the dominion of the air-第19节

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country　through　which　he　is　making　an　unwilling　steeplechase　may　be　difficult；　or　even　dangerous。　　Rivers；　railway　cuttings；　or　other　undesirable　obstacles　may　lie　ahead；　or；　worse　yet；　such　a　death　trap　as　in　such　circumstances　almost　any　part　of　Derbyshire　affords；　with　its　stone　walls；　its　precipitous　cliffs；　and　deep　rocky　dells。　　To　be　dragged　at　the　speed　of　an　express　train　through　territory　of　this　description　will　presently　mean　damage　to　something；　perhaps　to　telegraph　poles；　to　roofs；　or　crops；　and　if　not；　then　to　the　balloon　itself。　　Something　appertaining　to　it　must　be　victimised；　and　it　is　in　all　ways　best　that　this　should　be　the　fabric　of　the　balloon　itself。　　If　made　of　some　form；　or　at　least　some　proportion　of　linen；　this　will　probably　rend　ere　long；　and；　allowing　the　gas　to　escape；　will　soon　bring　itself　to　rest。　　On　the　other　hand；　if　the　balloon　proper　is　a　silk　one；　with　sound　net　and　in　good　condition；　it　is　probable　that　something　else　will　give　way　first；　and　that　something　may　prove　to　be　the　hapless　passenger　or　passengers。

And　here　be　it　laid　down　as　one　first　and　all…important　principle；　that　in　any　such　awkward　predicament　as　that　just　described；　if　there　be　more　than　one　passenger　aboard；　let　none　attempt　to　get　out。　　In　the　first　place；　he　may　very　probably　break　a　limb　in　so　doing；　inasmuch　as　the　tangle　of　the　ropes　will　not　allow　of　his　getting　cut　readily；　or；　when　actually　on　the　ground；　he　may　be　caught　and　impaled　by　the　anchor　charging　and　leaping　behind。　　But；　worse　than　all；　he　may；　in　any　case；　jeopardise　the　lives　of　his　companions；　who　stand　in　need　of　all　the　available　weight　and　help　that　the　car　contains　up　to　the　moment　Of　coming　to　final　rest。

We　have　already　touched　on　the　early　notions　as　to　the　means　of　steering　a　balloon。　　Oars　had　been　tested　without　satisfactory　result；　and　the　conception　of　a　rotary　screw　found　favour　among　theorists　at　this　time；　the　principle　being　actually　tried　with　success　in　working　models；　which；　by　mechanical　means；　could　be　made　to　flit　about　in　the　still　air　of　the　lecture　room；　but　the　only　feasible　method　advocated　was　that　already　alluded　to；　which　depended　on　the　undesirable　action　of　a　trail　rope　dragging　over　the　ground　or　through　water。　　The　idea　was；　of　course；　perfectly　practical；　and　was　simply　analogous　to　the　method　adopted　by　sailors；　who；　when　floating　with　the　stream　but　without　wind；　are　desirous　of　gaining　〃steerage　way。〃　　While　simply　drifting　with　the　flood；　they　are　unable　to　guide　their　vessel　in　any　way；　and　this；　in　practice；　is　commonly　effected　by　simply　propelling　the　vessel　faster　than　the　stream；　in　which　case　the　rudder　at　once　becomes　available。　　But　the　same　result　is　equally　well　obtained　by　slowing　the　vessel；　and　this　is　easily　accomplished　by　a　cable；　with　a　small　anchor　or　other　weight　attached；　dragging　below　the　vessel。　　This　cable　is　essentially　the　same　as　the　guide…rope　of　the　older　aeronauts。

It　is　when　we　come　to　consider　the　impressions　and　sensations　described　by　sky　voyagers　of　bygone　times　that　we　find　them　curiously　at　variance　with　our　own。　　As　an　instance；　we　may　state　that　the　earth；　as　seen　from　a　highflying　balloon；　used　to　be　almost　always　described　as　appearing　concave；　or　like　a　huge　basin；　and　ingenious　attempts　were　made　to　prove　mathematically　that　this　must　be　so。　　The　laws　of　refraction　are　brought　in　to　prove　the　fact；　or；　again；　the　case　is　stated　thus：　　Supposing　the　extreme　horizon　to　be　seen　when　the　balloon　is　little　more　than　a　mile　high；　the　range　of　view　on　all　sides　will　then　be；　roughly；　some　eighty　miles。　　If；　then；　a　line　were　drawn　from　the　aerial　observer　to　this　remote　distance；　that　line　would　be　almost　horizontal；　so　nearly　so　that　he　cannot　persuade　himself　that　his　horizon　is　otherwise　than　still　on　a　level　with　his　eye；　yet　the　earth　below　him　lies；　as　it　seems；　at　the　bottom　of　a　huge　gulf。　　Thus　the　whole　visible　earth　appears　as　a　vast　bowl　or　basin。　　This　is　extremely　ingenious　reasoning；　and　not　to　be　disregarded；　but　the　fact　remains　that　in　the　experience　of　the　writer　and　of　many　others　whom　he　has　consulted；　there　is　no　such　optical　illusion　as　I　have　just　discussed；　and　to　their　vision　it　is　impossible　to　regard　the　earth　as　anything　but　uniformly　flat。

Another　impression　invariably　insisted　on　by　early　balloonists　is　that　the　earth；　on　quitting　it；　appears　to　drop　away　into　an　abyss；　leaving　the　voyagers　motionless；　and　this　illusion　must；　indeed；　be　probably　universal。　　It　is　the　same　illusion　as　the　apparent　gliding　backwards　of　objects　to　a　traveller　in　a　railway　carriage；　only　in　this　latter　case　the　rattling　and　shaking　of　the　carriage　helps　the　mind　to　grasp　the　real　fact　that　the　motion　belongs　to　the　train　itself；　whereas　it　is　otherwise　with　a　balloon；　whose　motion　is　so　perfectly　smooth　as　to　be　quite　imperceptible。

Old　ideas；　formed　upon　insufficient　observations；　even　if　erroneous；　were　slow　to　die。　　Thus　it　used　to　be　stated　that　an　upper　cloud　floor　adapted　itself　to　the　contour　of　the　land　over　which　it　rested；　giving　what　Mr。　Monck　Mason　has　called　a　〃phrenological　estimate〃　of　the　character　of　the　earth　below；　the　clouds；　〃even　when　under　the　influence　of　rapid　motion；　seeming　to　accommodate　themselves　to　all　variations　of　form　in　the　surface　of　the　subjacent　soil；　rising　with　its　prominences　and　sinking　with　its　depressions。〃　　Probably　few　aeronauts　of　the　present　time　will　accept　the　statement。

It　used　commonly　to　be　asserted；　and　is　so　often　to　this　day；　that　a　feeling　as　of　sea…sickness　is　experienced　in　balloon　travel；　and　the　notion　has　undoubtedly　arisen　from　the　circumstances　attending　an　ascent　in　a　captive　balloon。　　It　were　well；　now　that　ballooning　bids　fair　to　become　popular；　to　disabuse　the　public　mind　of　such　a　wholly　false　idea。　　The　truth　is　that　a　balloon　let　up　with　a　lengthy　rope　and　held　captive　will；　with　a　fitful　breeze；　pitch　and　sway　in　a　manner　which　may　induce　all　the　unpleasant　feelings　attending　a　rough　passage　at　sea。　　It　may　do　worse；　and　even　be　borne　to　earth　with　a　puff　of　wind　which　may　come　unexpectedly；　and　considerably　unsettle　the　nerves　of　any　holiday　passenger。　　I　could　tell　of　a　〃captive〃　that　had　been　behaving　itself　creditably　on　a　not　very　settled　day　suddenly　swooping　over　a　roadway　and　down　into　public　gardens；　where　it　lay　incontinently　along　the　ground；　and　then；　before　the　astonished　passengers　could　attempt　to　alight；　it　was　seized　with　another　mood；　and；　mounting　once　again　majestically　skyward；　submitted　to　be　hauled　down　with　all　becoming　grace　and　ease。　　It　is　owing　to　their　vagaries　and　want　of　manageability　that；　as　will　be　shown；　〃captives〃　are　of　uncertain　use　in　war。　　On　the　other　hand；　a　free　balloon　is　exempt　from　such　disadvantages；　and　at　moderate　heights　not　the　smallest　feeling　of　nausea　is　ever　experienced。　　The　only　unpleasant　sensation；　and　that　not　of　any　gravity；　ever　complained　of；　is　a　peculiar　tension　in　the　ears　experienced　in　a　rapid　ascent；　or　more　often；　perhaps；　in　a　descent。　　The　cause；　which　is　trivial　and　easily　removed；　should　be　properly　understood；　and　cannot　be　given　in　clearer　language　than　that　used　by　Professor　Tyndall：〃Behind　the　tympanic　membrane　exists　a　cavitythe　drum　of　the　earin　part　crossed　by　a　series　of　bones；　and　in　part　occupied　by　air。　　This　cavity　communicates　with　the　mouth　by　means　of　a　duct　called　the　Eustachian　tube。　　This　tube　is　generally　closed；　the　air　space　behind　the　tympanic　membrane　being　thus　cut　off　from　the　external　air。　　If；　under　these　circumstances；　the　external　air　becomes　denser；　it　will　press　the　tympanic　membrane　inwards；　if；　on　the　other　hand；　the　air　on　the　other　side　becomes　rarer；　while　the　Eustachian　tube　becomes　closed；　the　membrane　will　be　pressed　outwards。　　Pain　is　felt　in　both　cases；　and　partial　deafness　is　experienced。。。。　By　the　act　of　swallowing　the　Eustachian　tube　is　opened；　and　thus　equilibrium　is　established　between　the　external　and　internal　pressure。〃

Founded　on　physical　facts　more　or　less　correct　in　themselves；　come　a　number　of　tales　of　olden　days；　which　are　at　least　more　marvellous　than　credible；　the　following　serving　as　an　example。　　The　scientific　truth　underlying　the　story　is　the　well…known　expedient　of　placing　a　shrivelled　apple　under　the　receiver　of　an　air　pump。　　As　the　air　becomes　rarefied　the　apple　swells；　smooths　itself　out；　and　presently　becomes　round　and　rosy　as　it　was　in　the　summer　time。　　It　is　recorded　that　on　one　occasion　a　man　of　mature　years　made　an　ascent；　accompanied　by　his　son；　and；　after　reaching　some　height；　the　youth　remarked　on　how　young　his　father　was　looking。　　They　still　continued　to　ascend；　and　the　same　remark　was　repeated　more　than　once。　　And　at　last；　having　now　reached　attenuated　regions；　the　son　cried　in　astonishment；　〃Why；　dad；　you　ought　to　be　at　school！〃　　The　cause　of　this　remark　was　that　in　the　rarefied　air　all　the　wrinkles　had　come　out　of　the　old　man's　face；　and　his　cheeks　were　as　chubby　as　his　son's。

This　discussion　of　old　ideas　should　not　be　closed　without　mention　of　a　plausible　plea　for　the　balloon　made　by　Wise　and　others　on　the　score　of　its　value　to　health。　　Lofty　ascents　have　proved　a　strain　on　even　robust　constitutionsthe　heart　may　begin　to　suffer；　or　i