles　of　air　expand　to　fill　all　the　space　when　the　bulk　of　air　as　a　whole　expands　under　the　influence　of　heat；　nor　can　we　conceive　that　the　particles　of　normal　air　are　in　actual　contact；　else　we　should　not　be　able　to　compress　the　air。　Hence　his　conclusion；　which；　as　we　have　seen；　he　makes　general　in　its　application　to　all　matter；　that　there　are　spaces；　or；　as　he　calls　them；　vacua；　between　the　particles　that　go　to　make　up　all　substances；　whether　liquid；　solid；　or　gaseous。　Here；　clearly　enough；　was　the　idea　of　the　〃atomic〃　nature　of　matter　accepted　as　a　fundamental　notion。　The　argumentative　attitude　assumed　by　Hero　shows　that　the　doctrine　could　not　be　expected　to　go　unchallenged。　But；　on　the　other　hand；　there　is　nothing　in　his　phrasing　to　suggest　an　intention　to　claim　originality　for　any　phase　of　the　doctrine。　We　may　infer　that　in　the　three　hundred　years　that　had　elapsed　since　the　time　of　Anaxagoras；　that　philosopher's　idea　of　the　molecular　nature　of　matter　had　gained　fairly　wide　currency。　As　to　the　expansive　power　of　gas；　which　Hero　describes　at　some　length　without　giving　us　a　clew　to　his　authorities；　we　may　assume　that　Ctesibius　was　an　original　worker；　yet　the　general　facts　involved　were　doubtless　much　older　than　his　day。　Hero；　for　example；　tells　us　of　the　cupping…glass　used　by　physicians；　which　he　says　is　made　into　a　vacuum　by　burning　up　the　air　in　it；　but　this　apparatus　had　probably　been　long　in　use；　and　Hero　mentions　it　not　in　order　to　describe　the　ordinary　cupping…glass　which　is　referred　to；　but　a　modification　of　it。　He　refers　to　the　old　form　as　if　it　were　something　familiar　to　all。　Again；　we　know　that　Empedocles　studied　the　pressure　of　the　air　in　the　fifth　century　B。C。；　and　discovered　that　it　would　support　a　column　of　water　in　a　closed　tube；　so　this　phase　of　the　subject　is　not　new。　But　there　is　no　hint　anywhere　before　this　work　of　Hero　of　a　clear　understanding　that　the　expansive　properties　of　the　air　when　compressed；　or　when　heated；　may　be　made　available　as　a　motor　power。　Hero；　however；　has　the　clearest　notions　on　the　subject　and　puts　them　to　the　practical　test　of　experiment。　Thus　he　constructs　numerous　mechanisms　in　which　the　expansive　power　of　air　under　pressure　is　made　to　do　work；　and　others　in　which　the　same　end　is　accomplished　through　the　expansive　power　of　heated　air。　For　example；　the　doors　of　a　temple　are　made　to　swing　open　automatically　when　a　fire　is　lighted　on　a　distant　altar；　closing　again　when　the　fire　dies　outeffects　which　must　have　filled　the　minds　of　the　pious　observers　with　bewilderment　and　wonder；　serving　a　most　useful　purpose　for　the　priests；　who　alone；　we　may　assume；　were　in　the　secret。　There　were　two　methods　by　which　this　apparatus　was　worked。　In　one　the　heated　air　pressed　on　the　water　in　a　close　retort　connected　with　the　altar；　forcing　water　out　of　the　retort　into　a　bucket；　which　by　its　weight　applied　a　force　through　pulleys　and　ropes　that　turned　the　standards　on　which　the　temple　doors　revolved。　When　the　fire　died　down　the　air　contracted；　the　water　was　siphoned　back　from　the　bucket；　which；　being　thus　lightened；　let　the　doors　close　again　through　the　action　of　an　ordinary　weight。　The　other　method　was　a　slight　modification；　in　which　the　retort　of　water　was　dispensed　with　and　a　leather　sack　like　a　large　football　substitued。　The　ropes　and　pulleys　were　connected　with　this　sack；　which　exerted　a　pull　when　the　hot　air　expanded；　and　which　collapsed　and　thus　relaxed　its　strain　when　the　air　cooled。　A　glance　at　the　illustrations　taken　from　Hero's　book　will　make　the　details　clear。　Other　mechanisms　utilized　a　somewhat　different　combination　of　weights；　pulleys；　and　siphons；　operated　by　the　expansive　power　of　air；　unheated　but　under　pressure；　such　pressure　being　applied　with　a　force…　pump；　or　by　the　weight　of　water　running　into　a　closed　receptacle。　One　such　mechanism　gives　us　a　constant　jet　of　water　or　perpetual　fountain。　Another　curious　application　of　the　principle　furnishes　us　with　an　elaborate　toy；　consisting　of　a　group　of　birds　which　alternately　whistle　or　are　silent；　while　an　owl　seated　on　a　neighboring　perch　turns　towards　the　birds　when　their　song　begins　and　away　from　them　when　it　ends。　The　〃singing〃　of　the　birds；　it　must　be　explained；　is　produced　by　the　expulsion　of　air　through　tiny　tubes　passing　up　through　their　throats　from　a　tank　below。　The　owl　is　made　to　turn　by　a　mechanism　similar　to　that　which　manipulates　the　temple　doors。　The　pressure　is　supplied　merely　by　a　stream　of　running　water；　and　the　periodical　silence　of　the　birds　is　due　to　the　fact　that　this　pressure　is　relieved　through　the　automatic　siphoning　off　of　the　water　when　it　reaches　a　certain　height。　The　action　of　the　siphon；　it　may　be　added；　is　correctly　explained　by　Hero　as　due　to　the　greater　weight　of　the　water　in　the　longer　arm　of　the　bent　tube。　As　before　mentioned；　the　siphon　is　repeatedly　used　in　these　mechanisms　of　Hero。　The　diagram　will　make　clear　the　exact　application　of　it　in　the　present　most　ingenious　mechanism。　We　may　add　that　the　principle　of　the　whistle　was　a　favorite　one　of　Hero。　By　the　aid　of　a　similar　mechanism　he　brought　about　the　blowing　of　trumpets　when　the　temple　doors　were　opened；　a　phenomenon　which　must　greatly　have　enhanced　the　mystification。　It　is　possible　that　this　principle　was　utilized　also　in　connection　with　statues　to　produce　seemingly　supernatural　effects。　This　may　be　the　explanation　of　the　tradition　of　the　speaking　statue　in　the　temple　of　Ammon　at　Thebes。　｛illustration　caption　=　DEVICE　FOR　CAUSING　THE　DOORS　OF　THE　TEMPLE　TO　OPEN　WHEN　THE　FIRE　ON　THE　ALTAR　IS　LIGHTED　（Air　heated　in　the　altar　F　drives　water　from　the　closed　receptacle　H　through　the　tube　KL　into　the　bucket　M；　which　descends　through　gravity；　thus　opening　the　doors。　When　the　altar　cools；　the　air　contracts；　the　water　is　sucked　from　the　bucket；　and　the　weight　and　pulley　close　the　doors。）｝　｛illustration　caption　=　THE　STEAM…ENGINE　OF　HERO　（The　steam　generated　in　the　receptacle　AB　passes　through　the　tube　EF　into　the　globe；　and　escapes　through　the　bent　tubes　H　and　K；　causing　the　globe　to　rotate　on　the　axis　LG。）｝

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