the dominion of the air-第30章

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d　his　danger　when　his　balloon　blew　up　〃with　an　awfully　grand　explosion；〃　and　he　was　reduced　to　the　extremity　of　jumping　for　his　life；　happily　escaping　the　mass　of　burning　silk　and　ropes。

The　awful　predicament　of　falling　into　the　sea；　which　has　been　illustrated　already；　and　which　will　recur　again　in　these　pages；　was　ably　and　successfully　met　by　Mr。　Cunningham；　who　made　an　afternoon　ascent　from　the　Artillery　Barracks　at　Clevedon；　reaching　Snake　Island　at　nightfall；　where；　owing　to　the　gathering　darkness；　he　felt　constrained　to　open　his　valve。　　He　quickly　commenced　descending　into　the　sea；　and　when　within　ten　feet　of　the　water；　turned　the　〃detaching　screw〃　which　connected　the　car　with　the　balloon。　　The　effect　of　this　was　at　once　to　launch　him　on　the　waves；　but；　being　still　able　to　keep　control　over　the　valve；　he　allowed　just　enough　gas　to　remain　within　the　silk　to　hold　the　balloon　above　water。　　He　then　betook　himself　to　the　paddles　with　which　his　craft　was　provided；　and　reached　Snake　Island　with　the　balloon　in　tow。　　Here　he　seems　to　have　found　good　use　for　a　further　portion　of　his　very　complete　equipment；　for；　lighting　a　signal　rocket；　he　presently　brought　a　four…oared　gig　to　his　succour　from　Portsmouth　Harbour。

The　teaching　of　the　above　incident　is　manifest　enough。　　If　it　should　be　contemplated　to　use　the　balloon　for　serious　or　lengthened　travel　anywhere　within　possible　reach　of　the　sea…boardand　this　must　apply　to　all　parts　of　the　British　Islesit　must　become　a　wise　precaution；　if　not　an　absolute　necessity；　to　adopt　some　form　of　car　that　would　be　of　avail　in　the　event　of　a　fall　taking　place　in　the　sea。　　Sufficient　confirmation　of　this　statement　will　be　shortly　afforded　by　a　memorable　voyage　accomplished　during　the　partnership　of　Messrs。　　Glaisher　and　Coxwell；　one　which　would　certainly　have　found　the　travellers　in　far　less　jeopardy　had　their　car　been　convertible　into　a　boat。　　We　have　already　seen　how　essential　Wise　considered　this　expedient　in　his　own　bolder　schemes；　and　it　may　further　be　mentioned　here　that　modern　air　ships　have　been　designed　with　the　intention　of　making　the　water　a　perfectly　safe　landing。

The　ballooning　exploits　which；　however；　we　have　now　to　recount　had　quite　another　and　more　special　object　consistently　in　viewthat　of　scientific　investigation；　and　we　would　here　premise　that　the　proper　appreciation　of　these　investigations　will　depend　on　a　due　understanding　of　the　attendant　circumstances；　as　also　of　the　constant　characteristic　behaviour　of　balloons；　whether　despatched　for　mere　travel　or　research。

First　let　us　regard　the　actual　path　of　a　balloon　in　space　when　being　manoeuvred　in　the　way　we　read　of　in　Mr。　Glaisher's　own　accounts。　　This　part　is　in　most　cases　approximately　indicated　in　that　most　attractive　volume　of　his　entitled；　〃Travels　in　the　Air；〃　by　diagrams　giving　a　sectional　presentment　of　his　more　important　voyages；　but　a　little　commonplace　consideration　may　take　the　place　of　diagrams。

It　has　been　common　to　assert　that　a　balloon　poised　in　space　is　the　most　delicate　balance　conceivable。　　Its　intrinsic　weight　must　be　exactly　equal　to　the　weight　of　the　air　it　displaces；　and　since　the　density　of　the　air　decreases　according　to　a　fixed　law；　amounting；　approximately；　to　a　difference　in　barometric　reading　of　0。1　inch　for　every　90　feet；　it　follows；　theoretically；　that　if　a　balloon　is　poised　at　1；000　feet　above　sea　level；　then　it　would　not　be　in　equilibrium　at　any　other　height；　so　long　as　its　weight　and　volume　remain　the　same。　　If　it　were　50　feet　higher　it　must　commence　descending；　and；　if　lower；　then　it　must　ascend　till　it　reaches　its　true　level；　and；　more　than　that；　in　the　event　of　either　such　excursion　mere　impetus　would　carry　it　beyond　this　level；　about　which　it　would　oscillate　for　a　short　time；　after　the　manner　of　the　pendulum。　　This　is　substantially　true；　but　it　must　be　taken　in　connection　with　other　facts　which　have　a　far　greater　influence　on　a　balloon's　position　or　motion。

For　instance；　in　the　volume　just　referred　to　it　is　stated　by　M。　Gaston　Tissandier　that　on　one　occasion　when　aloft　he　threw　overboard　a　chicken　bone；　and；　immediately　consulting　a　barometer；　had　to　admit　on　〃clearest　evidence　that　the　bone　had　caused　a　rise　of　from　twenty　to　thirty　yards；　so　delicately　is　a　balloon　equipoised　in　the　air。〃　　Here；　without　pausing　to　calculate　whether　the　discharge　of　an　ounce　or　so　would　suffice　to　cause　a　large　balloon　to　ascend　through　ninety　feet；　it　may　be　pointed　out　that　the　record　cannot　be　trustworthy；　from　the　mere　fact　that　a　free　balloon　is　from　moment　to　moment　being　subjected　to　other　potent　influences；　which　necessarily　affect　its　position　in　space。　　In　daytime　the　sun's　influence　is　an　all…important　factor；　and　whether　shining　brightly　or　partially　hidden　by　clouds；　a　slight　difference　in　obscuration　will　have　a　ready　and　marked　effect　on　the　balloon's　altitude。　　Again；　a　balloon　in　transit　may　pass　almost　momentarily　from　a　warmer　layer　of　air　to　a　colder；　or　vice　versa；　the　plane　of　demarcation　between　the　two　being　very　definite　and　abrupt；　and　in　this　case　altitude　is　at　once　affected；　or；　yet　again；　there　are　the　descending　and　ascending　currents；　met　with　constantly　and　unexpectedly；　which　have　to　be　reckoned　with。

Thus　it　becomes　a　fact　that　a　balloon's　vertical　course　is　subjected　to　constant　checks　and　vicissitudes　from　a　variety　of　causes；　and　these　will　have　to　be　duly　borne　in　mind　when　we　are　confronted　with　the　often　surprising　results　and　readings　which　are　supplied　by　scientific　observers。　　With　regard　to　the　close　proximity；　without　appreciable　intermingling；　of　widely　differing　currents；　it　should　be　mentioned　that　explorers　have　found　in　regions　where　winds　of　different　directions　pass　each　other　that　one　air　stream　appears　actually　to　drag　against　the　surface　of　the　other；　as　though　admitting　no　interspace　where　the　streams　might　mingle。　　Indeed；　trustworthy　observers　have　stated　that　even　a　hurricane　can　rage　over　a　tranquil　atmosphere　with　a　sharply　defined　surface　of　demarcation　between　calm　and　storm。　　Thus；　to　quote　the　actual　words　of　Charles　Darwin；　than　whom　it　is　impossible　to　adduce　a　more　careful　witness；　we　find　him　recording　how　on　mountain　heights　he　met　with　winds　turbulent　and　unconfined；　yet　holding　courses　〃like　rivers　within　their　beds。〃

It　is　in　tracing　the　trend　of　upper　air　streams；　to　whose　wayward　courses　and　ever　varying　conditions　we　are　now　to　be　introduced；　that　much　of　our　most　valuable　information　has　come；　affecting　the　possibility　of　forecasting　British　wind　and　weather。　　It　should　need　no　insisting　on　that　the　data　required　by　meteorologists　are　not　sufficiently　supplied　by　the　readings　of　instruments　placed　on　or　near　the　ground；　or　by　the　set　of　the　wind　as　determined　by　a　vane　planted　on　the　top　of　a　pole　or　roof　of　a　building。　　The　chief　factors　in　our　meteorology　are　rather　those　broader　and　deeper　conditions　which　obtain　in　higher　regions　necessarily　beyond　our　ken；　until　those　regions　are　duly　and　diligently　explored。

Mr。　Glaisher's　estimate　of　the　utility　of　the　balloon　as　an　instrument　of　research；　formed　at　the　conclusion　of　his　aeronautical　labours；　has　a　special　value　and　significance。　　Speaking　with　all　the　weight　attaching　to　so　trained　and　eminent　an　observer；　he　declares；　〃The　balloon；　considered　as　an　instrument　for　vertical　exploration；　presents　itself　to　us　under　a　variety　of　aspects；　each　of　which　is　fertile　in　suggestions。　　Regarding　the　atmosphere　as　the　great　laboratory　of　changes　which　contain　the　germ　of　future　dis　discoveries；　to　belong　respectively；　as　they　unfold；　to　the　chemist　and　meteorologist；　the　physical　relation　to　animal　life　of　different　heights；　the　form　of　death　which　at　certain　elevations　waits　to　accomplish　its　destruction；　the　effect　of　diminished　pressure　upon　individuals　similarly　placed；　the　comparison　of　mountain　ascents　with　the　experiences　of　aeronauts；　are　some　of　the　questions　which　suggest　themselves　and　faintly　indicate　enquiries　which　naturally　ally　themselves　to　the　course　of　balloon　experiments。　　Sufficiently　varied　and　important；　they　will　be　seen　to　rank　the　balloon　as　a　valuable　aid　to　the　uses　of　philosophy；　and　rescue　it　from　the　impending　degradation　of　continuing　a