a history of science-2-第17章

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ertical　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　refraction　by　means　of　a　simple　yet　ingenious　trough…like　apparatus　which　enabled　him　to　compare　readily　the　direct　and　refracted　rays。　He　discovered　that　when　a　ray　of　light　passes　through　a　glass　plate；　if　it　strikes　the　farther　surface　of　the　glass　at　an　angle　greater　than　45　degrees　it　will　be　totally　refracted　instead　of　passing　through　into　the　air。　He　could　not　well　fail　to　know　that　different　mediums　refract　light　differently；　and　that　for　the　same　medium　the　amount　of　light　valies　with　the　change　in　the　angle　of　incidence。　He　was　not　able；　however；　to　generalize　his　observations　as　he　desired；　and　to　the　last　the　law　that　governs　refraction　escaped　him。　It　remained　for　Willebrord　Snell；　a　Dutchman；　about　the　year　1621；　to　discover　the　law　in　question；　and　for　Descartes；　a　little　later；　to　formulate　it。　Descartes；　indeed；　has　sometimes　been　supposed　to　be　the　discoverer　of　the　law。　There　is　reason　to　believe　that　he　based　his　generalizations　on　the　experiment　of　Snell；　though　he　did　not　openly　acknowledge　his　indebtedness。　The　law；　as　Descartes　expressed　it；　states　that　the　sine　of　the　angle　of　incidence　bears　a　fixed　ratio　to　the　sine　of　the　angle　of　refraction　for　any　given　medium。　Here；　then；　was　another　illustration　of　the　fact　that　almost　infinitely　varied　phenomena　may　be　brought　within　the　scope　of　a　simple　law。　Once　the　law　had　been　expressed；　it　could　be　tested　and　verified　with　the　greatest　ease；　and；　as　usual；　the　discovery　being　made；　it　seems　surprising　that　earlier　investigatorsin　particular　so　sagacious　a　guesser　as　Keplershould　have　missed　it。　Galileo　himself　must　have　been　to　some　extent　a　student　of　light；　since；　as　we　have　seen；　he　made　such　notable　contributions　to　practical　optics　through　perfecting　the　telescope；　but　he　seems　not　to　have　added　anything　to　the　theory　of　light。　The　subject　of　heat；　however；　attracted　his　attention　in　a　somewhat　different　way；　and　he　was　led　to　the　invention　of　the　first　contrivance　for　measuring　temperatures。　His　thermometer　was　based　on　the　afterwards　familiar　principle　of　the　expansion　of　a　liquid　under　the　influence　of　heat；　but　as　a　practical　means　of　measuring　temperature　it　was　a　very　crude　affair；　because　the　tube　that　contained　the　measuring　liquid　was　exposed　to　the　air；　hence　barometric　changes　of　pressure　vitiated　the　experiment。　It　remained　for　Galileo's　Italian　successors　of　the　Accademia　del　Cimento　of　Florence　to　improve　upon　the　apparatus；　after　the　experiments　of　Torricellito　which　we　shall　refer　in　a　momenthad　thrown　new　light　on　the　question　of　atmospheric　pressure。　Still　later　the　celebrated　Huygens　hit　upon　the　idea　of　using　the　melting　and　the　boiling　point　of　water　as　fixed　points　in　a　scale　of　measurements；　which　first　gave　definiteness　to　thermometric　tests。

TORRICELLI　In　the　closing　years　of　his　life　Galileo　took　into　his　family；　as　his　adopted　disciple　in　science；　a　young　man；　Evangelista　Torricelli　（1608…1647）；　who　proved　himself；　during　his　short　lifetime；　to　be　a　worthy　follower　of　his　great　master。　Not　only　worthy　on　account　of　his　great　scientific　discoveries；　but　grateful　as　well；　for　when　he　had　made　the　great　discovery　that　the　〃suction〃　made　by　a　vacuum　was　really　nothing　but　air　pressure；　and　not　suction　at　all；　he　regretted　that　so　important　a　step　in　science　might　not　have　been　made　by　his　great　teacher；　Galileo；　instead　of　by　himself。　〃This　generosity　of　Torricelli；〃　says　Playfair；　〃was；　perhaps；　rarer　than　his　genius：　there　are　more　who　might　have　discovered　the　suspension　of　mercury　in　the　barometer　than　who　would　have　been　willing　to　part　with　the　honor　of　the　discovery　to　a　master　or　a　friend。〃　Torricelli's　discovery　was　made　in　1643；　less　than　two　years　after　the　death　of　his　master。　Galileo　had　observed　that　water　will　not　rise　in　an　exhausted　tube；　such　as　a　pump；　to　a　height　greater　than　thirty…three　feet；　but　he　was　never　able　to　offer　a　satisfactory　explanation　of　the　principle。　Torricelli　was　able　to　demonstrate　that　the　height　at　which　the　water　stood　depended　upon　nothing　but　its　weight　as　compared　with　the　weight　of　air。　If　this　be　true；　it　is　evident　that　any　fluid　will　be　supported　at　a　definite　height；　according　to　its　relative　weight　as　compared　with　air。　Thus　mercury；　which　is　about　thirteen　times　more　dense　than　water；　should　only　rise　to　one…thirteenth　the　height　of　a　column　of　waterthat　is；　about　thirty　inches。　Reasoning　in　this　way；　Torricelli　proceeded　to　prove　that　his　theory　was　correct。　Filling　a　long　tube；　closed　at　one　end；　with　mercury；　he　inverted　the　tube　with　its　open　orifice　in　a　vessel　of　mercury。　The　column　of　mercury　fell　at　once；　but　at　a　height　of　about　thirty　inches　it　stopped　and　remained　stationary；　the　pressure　of　the　air　on　the　mercury　in　the　vessel　maintaining　it　at　that　height。　This　discovery　was　a　shattering　blow　to　the　old　theory　that　had　dominated　that　field　of　physics　for　so　many　centuries。　It　was　completely　revolutionary　to　prove　that；　instead　of　a　mysterious　something　within　the　tube　being　responsible　for　the　suspension　of　liquids　at　certain　heights；　it　was　simply　the　ordinary　atmospheric　pressure　mysterious　enough；　it　is　truepushing　upon　them　from　without。　The　pressure　exerted　by　the　atmosphere　was　but　little　understood　at　that　time；　but　Torricelli's　discovery　aided　materially　in　solving　the　mystery。　The　whole　class　of　similar　phenomena　of　air　pressure；　which　had　been　held　in　the　trammel　of　long…established　but　false　doctrines；　was　now　reduced　to　one　simple　law；　and　the　door　to　a　solution　of　a　host　of　unsolved　problems　thrown　open。　It　had　long　been　suspected　and　believed　that　the　density　of　the　atmosphere　varies　at　certain　times。　That　the　air　is　sometimes　〃heavy〃　and　at　other　times　〃light〃　is　apparent　to　the　senses　without　scientific　apparatus　for　demonstration。　It　is　evident；　then；　that　Torricelli's　column　of　mercury　should　rise　and　fall　just　in　proportion　to　the　lightness　or　heaviness　of　the　air。　A　short　series　of　observations　proved　that　it　did　so；　and　with　those　observations　went　naturally　the　observations　as　to　changes　in　the　weather。　It　was　only　necessary；　therefore；　to　scratch　a　scale　on　the　glass　tube；　indicating　relative　atmospheric　pressures；　and　the　Torricellian　barometer　was　complete。　Such　a　revolutionary　theory　and　such　an　important　discovery　were；　of　course；　not　to　be　accepted　without　controversy；　but　the　feeble　arguments　of　the　opponents　showed　how　untenable　the　old　theory　had　become。　In　1648　Pascal　suggested　that　if　the　theory　of　the　pressure　of　air　upon　the　mercury　was　correct；　it　could　be　demonstrated　by　ascending　a　mountain　with　the　mercury　tube。　As　the　air　was　known　to　get　progressively　lighter　from　base　to　summit；　the　height　of　the　column　should　be　progressively　lessened　as　the　ascent　was　made；　and　increase　again　on　the　descent　into　the　denser　air。　The　experiment　was　made　on　the　mountain　called　the　Puy…de…Dome；　in　Auvergne；　and　the　column　of　mercury　fell　and　rose　progressively　through　a　space　of　about　three　inches　as　the　ascent　and　descent　were　made。　This　experiment　practically　sealed　the　verdict　on　the　new　theory；　but　it　also　suggested　something　more。　If　the