a history of science-2-第17部分

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himedes。　But　it　remains　to　explain　the　fact　that　bodies　of　a　certain　shape　will　float；　while　bodies　of　the　same　material　and　weight；　but　of　a　different　shape；　will　sink。　We　shall　see　what　explanation　Galileo　finds　of　this　anomaly　as　we　proceed。　In　the　first　place；　Galileo　makes　a　cone　of　wood　or　of　wax；　and　shows　that　when　it　floats　with　either　its　point　or　its　base　in　the　water；　it　displaces　exactly　the　same　amount　of　fluid；　although　the　apex　is　by　its　shape　better　adapted　to　overcome　the　resistance　of　the　water；　if　that　were　the　cause　of　buoyancy。　Again；　the　experiment　may　be　varied　by　tempering　the　wax　with　filings　of　lead　till　it　sinks　in　the　water；　when　it　will　be　found　that　in　any　figure　the　same　quantity　of　cork　must　be　added　to　it　to　raise　the　surface。　〃But；〃　says　Galileo；　〃this　silences　not　my　antagonists；　they　say　that　all　the　discourse　hitherto　made　by　me　imports　little　to　them；　and　that　it　serves　their　turn；　that　they　have　demonstrated　in　one　instance；　and　in　such　manner　and　figure　as　pleases　them　best　namely；　in　a　board　and　in　a　ball　of　ebonythat　one　when　put　into　the　water　sinks　to　the　bottom；　and　that　the　other　stays　to　swim　on　the　top；　and　the　matter　being　the　same；　and　the　two　bodies　differing　in　nothing　but　in　figure；　they　affirm　that　with　all　perspicuity　they　have　demonstrated　and　sensibly　manifested　what　they　undertook。　Nevertheless；　I　believe；　and　think　I　can　prove；　that　this　very　experiment　proves　nothing　against　my　theory。　And　first；　it　is　false　that　the　ball　sinks　and　the　board　not；　for　the　board　will　sink；　too；　if　you　do　to　both　the　figures　as　the　words　of　our　question　require；　that　is；　if　you　put　them　both　in　the　water；　for　to　be　in　the　water　implies　to　be　placed　in　the　water；　and　by　Aristotle's　own　definition　of　place；　to　be　placed　imports　to　be　environed　by　the　surface　of　the　ambient　body；　but　when　my　antagonists　show　the　floating　board　of　ebony；　they　put　it　not　into　the　water；　but　upon　the　water；　where；　being　detained　by　a　certain　impediment　（of　which　more　anon）；　it　is　surrounded；　partly　with　water；　partly　with　air；　which　is　contrary　to　our　agreement；　for　that　was　that　bodies　should　be　in　the　water；　and　not　part　in　the　water；　part　in　the　air。　〃I　will　not　omit　another　reason；　founded　also　upon　experience；　and；　if　I　deceive　not　myself；　conclusive　against　the　notion　that　figure；　and　the　resistance　of　the　water　to　penetration；　have　anything　to　do　with　the　buoyancy　of　bodies。　Choose　a　piece　of　wood　or　other　matter；　as；　for　instance；　walnut…wood；　of　which　a　ball　rises　from　the　bottom　of　the　water　to　the　surface　more　slowly　than　a　ball　of　ebony　of　the　same　size　sinks；　so　that；　clearly；　the　ball　of　ebony　divides　the　water　more　readily　in　sinking　than　the　ball　of　wood　does　in　rising。　Then　take　a　board　of　walnut…tree　equal　to　and　like　the　floating　one　of　my　antagonists；　and　if　it　be　true　that　this　latter　floats　by　reason　of　the　figure　being　unable　to　penetrate　the　water；　the　other　of　walnut…tree；　without　a　question；　if　thrust　to　the　bottom；　ought　to　stay　there；　as　having　the　same　impeding　figure；　and　being　less　apt　to　overcome　the　said　resistance　of　the　water。　But　if　we　find　by　experience　that　not　only　the　thin　board；　but　every　other　figure　of　the　same　walnut…tree；　will　return　to　float；　as　unquestionably　we　shall；　then　I　must　desire　my　opponents　to　forbear　to　attribute　the　floating　of　the　ebony　to　the　figure　of　the　board；　since　the　resistance　of　the　water　is　the　same　in　rising　as　in　sinking；　and　the　force　of　ascension　of　the　walnut…tree　is　less　than　the　ebony's　force　for　going　to　the　bottom。　〃Now　let　us　return　to　the　thin　plate　of　gold　or　silver；　or　the　thin　board　of　ebony；　and　let　us　lay　it　lightly　upon　the　water；　so　that　it　may　stay　there　without　sinking；　and　carefully　observe　the　effect。　It　will　appear　clearly　that　the　plates　are　a　considerable　matter　lower　than　the　surface　of　the　water；　which　rises　up　and　makes　a　kind　of　rampart　round　them　on　every　side。　But　if　it　has　already　penetrated　and　overcome　the　continuity　of　the　water；　and　is　of　its　own　nature　heavier　than　the　water；　why　does　it　not　continue　to　sink；　but　stop　and　suspend　itself　in　that　little　dimple　that　its　weight　has　made　in　the　water？　My　answer　is；　because　in　sinking　till　its　surface　is　below　the　water；　which　rises　up　in　a　bank　round　it；　it　draws　after　and　carries　along　with　it　the　air　above　it；　so　that　that　which；　in　this　case；　descends　in　the　water　is　not　only　the　board　of　ebony　or　the　plate　of　iron；　but　a　compound　of　ebony　and　air；　from　which　composition　results　a　solid　no　longer　specifically　heavier　than　the　water；　as　was　the　ebony　or　gold　alone。　But；　gentlemen；　we　want　the　same　matter；　you　are　to　alter　nothing　but　the　shape；　and；　therefore；　have　the　goodness　to　remove　this　air；　which　may　be　done　simply　by　washing　the　surface　of　the　board；　for　the　water　having　once　got　between　the　board　and　the　air　will　run　together；　and　the　ebony　will　go　to　the　bottom；　and　if　it　does　not；　you　have　won　the　day。　〃But　methinks　I　hear　some　of　my　antagonists　cunningly　opposing　this；　and　telling　me　that　they　will　not　on　any　account　allow　their　boards　to　be　wetted；　because　the　weight　of　the　water　so　added；　by　making　it　heavier　than　it　was　before；　draws　it　to　the　bottom；　and　that　the　addition　of　new　weight　is　contrary　to　our　agreement；　which　was　that　the　matter　should　be　the　same。　〃To　this　I　answer；　first；　that　nobody　can　suppose　bodies　to　be　put　into　the　water　without　their　being　wet；　nor　do　I　wish　to　do　more　to　the　board　than　you　may　do　to　the　ball。　Moreover；　it　is　not　true　that　the　board　sinks　on　account　of　the　weight　of　the　water　added　in　the　washing；　for　I　will　put　ten　or　twenty　drops　on　the　floating　board；　and　so　long　as　they　stand　separate　it　shall　not　sink；　but　if　the　board　be　taken　out　and　all　that　water　wiped　off；　and　the　whole　surface　bathed　with　one　single　drop；　and　put　it　again　upon　the　water；　there　is　no　question　but　it　will　sink；　the　other　water　running　to　cover　it；　being　no　longer　hindered　by　the　air。　In　the　next　place；　it　is　altogether　false　that　water　can　in　any　way　increase　the　weight　of　bodies　immersed　in　it；　for　water　has　no　weight　in　water；　since　it　does　not　sink。　Now　just　as　he　who　should　say　that　brass　by　its　own　nature　sinks；　but　that　when　formed　into　the　shape　of　a　kettle　it　acquires　from　that　figure　the　virtue　of　lying　in　water　without　sinking；　would　say　what　is　false；　because　that　is　not　purely　brass　which　then　is　put　into　the　water；　but　a　compound　of　brass　and　air；　so　is　it　neither　more　nor　less　false　that　a　thin　plate　of　brass　or　ebony　swims　by　virtue　of　its　dilated　and　broad　figure。　Also；　I　cannot　omit　to　tell　my　opponents　that　this　conceit　of　refusing　to　bathe　the　surface　of　the　board　might　beget　an　opinion　in　a　third　person　of　a　poverty　of　argument　on　their　side；　especially　as　the　conversation　began　about　flakes　of　ice；　in　which　it　would　be　simple　to　require　that　the　surfaces　should　be　kept　dry；　not　to　mention　that　such　pieces　of　ice；　whether　wet　or　dry；　always　float；　and　so　my　antagonists　say；　because　of　their　shape。　〃Some　may　wonder　that　I　affirm　this　power　to　be　in　the　air　of　keeping　plate　of　brass　or　silver　above　water；　as　if　in　a　certain　sense　I　would　attribute　to　the　air　a　kind　of　magnetic　virtue　for　sustaining　heavy　bodies　with　which　it　is　in　contact。　To　satisfy　all　these　doubts　I　have　contrived　the　following　experiment　to　demonstrate　how　truly　the　air　does　support　these　bodies；　for　I　have　found；　when　one　of　these　bodies　which　floats　when　placed　lightly　on　the　water　is　thoroughly　bathed　and　sunk　to　the　bottom；　that　by　carrying　down　to　it　a　little　air　without　otherwise　touching　it　in　the　least；　I　am　able　to　raise　and　carry　it　back　to　the　top；　where　it　floats　as　before。　To　this　effect；　I　take　a　ball　of　wax；　and　with　a　little　lead　make　it　just　heavy　enough　to　sink　very　slowly　to　the　bottom；　taking　care　that　its　surface　be　quite　smooth　and　even。　This；　if　put　gently　into　the　water；　submerges　almost　entirely；　there　remaining　visible　only　a　little　of　the　very　top；　which；　so　long　as　it　is　joined　to　the　air；　keeps　the　ball　afloat；　but　if　we　take　away　the　contact　of　the　air　by　wetting　this　top；　the　ball　sinks　to　the　bottom　and　remains　there。　Now　to　make　it　return　to　the　surface　by　virtue　of　the　air　which　before　sustained　it；　thrust　into　the　water　a　glass　with　the　mouth　downward；　which　will　carry　with　it　the　air　it　contains；　and　move　this　down　towards　the　ball　until　you　see；　by　the　transparency　of　the　glass；　that　the　air　has　reached　the　top　of　it；　then　gently　draw　the　glass　upward；　and　you　will　see　the　ball　rise；　and　afterwards　stay　on　the　top　of　the　water；　if　you　carefully　part　the　glass　and　water　without　too　much　disturbing　it。〃＇3＇　It　will　be　seen　that　Galileo；　while　holding　in　the　main　to　a　correct　thesis；　yet　mingles　with　it　some　false　ideas。　At　the　very　outset；　of　course；　it　is　not　true　that　water　has　no　resistance　to　penetration；　it　is　true；　however；　in　the　sense　in　which　Galileo　uses　the　termthat　is　to　say；　the　resistance　of　the　water　to　penetration　is　not　the　determining　factor　ordinarily　in　deciding　whether　a　body　sinks　or　floats。　Yet　in　the　case　of　the　flat　body　it　is　not　altogether　inappropriate　to　say　that　the　water　resists　penetration　and　thus　supports　the　body。　The　modern　physicist　explains　the　phenomenon　as　due　to　surface…tension　of　the　fluid。　Of　course；　Galileo's　disquisition　on　the　mixing　of　air　with　the　floating　body　is　utterly　fanciful。　His　experiments　were　beautifully　exact；　his　theorizing　from　them　was；　in　this　instance；　altogether　fallacious。　Thus；　as　already　intimated；　his　paper　is　admirably　adapted　to　convey　a　double　lesson　to　the　student　of　science。

WILLIAM　GILBERT　AND　THE　STUDY　OF　MAGNETISM　It　will　be　observed　that　the　studies　of　Galileo　and　Stevinus　were　chiefly　concerned　with　the　force　of　gravitation。　Meanwhile；　there　was　an　English　philosopher　of　corresponding　genius；　whose　attention　was　directed　towards　investigation　of　the　equally　mysterious　force　of　terrestrial　magnetism。　With　the　doubtful　exception　of　Bacon；　Gilbert　was　the　most　distinguished　man　of　science　in　England　during　the　reign　of　Queen　Elizabeth。　He　was　for　many　years　court　physician；　and　Queen　Elizabeth　ultimately　settled　upon　him　a　pension　that　enabled　him　to　continue　his　researches　in　pure　science。　His　investigations　in　chemistry；　although　supposed　to　b