a history of science-2-第20部分

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t；　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　them　the　dip　had　just　been　discovered；　and　had　not　been　studied　except　at　London。　His　theory　of　the　dip　was；　therefore；　a　scientific　prediction；　based　on　a　preconceived　hypothesis。　Gilbert　found　the　dip　to　be　72　degrees　at　London；　eight　years　later　Hudson　found　the　dip　at　75　degrees　22'　north　latitude　to　be　89　degrees　30'；　but　it　was　not　until　over　two　hundred　years　later；　in　1831；　that　the　vertical　dip　was　first　observed　by　Sir　James　Ross　at　about　70　degrees　5'　north　latitude；　and　96　degrees　43'　west　longitude。　This　was　not　the　exact　point　assumed　by　Gilbert；　and　his　scientific　predictions；　therefore；　were　not　quite　correct；　but　such　comparatively　slight　and　excusable　errors　mar　but　little　the　excellence　of　his　work　as　a　whole。　A　brief　epitome　of　some　of　his　other　important　discoveries　suffices　to　show　that　the　exalted　position　in　science　accorded　him　by　contemporaries；　as　well　as　succeeding　generations　of　scientists；　was　well　merited。　He　was　first　to　distinguish　between　magnetism　and　electricity；　giving　the　latter　its　name。　He　discovered　also　the　〃electrical　charge；〃　and　pointed　the　way　to　the　discovery　of　insulation　by　showing　that　the　charge　could　be　retained　some　time　in　the　excited　body　by　covering　it　with　some　non…conducting　substance；　such　as　silk；　although；　of　course；　electrical　conduction　can　hardly　be　said　to　have　been　more　than　vaguely　surmised；　if　understood　at　all　by　him。　The　first　electrical　instrument　ever　made；　and　known　as　such；　was　invented　by　him；　as　was　also　the　first　magnetometer；　and　the　first　electrical　indicating　device。　Although　three　centuries　have　elapsed　since　his　death；　the　method　of　magnetizing　iron　first　introduced　by　him　is　in　common　use　to…day。　He　made　exhaustive　experiments　with　a　needle　balanced　on　a　pivot　to　see　how　many　substances　he　could　find　which；　like　amber；　on　being　rubbed　affected　the　needle。　In　this　way　he　discovered　that　light　substances　were　attracted　by　alum；　mica；　arsenic；　sealing…wax；　lac　sulphur；　slags；　beryl；　amethyst；　rock…crystal；　sapphire；　jet；　carbuncle；　diamond；　opal；　Bristol　stone；　glass；　glass　of　antimony；　gum…mastic；　hard　resin；　rock…salt；　and；　of　course；　amber。　He　discovered　also　that　atmospheric　conditions　affected　the　production　of　electricity；　dryness　being　unfavorable　and　moisture　favorable。　Galileo's　estimate　of　this　first　electrician　is　the　verdict　of　succeeding　generations。　〃I　extremely　admire　and　envy　this　author；〃　he　said。　〃I　think　him　worthy　of　the　greatest　praise　for　the　many　new　and　true　observations　which　he　has　made；　to　the　disgrace　of　so　many　vain　and　fabling　authors。〃

STUDIES　OF　LIGHT；　HEAT；　AND　ATMOSPHERIC　PRESSURE　We　have　seen　that　Gilbert　was　by　no　means　lacking　in　versatility；　yet　the　investigations　upon　which　his　fame　is　founded　were　all　pursued　along　one　line；　so　that　the　father　of　magnetism　may　be　considered　one　of　the　earliest　of　specialists　in　physical　science。　Most　workers　of　the　time；　on　the　other　band；　extended　their　investigations　in　many　directions。　The　sum　total　of　scientific　knowledge　of　that　day　had　not　bulked　so　large　as　to　exclude　the　possibility　that　one　man　might　master　it　all。　So　we　find　a　Galileo；　for　example；　making　revolutionary　discoveries　in　astronomy；　and　performing　fundamental　experiments　in　various　fields　of　physics。　Galileo's　great　contemporary；　Kepler；　was　almost　equally　versatile；　though　his　astronomical　studies　were　of　such　pre…eminent　importance　that　his　other　investigations　sink　into　relative　insignificance。　Yet　he　performed　some　notable　experiments　in　at　least　one　department　of　physics。　These　experiments　had　to　do　with　the　refraction　of　light；　a　subject　which　Kepler　was　led　to　investigate；　in　part　at　least；　through　his　interest　in　the　telescope。　We　have　seen　that　Ptolemy　in　the　Alexandrian　time；　and　Alhazen；　the　Arab；　made　studies　of　refraction。　Kepler　repeated　their　experiments；　and；　striving　as　always　to　generalize　his　observations；　he　attempted　to　find　the　law　that　governed　the　observed　change　of　direction　which　a　ray　of　light　assumes　in　passing　from　one　medium　to　another。　Kepler　measured　the　angle　of　ref