a history of science-2-第40部分

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　in　use。　He　was　led　to　this　invention　by　his　experiments　with　light　and　colors。　In　1671　he　presented　to　the　Royal　Society　a　second　and　somewhat　larger　telescope；　which　he　had　made；　and　this　type　of　instrument　was　little　improved　upon　until　the　introduction　of　the　achromatic　telescope；　invented　by　Chester　Moor　Hall　in　1733。　As　is　generally　known；　the　element　of　accurate　measurements　of　time　plays　an　important　part　in　the　measurements　of　the　movements　of　the　heavenly　bodies。　In　fact；　one　was　scarcely　possible　without　the　other；　and　as　it　happened　it　was　the　same　man；　Huygens；　who　perfected　Kepler's　telescope　and　invented　the　pendulum　clock。　The　general　idea　had　been　suggested　by　Galileo；　or；　better　perhaps；　the　equal　time　occupied　by　the　successive　oscillations　of　the　pendulum　had　been　noted　by　him。　He　had　not　been　able；　however；　to　put　this　discovery　to　practical　account。　But　in　1656　Huygens　invented　the　necessary　machinery　for　maintaining　the　motion　of　the　pendulum　and　perfected　several　accurate　clocks。　These　clocks　were　of　invaluable　assistance　to　the　astronomers；　affording　as　they　did　a　means　of　keeping　time　〃more　accurate　than　the　sun　itself。〃　When　Picard　had　corrected　the　variation　caused　by　heat　and　cold　acting　upon　the　pendulum　rod　by　combining　metals　of　different　degrees　of　expansibility；　a　high　degree　of　accuracy　was　possible。　But　while　the　pendulum　clock　was　an　unequalled　stationary　time…piece；　it　was　useless　in　such　unstable　situations　as；　for　example；　on　shipboard。　But　here　again　Huygens　played　a　prominent　part　by　first　applying　the　coiled　balance…spring　for　regulating　watches　and　marine　clocks。　The　idea　of　applying　a　spring　to　the　balance…wheel　was　not　original　with　Huygens；　however；　as　it　had　been　first　conceived　by　Robert　Hooke；　but　Huygens's　application　made　practical　Hooke's　idea。　In　England　the　importance　of　securing　accurate　watches　or　marine　clocks　was　so　fully　appreciated　that　a　reward　of　L20；000　sterling　was　offered　by　Parliament　as　a　stimulus　to　the　inventor　of　such　a　time…piece。　The　immediate　incentive　for　this　offer　was　the　obvious　fact　that　with　such　an　instrument　the　determination　of　the　longitude　of　places　would　be　much　simplified。　Encouraged　by　these　offers；　a　certain　carpenter　named　Harrison　turned　his　attention　to　the　subject　of　watch…making；　and；　after　many　years　of　labor；　in　1758　produced　a　spring　time…keeper　which；　during　a　sea…voyage　occupying　one　hundred　and　sixty…one　days；　varied　only　one　minute　and　five　seconds。　This　gained　for　Harrison　a　reward　Of　L5000　sterling　at　once；　and　a　little　later　L10；000　more；　from　Parliament。　While　inventors　were　busy　with　the　problem　of　accurate　chronometers；　however；　another　instrument　for　taking　longitude　at　sea　had　been　invented。　This　was　the　reflecting　quadrant；　or　sextant；　as　the　improved　instrument　is　now　called；　invented　by　John　Hadley　in　1731；　and　independently　by　Thomas　Godfrey；　a　poor　glazier　of　Philadelphia；　in　1730。　Godfrey's　invention；　which　was　constructed　on　the　same　principle　as　that　of　the　Hadley　instrument；　was　not　generally　recognized　until　two　years　after　Hadley's　discovery；　although　the　instrument　was　finished　and　actually　in　use　on　a　sea…voyage　some　months　before　Hadley　reported　his　invention。　The　principle　of　the　sextant；　however；　seems　to　have　been　known　to　Newton；　who　constructed　an　instrument　not　very　unlike　that　of　Hadley；　but　this　invention　was　lost　sight　of　until　several　years　after　the　philosopher's　death　and　some　time　after　Hadley's　invention。　The　introduction　of　the　sextant　greatly　simplified　taking　reckonings　at　sea　as　well　as　facilitating　taking　the　correct　longitude　of　distant　places。　Before　that　time　the　mariner　was　obliged　to　depend　upon　his　compass；　a　cross…staff；　or　an　astrolabe；　a　table　of　the　sun's　declination　and　a　correction　for　the　altitude　of　the　polestar；　and　very　inadequate　and　incorrect　charts。　Such　were　the　instruments　used　by　Columbus　and　Vasco　da　Gama　and　their　immediate　successors。　During　the　Newtonian　period　the　microscopes　generally　in　use　were　those　constructed　of　simple　lenses；　for　although　compound　microscopes　were　known；　the　difficulties　of　correcting　aberration　had　not　been　surmounted；　and　a　much　clearer　field　was　given　by　the　simple　instrument。　The　results　obtained　by　the　use　of　such　instruments；　however；　were　very　satisfactory　in　many　ways。　By　referring　to　certain　plates　in　this　volume；　which　reproduce　illustrations　from　Robert　Hooke's　work　on　the　microscope；　it　will　be　seen　that　quite　a　high　degree　of　effectiveness　had　been　attained。　And　it　should　be　recalled　that　Antony　von　Leeuwenboek；　whose　death　took　place　shortly　before　Newton's；　had　discovered　such　micro…organisms　as　bacteria；　had　seen　the　blood　corpuscles　in　circulation；　and　examined　and　described　other　microscopic　structures　of　the　body。

XIV。　PROGRESS　IN　ELECTRICITY　FROM　GILBERT　AND　VON　GUERICKE　TO　FRANKLIN　We　have　seen　how　Gilbert；　by　his　experiments　with　magnets；　gave　an　impetus　to　the　study　of　magnetism　and　electricity。　Gilbert　himself　demonstrated　some　facts　and　advanced　some　theories；　but　the　system　of　general　laws　was　to　come　later。　To　this　end　the　discovery　of　electrical　repulsion；　as　well　as　attraction；　by　Von　Guericke；　with　his　sulphur　ball；　was　a　step　forward；　but　something　like　a　century　passed　after　Gilbert's　beginning　before　anything　of　much　importance　was　done　in　the　field　of　electricity。　In　1705；　however；　Francis　Hauksbee　began　a　series　of　experiments　that　resulted　in　some　startling　demonstrations。　For　many　years　it　had　been　observed　that　a　peculiar　light　was　seen　sometimes　in　the　mercurial　barometer；　but　Hauksbee　and　the　other　scientific　investigators　supposed　the　radiance　to　be　due　to　the　mercury　in　a　vacuum；　brought　about；　perhaps；　by　some　agitation。　That　this　light　might　have　any　connection　with　electricity　did　not；　at　first；　occur　to　Hauksbee　any　more　than　it　had　to　his　predecessors。　The　problem　that　interested　him　was　whether　the　vacuum　in　the　tube　of　the　barometer　was　essential　to　the　light；　and　in　experimenting　to　determine　this；　he　invented　his　〃mercurial　fountain。〃　Having　exhausted　the　air　in　a　receiver　containing　some　mercury；　he　found　that　by　allowing　air　to　rush　through　the　mercury　the　metal　became　a　jet　thrown　in　all　directions　against　the　sides　of　the　vessel；　making　a　great；　flaming　shower；　〃like　flashes　of　lightning；〃　as　he　said。　But　it　seemed　to　him　that　there　was　a　difference　between　this　light　and　the　glow　noted　in　the　barometer。　This　was　a　bright　light；　whereas　the　barometer　light　was　only　a　glow。　Pondering　over　this；　Hauksbee　tried　various　experiments；　revolving　pieces　of　amber；　flint；　steel；　and　other　substances　in　his　exhausted　air…pump　receiver；　with　negative；　or　unsatisfactory；　results。　Finally；　it　occurred　to　him　to　revolve　an　exhausted　glass　tube　itself。　Mounting　such　a　globe　of　glass　on　an　axis　so　that　it　could　be　revolved　rapidly　by　a　belt　running　on　a　large　wheel；　he　found　that　by　holding　his　fingers　against　the　whirling　globe　a　purplish　glow　appeared；　giving　sufficient　light　so　that　coarse　print　could　be　read；　and　the　walls　of　a　dark　room　sensibly　lightened　several　feet　away。　As　air　was　admitted　to　the　globe　the　light　gradually　diminished；　and　it　seemed　to　him　that　this　diminished　glow　was　very　similar　in　appearance　to　the　pale　light　seen　in　the　mercurial　barometer。　Could　it　be　that　it　was　the　glass；　and　not　the　mercury；　that　caused　it？　Going　to　a　barometer　he　proceeded　to　rub　the　glass　above　the　column　of　mercury　over　the　vacuum；　without　disturbing　the　mercury；　when；　to　his　astonishment；　the　same　faint　light；　to　all　appearances　identical　with　the　glow　seen　in　the　whirling　globe；　was　produced。　Turning　these　demonstrations　over　in　his　mind；　he　recalled　the　well…known　fact　that　rubbed　glass　attracted　bits　of　paper；　leaf…brass；　and　other　light　substances；　and　that　this　phenomenon　was　supposed　to　be　electrical。　This　led　him　finally　to　determine　the　hitherto　unsuspected　fact；　that　the　glow　in　the　barometer　was　electrical　as　was　also　the　glow　seen　in　his　whirling　globe。　Continuing　his　investigations；　he　soon　discovered　that　solid　glass　rods　when　rubbed　produced　the　same　effects　as　the　tube。　By　mere　chance；　happening　to　hold　a　rubbed　tube　to　his　cheek；　he　felt　the　effect　of　electricity　upon　the　skin　like　〃a　number　of　fine；　limber　hairs；〃　and　this　suggested　to　him　that；　since　the　mysterious　manifestation　was　so　plain；　it　could　be　made　to　show　its　effects　upon　various　substances。　Suspending　some　woollen　threads　over　the　whirling　glass　cylinder；　he　found　that　as　soon　as　he　touched　the　glass　with　his　hands　the　threads；　which　were　waved　about　by　the　wind　of　the　revolution；　suddenly　straightened　themselves　in　a　peculiar　manner；　and　stood　in　a　radical　position；　pointing　to　the　axis　of　the　cylinder。　Encouraged　by　these　successes；　he　continued　his　experiments　with　breathless　expectancy；　and　soon　made　another　important　discovery；　that　of　〃induction；〃　although　the　real　significance　of　this　discovery　was　not　appreciated　by　him　or；　for　that　matter；　by　any　one　else　for　several　generations　following。　This　discovery　was　made　by　placing　two　revolving　cylinders　within　an　inch　of　each　other；　one　with　the　air　exhausted　and　the　other　unexhausted。　Placing　his　hand　on　the　unexhausted　tube　caused　the　light　to　appear　not　only　upon　it；　but　on　the　other　tube　as　well。　A　little　later　he　discovered　that　it　is　not　necessary　to　whirl　the　exhausted　tube　to　produce　this　effect；　but　simply　to　place　it　in　close　proximity　to　the　other　whirling　cylinder。　These　demonstrations　of　Hauksbee　attracted　wide　attention　and　gave　an　impetus　to　investigators　in　the　field　of　electricity；　but　still　no　great　advance　was　made　for　something　like　a　quarter　of　a　century。　Possibly　the　energies　of　the　scientists　were　exhausted　for　the　moment　in　exploring　the　new　fields　thrown　open　to　investigation　by　the　colossal　work　of　Newton。

THE　EXPERIMENTS　OF　STEPHEN　GRAY　In　1729　Stephen　Gray　（died　in　1736）；　an　eccentric　and　irascible　old　pensioner　of　the　Charter　House　in　London；　undertook　some　investigations　along　lines　similar　to　those　of　Hauksbee。　While　experimenting　with　a　glass　tube　for　producing　electricity；　as　Hauksbee　had　done；　he　noticed　that　the　corks　with　which　he　had　stopped　the　ends　of　the　tube　to　exclude　the　dust；　seemed　to　attract　bits　of　paper　and　leaf…brass　as　well