the dominion of the air-第50部分

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Standing　on　an　open　heath　on　a　gusty　day；　we　may　often　note　the　rhythmic　buffeting　of　the　wind；　resembling　the　assault　of　rolling　billows　of　air。　　The　evidence　of　these　billows　has　been　actually　traced　far　aloft　in　balloon　travel；　when　aeronauts；　looking　down　on　a　wind…swept　surface　of　cloud；　have　observed　this　surface　to　be　thrown　into　a　series　of　rolls　of　vapour；　which　were　but　vast　and　veritable　waves　of　air。　　The　interval　between　successive　crests　of　these　waves　has　on　one　occasion　been　estimated　at　approximately　half　a　mile。　　We　have　seen　how　these　air　streams　sometimes　hold　wide　and　independent　sway　at　different　levels。　　We　have　seen；　too；　how　they　sometimes　meet　and　mingle；　not　infrequently　attended　with　electrical　disturbance

Through　broad　drifts　of　air　minor　air　streams　would　seem　often　literally　to　〃thread〃　their　way；　breakng　up　into　filaments　or　wandering　rills　of　air。　　In　the　voyage　across　Salisbury　Plain　lately　described；　while　the　balloon　was　being　carried　with　the　more　sluggish　current；　a　number　of　small　parachutes　were　dropped　out　at　frequent　intervals　and　carefully　watched。　　These　would　commonly　attend　the　balloon　for　a　little　while；　until；　getting　into　some　minor　air　stream；　they　would　suddenly　and　rapidly　diverge　at　such　wide　angles　as　to　suggest　that　crossing　our　actual　course　there　were　side　paths；　down　which　the　smaller　bodies　became　wafted。

On　another　occasion　the　writer　met　with　strongly　marked　and　altogether　exceptional　evidence　of　the　vehemence　and　persistence　of　these　minor　aerial　streamlets。　　It　was　on　an　occasion　in　April　weather；　when　a　heavy　overcast　sky　blotted　out　the　upper　heavens。　　In　the　cloud　levels　the　wind　was　somewhat　sluggish；　and　for　an　hour　we　travelled　at　an　average　speed　of　a　little　over　twenty　miles　an　hour；　never　higher　than　3；000　feet。　　At　this　point；　while　flying　over　Hertfordshire；　we　threw　out　sufficient　ballast　to　cause　the　balloon　to　rise　clear　of　the　hazy　lower　air；　and　coming　under　the　full　influence　of　the　sun；　then　in　the　meridian；　we　shot　upwards　at　considerable　speed；　and　soon　attained　an　altitude　of　three　miles。　　But　for　a　considerable　portion　of　this　climbwhile；　in　fact；　we　were　ascending　through　little　less　than　a　mile　of　our　upward　coursewe　were　assailed　by　impetuous　cross　currents；　which　whistled　through　car　and　rigging　and　smote　us　fairly　on　the　cheek。　　It　was　altogether　a　novel　experience；　and　the　more　remarkable　from　the　fact　that　our　main　onward　course　was　not　appreciably　diverted。

Then　we　got　above　these　currents；　and　remained　at　our　maximum　level；　while　we　floated；　still　at　only　a　moderate　speed；　the　length　of　a　county。　　The　descent　then　began；　and　once　again；　while　we　dropped　through　the　same　　disturbed　region；　the　same　far…reaching　and　obtrusive　cross…current　assailed　us。　　It　was　quite　obvious　that　the　vehement　currents　were　too　slender　to　tell　largely　upon　the　huge　surface　of　the　balloon；　as　it　was　being　swept　steadily　onwards　by　the　main　wind；　which　never　varied　in　direction　from　ground　levels　up　to　the　greatest　height　attained。

This　experience　is　but　confirmation　of　the　story　of　the　wind　told　by　the　wind　gauges　on　the　Forth　Bridge。　　Here　the　maximum　pressure　measured　on　the　large　gauge　of　300　square　feet　is　commonly　considerably　less　than　that　on　the　smaller　gauge；　suggesting　that　the　latter　must　be　due　to　threads　of　air　of　limited　area　and　high　velocity。

Further　and　very　valuable　light　is　thrown　on　the　peculiar　ways　of　the　wind；　now　being　considered；　by　Professor　Langley　in　the　special　researches　of　his　to　which　reference　has　already　been　made。　　This　eminent　observer　and　mathematician；　suspecting　that　the　old…fashioned　instruments；　which　only　told　what　the　wind　had　been　doing　every　hour；　or　at　best　every　minute；　gave　but　a　most　imperfect　record；　constructed　delicate　gauges；　which　would　respond　to　every　impulse　and　give　readings　from　second　to　second。

In　this　way　he　established　the　fact　that　the　wind；　far　from　being　a　body　of　even　approximate　uniformity；　is　under　most　ordinary　conditions　irregular　almost　beyond　conception。　　Further；　that　the　greater　the　speed　the　greater　the　fluctuations；　so　that　a　high　wind　has　to　be　regarded　as　〃air　moving　in　a　tumultuous　mass；〃　the　velocity　at　one　moment　perhaps　forty　miles　an　hour；　then　diminishing　to　an　almost　instantaneous　calm；　and　then　resuming。〃　　In　fact；　in　the　very　nature　of　the　case；　wind　is　not　the　result　of　one　simple　cause；　but　of　an　infinite　number　of　impulses　and　changes；　perhaps　long　passed；　which　are　preserved　in　it；　and　which　die　only　slowly　away。〃

When　we　come　to　take　observations　of　temperature　we　find　the　conditions　in　the　atmosphere　above　us　to　be　at　first　sight　not　a　little　complex；　and　altogether　different　in　day　and　night　hours。　　From　observations　already　recorded　in　this　volumenotably　those　of　Gay　Lussac；　Welsh；　and　Glaisherit　has　been　made　to　appear　that；　in　ascending　into　the　sky　in　daytime；　the　temperature　usually　falls　according　to　a　general　law；　but　there　are　found　regions　where　the　fall　of　temperature　becomes　arrested；　such　regions　being　commonly；　though　by　no　means　invariably；　associated　with　visible　cloud。　　It　is　probable；　however；　that　it　would　be　more　correct　not　to　interpret　the　presence　of　cloud　as　causing　manifestation　of　cold；　but　rather　to　regard　the　meeting　of　warm　and　cold　currents　as　the　cause　of　cloud。

The　writer　has　experimented　in　the　upper　regions　with　a　special　form　of　air　thermometer　of　great　sensibility；　designed　to　respond　rapidly　to　slight　variations　of　temperature。　　Testing　this　instrument　on　one　occasion　in　a　room　of　equable　warmth；　and　without　draughts；　he　was　puzzled　by　seeing　the　index　in　a　capillary　tube　suddenly　mounting　rapidly；　due　to　some　cause　which　was　not　apparent；　till　it　was　noticed　that　the　parlour　cat；　attracted　by　the　proceedings；　had　approached　near　the　apparatus。　　The　behaviour　of　this　instrument　when　slung　in　the　clear　some　distance　over　the　side　of　the　balloon　car；　and　carefully　watched；　suggests　by　its　fitful；　sudden；　and　rapid　changes　that　warmer　currents　are　often　making　their　way　in　such　slender　wandering　rills　as　have　been　already　pictured　as　permeating　the　broader　air　streams。　　During　night　hours　conditions　are　reversed。　　The　warmer　air　radiated　off　the　earth　through　the　day　has　then　ascended。　　It　will　be　found　at　different　heights；　lying　in　pools　or　strata；　possibly　resembling　in　form；　could　they　be　seen；　masses　of　visible　cloud。

The　writer　has　gathered　from　night　voyages　instructive　and　suggestive　facts　with　reference　to　the　ascent　of　air　streams；　due　to　differences　of　temperature；　particularly　over　London　and　the　suburbs；　and　it　is　conceivable　that　in　such　ascending　streams　may　lie　a　means　of　dealing　successfully　with　visitations　of　smoke　and　fog。

One　lesson　taught　by　balloon　travel　has　been　that　fog　or　haze　will　come　or　go　in　obedience　to　temperature　variations　at　low　levels。　　Thus　thick　haze　has　lain　over　London；　more　particularly　over　the　lower　parts；　at　sundown。　　Then　through　night　hours；　as　the　temperature　of　the　lower　air　has　become　equalised；　the　haze　has　completely　disappeared；　but　only　to　reassert　itself　at　dawn。

A　description　of　the　very　impressive　experience　of　a　night　sail　over　London　has　been　reserved；　but　should　not　be　altogether　omitted。　　Glaisher；　writing　of　the　spectacle　as　he　observed　it　nearly　forty　years　ago；　describes　London　seen　at　night　from　a　balloon　at　a　distance　as　resembling　a　vast　conflagration。　　When　actually　over　the　town；　a　main　thoroughfare　like　the　Commercial　Road　shone　up　like　a　line　of　brilliant　fire；　but；　travelling　westward；　Oxford　Street　presented　an　appearance　which　puzzled　him。　　〃Here　the　two　thickly　studded　rows　of　brilliant　lights　were　seen　on　either　side　of　the　street；　with　a　narrow；　dark　space　between；　and　this　dark　space　was　bounded；　as　it　were；　on　both　sides　by　a　bright　fringe　like　frosted　silver。〃　　Presently　he　discovered　that　　this　rich　effect　was　caused　by　the　bright　illumination　of　the　shop　lights　on　the　pavements。〃

London；　as　seen　from　a　balloon　on　a　clear　moonlight　night　in　August　a　year　ago　（1901）；　wore　a　somewhat　altered　appearance。　　There　were　the　fairy　lamps　tracing　out　the　streets；　which；　though　dark　centred；　wore　their　silver　lining；　but　in　irregular　patches　a　whiter　light　from　electric　arc　lamps　broadened　and　brightened　and　shone　out　like　some　pyrotechnic　display　above　the　black　housetops。　　Through　the　vast　town　ran　a　blank；　black　channel；　the　river；　winding　on　into　distance；　crossed　here　and　there　by　bridges　showing　as　bright　bands；　and　with　bright　spots　occasionally　to　mark　where　lay　the　river　craft。　　But　what　was　most　striking　was　the　silence。　　Though　the　noise　of　London　traffic　as　heard　from　a　balloon　has　diminished　of　late　years　owing　to　the　better　paving；　yet　in　day　hours　the　roar　of　the　streets　is　heard　up　to　a　great　height　as　a　hard；　harsh；　grinding　din。　　But　at　night；　after　the　last　'bus　has　ceased　to　ply；　and　before　the　market　carts　begin　lumbering　in；　the　balloonist；　as　he　sails　over　the　town；　might　imagine　that　he　was　traversing　a　City　of　the　Dead。

It　is　at　such　times　that　a　shout　through　a　speaking　trumpet　has　a　most　startling　effect；　and　more　particularly　a　blast　on　a　horn。　　In　this　case　after　an　interval　of　some　seconds　a　wild　note　will　be　flung　back　from　the　house…tops　below；　answered　and　re…answered　on　all　sides　as　it　echoes　from　roof　to　roofa　wild；　weird　uproar　that　awakes　suddenly；　and　then　dies　out　slowly　far　away。

Experiments　with　echoes　from　a　balloon　have　proved　instructive。　　If；　when　riding　at　a　height；　say；　of　2；000　feet；　a　charge　of　gun…cotton　be　fired　electrically　100　feet　below　the　car；　the　report；　though　really　as　loud　as　a　cannon；　sounds　no　more　than　a　mere　pistol　shot；　possibly　partly　owing　to　the　greater　rarity　of　the　air；　but　chiefly　because　the　sound；　having　no　background　to　reflect　it；　simply　spends　itself　in　the　air。　　Then；　always　and　under　all　conditions　of　atmosphere　soever；　there　ensues　absolute　silence　until　the　time　for　the　echo　back　from　earth　has　fully　elapsed；　when　a　deafening　outburst　of　thunder　rises　from　below；　rolling　on　often　for　more　than　half　a　minute。　　Two　noteworthy　facts；　at　least；　the　writer　has　established　from　a　very　large　number　of　trials：　first；　that　the　theory　of　aerial　echoes　thrown　back　fr