the dominion of the air-第62章

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wetting　cloud；　and；　emerging　in　bright　sunshine；　continued　to　drift　for　two　hours　at　an　average　altitude　of　some　3；000　feet；　1；000　feet　below　us　was　the　ill…defined；　ever　changing　upper　surface　of　the　dense　cloud　floor；　and　it　was　no　longer　possible　to　determine　our　course；　which　we　therefore　assumed　to　have　remained　unchanged。　　At　length；　however；　as　a　measure　of　prudence；　we　determined　to　descend　through　the　clouds　sufficiently　to　learn　something　of　our　whereabouts；　which　we　reasonably　expected　to　be　somewhere　in　Surrey　or　Berks。　　On　emerging；　however；　below　the　cloud；　the　first　object　that　loomed　out　of　the　mist　irnmediately　below　us　was　a　cargo　vessel；　in　the　rigging　of　which　our　trail　rope　was　entangling　itself。　　Only　by　degrees　the　fact　dawned　upon　us　that　we　were　in　the　estuary　of　the　Thames；　and　beating　up　towards　London　once　again　with　an　cast　wind。　　Thus　it　became　evident　that　at　the　higher　level；　unknown　to　ourselves；　we　had　been　headed　back　on　our　course；　for　two　hours；　by　a　wind　diametrically　opposed　to　that　blowing　on　the　ground。

Two　recent　developments　of　the　hot…air　war　balloon　suggest　great　possibilities　in　the　near　future。　　One　takes　the　form　of　a　small　captive；　carrying　aloft　a　photographic　camera　directed　and　operated　electrically　from　the　ground。　　The　other　is　a　self…contained　passenger　balloon　of　large　dimensions；　carrying　in　complete　safety　a　special　petroleum　burner　of　great　power。　　These　new　and　important　departures　are　mainly　due　to　the　mechanical　genius　of　Mr。　J。　N。　Maskelyne；　who　has　patented　and　perfected　them　in　conjunction　with　the　writer。



CHAPTER　XXVIII。　　THE　CONSTITUTION　OF　THE　AIR。


Some　fair　idea　of　the　conditions　prevailing　in　the　upper　air　may　have　been　gathered　from　the　many　and　various　observations　already　recorded。　　Stating　the　case　broadly；　we　may　assert　that　the　same　atmospheric　changes　with　which　we　are　familiar　at　the　level　of　the　earth　are　to　be　found　also　at　all　accessible　heights；　equally　extensive　and　equally　sudden。

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