a history of science-2-第16章

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etted；　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　be　of　great　importance；　are　mostly　lost；　but　his　great　work；　De　Magnete；　on　which　he　labored　for　upwards　of　eighteen　years；　is　a　work　of　sufficient　importance；　as　Hallam　says；　〃to　raise　a　lasting　reputation　for　its　author。〃　From　its　first　appearance　it　created　a　profound　impression　upon　the　learned　men　of　the　continent；　although　in　England　Gilbert's　theories　seem　to　have　been　somewhat　less　favorably　received。　Galileo　freely　expressed　his　admiration　for　the　work　and　its　author；　Bacon；　who　admired　the　author；　did　not　express　the　same　admiration　for　his　theories；　but　Dr。　Priestley；　later；　declared　him　to　be　〃the　father　of　modern　electricity。〃　Strangely　enough；　Gilbert's　book　had　never　been　translated　into　English；　or　apparently　into　any　other　language；　until　recent　years；　although　at　the　time　of　its　publication　certain　learned　men；　unable　to　read　the　book　in　the　original；　had　asked　that　it　should　be。　By　this　neglect；　or　oversight；　a　great　number　of　general　readers　as　well　as　many　scientists；　through　succeeding　centuries；　have　been　deprived　of　the　benefit　of　writings　that　contained　a　good　share　of　the　fundamental　facts　about　magnetism　as　known　to…day。　Gilbert　was　the　first　to　discover　that　the　earth　is　a　great　magnet；　and　he　not　only　gave　the　name　of　〃pole〃　to　the　extremities　of　the　magnetic　needle；　but　also　spoke　of　these　〃poles〃　as　north　and　south　pole；　although　he　used　these　names　in　the　opposite　sense　from　that　in　which　we　now　use　them；　his　south　pole　being　the　extremity　which　pointed　towards　the　north；　and　vice　versa。　He　was　also　first　to　make　use　of　the　terms　〃electric　force；〃　〃electric　emanations；〃　and　〃electric　attractions。〃　It　is　hardly　necessary　to　say　that　some　of　the　views　taken　by　Gilbert；　many　of　his　theories；　and　the　accuracy　of　some　of　his　experiments　have　in　recent　times　been　found　to　be　erroneous。　As　a　pioneer　in　an　unexplored　field　of　science；　however；　his　work　is　remarkably　accurate。　〃On　the　whole；〃　says　Dr。　John　Robinson；　〃this　performance　contains　more　real　information　than　any　writing　of　the　age　in　which　he　lived；　and　is　scarcely　exceeded　by　any　that　has　appeared　since。〃＇4＇　In　the　preface　to　his　work　Gilbert　says：　〃Since　in　the　discovery　of　secret　things；　and　in　the　investigation　of　hidden　causes；　stronger　reasons　are　obtained　from　sure　experiments　and　demonstrated　arguments　than　from　probable　conjectures　and　the　opinions　of　philosophical　speculators　of　the　common　sort；　therefore；　to　the　end　of　that　noble　substance　of　that　great　loadstone；　our　common　mother　（the　earth）；　still　quite　unknown；　and　also　that　the　forces　extraordinary　and　exalted　of　this　globe　may　the　better　be　understood；　we　have　decided；　first；　to　begin　with　the　common　stony　and　ferruginous　matter；　and　magnetic　bodies；　and　the　part　of　the　earth　that　we　may　handle　and　may　perceive　with　senses；　and　then　to　proceed　with　plain　magnetic　experiments；　and　to　penetrate　to　the　inner　parts　of　the　earth。〃＇5＇　Before　taking　up　the　demonstration　that　the　earth　is　simply　a　giant　loadstone；　Gilbert　demonstrated　in　an　ingenious　way　that　every　loadstone；　of　whatever　size；　has　definite　and　fixed　poles。　He　did　this　by　placing　the　stone　in　a　metal　lathe　and　converting　it　into　a　sphere；　and　upon　this　sphere　demonstrated　how　the　poles　can　be　found。　To　this　round　loadstone　he　gave　the　name　of　terrellathat　is；　little　earth。　〃To　find；　then；　poles　answering　to　the　earth；〃　he　says；　〃take　in　your　hand　the　round　stone；　and　lay　on　it　a　needle　or　a　piece　of　iron　wire：　the　ends　of　the　wire　move　round　their　middle　point；　and　suddenly　come　to　a　standstill。　Now；　with　ochre　or　with　chalk；　mark　where　the　wire　lies　still　and　sticks。　Then　move　the　middle　or　centre　of　the　wire　to　another　spot；　and　so　to　a　third　and　fourth；　always　marking　the　stone　along　the　length　of　the　wire　where　it　stands　still；　the　lines　so　marked　will　exhibit　meridian　circles；　or　circles　like　meridians；　on　the　stone　or　terrella；　and　manifestly　they　will　all　come　together　at　the　poles　of　the　stone。　The　circle　being　continued　in　this　way；　the　poles　appear；　both　the　north　and　the　south；　and　betwixt　these；　midway；　we　may　draw　a　large　circle　for　an　equator；　as　is　done　by　the　astronomer　in　the　heavens　and　on　his　spheres；　and　by　the　geographer　on　the　terrestrial　globe。〃＇6＇　Gilbert　had　tried　the　familiar　experiment　of　placing　the　loadstone　on　a　float　in　water；　and　observed　that　the　poles　always　revolved　until　they　pointed　north　and　south；　which　he　explained　as　due　to　the　earth's　magnetic　attraction。　In　this　same　connection　he　noticed　that　a　piece　of　wrought　iron　mounted　on　a　cork　float　was　attracted　by　other　metals　to　a　slight　degree；　and　he　observed　also　that　an　ordinary　iron　bar；　if　suspended　horizontally　by　a　thread；　assumes　invariably　a　north　and　south　direction。　These；　with　many　other　experiments　of　a　similar　nature；　convinced　him　that　the　earth　〃is　a　magnet　and　a　loadstone；〃　which　he　says　is　a　〃new　and　till　now　unheard…of　view　of　the　earth。〃　Fully　to　appreciate　Gilbert's　revolutionary　views　concerning　the　earth　as　a　magnet；　it　should　be　remembered　that　numberless　theories　to　explain　the　action　of　the　electric　needle　had　been　advanced。　Columbus　and　Paracelsus；　for　example；　believed　that　the　magnet　was　attracted　by　some　point　in　the　heavens；　such　as　a　magnetic　star。　Gilbert　himself　tells　of　some　of　the　beliefs　that　had　been　held　by　his　predecessors；　many　of　whom　he　declares　〃wilfully　falsify。〃　One　of　his　first　steps　was　to　refute　by　experiment　such　assertions　as　that　of　Cardan；　that　〃a　wound　by　a　magnetized　needle　was　painless〃；　and　also　the　assertion　of　Fracastoni　that　loadstone　attracts　silver；　or　that　of　Scalinger；　that　the　diamond　will　attract　iron；　and　the　statement　of　Matthiolus　that　〃iron　rubbed　with　garlic　is　no　longer　attracted　to　the　loadstone。〃　Gilbert　made　extensive　experiments　to　explain　the　dipping　of　the　needle；　which　had　been　first　noticed　by　William　Norman。　His　deduction　as　to　this　phenomenon　led　him　to　believe　that　this　was　also　explained　by　the　magnetic　attraction　of　the　earth；　and　to　predict　where　the　vertical　dip　would　be　found。　These　deductions　seem　the　more　wonderful　because　at　the　time　he　made　the