or　sparks。〃
  CHAPTER　XI。　THE　BALLOON　IN　THE　SERVICE　OF　SCIENCE。
  At　this　point　we　must；　for　a　brief　while；　drop　the　history　of　the　famous　aeronaut　whose　early　career　we　have　been　briefly　sketching　in　the　last　chapter；　and　turn　our　attention　to　a　new　feature　of　English　ballooning。　　We　have；　at　last；　to　record　some　genuinely　scientific　ascents；　which　our　country　now；　all　too　tardily；　instituted。　　It　was　the　British　Association　that　took　the　initiative；　and　the　two　men　they　chose　for　their　purpose　were　both　exceptionally　qualified　for　the　task　they　had　in　hand。　　The　practical　balloonist　was　none　other　than　the　veteran　Charles　Green；　now　in　his　sixty…seventh　year；　but　destined　yet　to　enjoy　nearly　twenty　years　more　of　life。　　The　scientific　expert　was　Mr。　John　Welsh；　well　fitted　for　the　projected　work　by　long　training　at　Kew　Observatory。　　The　balloon　which　they　used　is　itself　worthy　of　mention；　being　the　great　Nassau　Balloon　of　olden　fame。
  Welsh　was　quick　to　realise　more　clearly　than　any　former　experimentalist　that　on　account　of　the　absence　of　breeze　in　a　free　balloon；　as　also　on　account　of　great　solar　radiation；　the　indications　of　thermometers　would；　without　special　precautions；　be　falsified。　　He　therefore　invented　a　form　of　aspirating　thermometer；　the　earliest　to　be　met　with；　and　far　in　advance　of　any　that　were　subsequently　used　by　other　scientists。　　It　consisted　of　a　polished　tube；　in　which　thermometers　were　enclosed；　and　through　which　a　stream　of　air　was　forced　by　bellows。
  The　difficulty　of　obtaining　really　accurate　readings　where　thermometers　are　being　quickly　transported　through　varying　temperatures　is　generally　not　duly　appreciated。　　In　the　case　of　instruments　carried　m　a　balloon　it　should　be　remembered　that　the　balloon　itself　conveys；　clinging　about　it；　no　inconsiderable　quantity　of　air；　brought　from　other　levels；　while　the　temperature　of　its　own　mass　will　be　liable　to　affect　any　thermometer　in　close　neighbourhood。　　Moreover；　any　ordinary　form　of　thermometer　is　necessarily　sluggish　in　action；　as　may　be　readily　noticed。　　If；　for　example；　one　be　carried　from　a　warm　room　to　a　cold　passage；　or　vice　versa　it　will　be　seen　that　the　column　moves　very　deliberately；　and　quite　a　long　interval　will　elapse　before　it　reaches　its　final　position；　the　cause　being　that　the　entire　instrument；　with　any　stand　or　mounting　that　it　may　have；　will　have　to　adapt　itself　to　the　change　of　temperature　before　a　true　record　will　be　obtained。　　This　difficulty　applies　unavoidably　to　all　thermometers　in　some　degree；　and　the　skill　of　instrument　makers　has　been　taxed　to　reduce　the　errors　to　a　minimum。　　It　is　necessary；　in　any　case；　that　a　constant　stream　of　surrounding　air　should　play　upon　the　instrument；　and　though　this　is　most　readily　effected　when　instruments　are　carried　aloft　by　kites；　yet　even　thus　it　is　thought　that　an　interval　of　some　minutes　has　to　elapse　before　any　form　of　thermometer　will　faithfully　record　any　definite　change　of　temperature。　　It　is　on　this　account　that　some　allowance　must　be　made　for　observations　which　will；　in　due　place；　be　recorded　of　scientific　explorers；　the　point　to　be　borne　in　mind　being　that；　as　was　mentioned　in　a　former　chapter；　such　observations　will　have　to　be　regarded　as　giving　readings　which　are　somewhat　too　high　in　ascents　and　too　low　in　descents。　　Two　forms　of　thermometers　at　extremely　simple　construction；　yet　possessed　of　great　sensibility；　will　be　discussed　in　later　chapters。
  The　thermometers　that　Welsh　used　were　undoubtedIy　far　superior　to　any　that　were　devised　before　his　time　and　it　is　much　to　be　regretted　that　they　were　allowed　to　fall　into　disuse。　　Perhaps　the　most　important　stricture　on　the　observations　that　will　have　to　be　recorded　is　that　the　observers　were　not　provided　with　a　base　station；　on　which　account　the　value　of　results　was　impaired。　　It　was　not　realised　that　it　was　necessary　to　make　observations　on　the　ground　to　compare　with　those　that　were　being　made　at　high　altitudes。
  Welsh　made；　in　all；　four　ascents　in　the　summer　and　autumn　of　1852　and　in　his　report　he　is　careful　to　give　the　highest　praise　to　his　colleague；　Green；　whose　control　over　his　balloon　he　describes　as　〃so　complete　that　none　who　accompanied　him　can　be　otherwise　than　relieved　from　all　apprehension；　and　free　to　devote　attention　calmly　to　the　work　before　him。〃
  The　first　ascent　was　made　at　3。49　p。m。　on　August　the　17th；　under　a　south　wind　and　with　clouds　covering　some　three…quarters　of　the　sky。　　Welsh's　first　remark　significant；　and　will　be　appreciated　by　anyone　who　has　attempted　observational　work　in　a　balloon。　　He　states　naively　that　〃a　short　time　was　lost　at　first　in　an　attempt　to　put　the　instruments　into　more　convenient　order；　and　also　from　the　novelty　of　the　situation。〃　　Then　he　mentions　an　observation　which；　in　the　experience　of　the　writer；　is　a　common　one。　　The　lowest　clouds；　which　were　about　2；500　feet　high　and　not　near　the　balloon；　were　passed　without　being　noticed；　other　clouds　were　passed　at　different　heights；　and；　finally；　a　few　star…shaped　crystals　of　snow；　but　the　sun　shone　almost　constantly。　　Little　variation　occurred　in　the　direction　of　travel；　which　averaged　thirty…eight　miles　an　hour；　and　the　descent　took　place　at　5。20　p。m。　at　Swavesey；　near　Cambridge。
  The　second　ascent　took　place　at　4。43　p。m。　on　August　26th；　under　a　gentle　east　wind　and　a　partially　obscured　sky。　　The　clouds　were　again　passed　without　being　perceived。　　This　was　at　the　height　of　3；000　feet；　beyond　which　was　very　clear　sky　of　deep　blue。　　The　air　currents　up　to　the　limits　of　12；000　feet　set　from　varying　directions。　　The　descent　occurred　near　Chesham　at　7。45　p。m。
  The　third　ascent；　at　2。35　p。m。　on　October　the　21st　was　made　into　a　sky　covered　with　dense　cloud　masses　lying　within　3；000　and　3；700　feet。　　The　sun　was　then　seen　shining　through　cirrus　far　up。　　The　shadow　of　the　balloon　was　also　seen　on　the　cloud；　fringed　with　a　glory；　and　about　this　time　there　was　seen　〃stretching　for　a　considerable　length　in　a　serpentine　course；　over　the　surface　of　the　cloud；　a　well…defined　belt；　having　the　appearance　of　a　broad　road。〃
  Being　now　at　12；000　feet；　Green　thought　it　prudent　to　reconnoitre　his　position；　and；　finding　they　were　near　the　sea；　descended　at　4。20　p。m。　at　Rayleigh；　in　Essex。　　Some　important　notes　on　the　polarisation　of　the　clouds　were　made。
  The　fourth　and　final　voyage　was　made　in　a　fast　wind　averaging　fifty　knots　from　the　north…east。　　Thin　scud　was　met　at　1；900　feet；　and　an　upper　stratum　at　4；500　feet；　beyond　which　was　bright　sun。　　The　main　shift　of　wind　took　place　just　as　the　upper　surface　of　the　first　stratum　was　reached。　　In　this　ascent　Welsh　reached　his　greatest　elevation；　22；930　feet；　when　both　Green　and　himself　experienced　considerable　difficulty　in　respiration　and　much　fatigue。　　The　sea　being　now　perceived　rapidly　approaching；　a　hasty　descent　was　made；　and　many　of　the　instruments　were　broken。
  In　summarising　his　results　Welsh　states　that　〃the　temperature　of　the　air　decreases　uniformly　with　height　above　the　earth's　surface　until　at　a　certain　elevation；　varying　on　different　days；　decrease　is　arrested；　and　for　the　space　of　2；000　or　3；000　feet　the　temperature　remains　nearly　constant；　or　even　increases；　the　regular　diminution　being　again　resumed　and　generally　maintained　at　a　rate　slightly　less　rapid　than　in　the　lower　part　of　the　atmosphere；　and　commencing　from　a　higher　temperature　than　would　have　existed　but　for　the　interruption　noticed。〃　　The　analysis　of　the　upper　air　showed　the　proportion　of　oxygen　and　nitrogen　to　vary　scarcely　more　than　at　different　spots　on　the　earth。
  As　it　is　necessary　at　this　point　to　take　leave　of　the　veteran　Green　as　a　practical　aeronaut；　we　may　here　refer　to　one　or　two　noteworthy　facts　and　incidents　relating　to　his　eventful　career。　　In　1850　M。　Poitevin　is　said　to　have　attracted　140；000　people　to　Paris　to　look　at　an　exhibition　of　himself　ascending　in　a　balloon　seated　on　horseback；　after　which　Madame　Poitevin　ascended　from　Cremorne　Gardens　in　the　same　manner；　the　exhibition　being　intended　as　a　representation　of　〃Europa　on　a　Bull。〃　　This；　however；　was　discountenanced　by　the　authorities　and　withdrawn。　　The　feats　were；　in　reality；　merely　the　repetitions　of　one　that　had　been　conceived　and　extremely　well　carried　out　by　Green　many　years　beforeas　long　ago；　in　fact；　as　1828；　when　he　arranged　to　make　an　ascent　from　the　Eagle　Tavern；　City　Road；　seated　on　a　pony。　　To　carry　out　his　intention；　he　discarded　the　ordinary　car；　replacing　it　with　a　small　platform；　which　was　provided　with　places　to　receive　the　pony's　feet；　while　straps　attached　to　the　hoop　were　passed　under　the　animal's　body；　preventing　it　from　lying　down　or　from　making　any　violent　movement。　　This　the　creature　seemed　in　no　way　disposed　to　attempt；　and　when　all　had　been　successfully　carried　out　and　an　easy　descent　effected　at　Beckenham；　the　pony　was　discovered　eating　a　meal　of　beans　with　which　it　had　been　supplied。
  Several　interesting　observations　have　been　recorded　by　Green　on　different　occasions；　some　of　which　are　highly　instructive　from　a　practical　or　scientific　point　of　view。　　On　an　ascent　from　Vauxhall；　in　which　he　was　accompanied　by　his　friend　Spencer　and　Mr。　Rush；　he　recorded　how；　as　he　constantly　and　somewhat　rapidly　rose；　the　wind　changed　its　direction　from