the dominion of the air-第47章

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ionately　greater　winged　surface；　can　carry　about　250　lbs。　per　horse　power。

Professor　Langley；　of　Washington；　working　contemporaneously；　but　independently　of　Mr。　Maxim；　had　tried　exhaustive　experiments　on　a　rotating　arm　（characteristically　designated　by　Mr。　Maxim　a　〃merry…go…round〃）；　thirty　feet　long；　applying　screw　propellers。　　He　used；　for　the　most　part；　small　planes；　carrying　loads　of　only　two　or　three　pounds；　and；　under　these　circumstances；　the　weight　carried　was　at　the　rate　of　250　lbs。　per　horse　power。　　His　important　statements　with　regard　to　these　trials　are　that　one…horse　power　will　transport　a　larger　weight　at　twenty　miles　an　hour　than　at　ten；　and　a　still　larger　at　forty　miles　than　at　twenty；　and　so　on；　that　〃the　sustaining　pressure　of　the　air　on　a　plane　moving　at　a　small　angle　of　inclination　to　a　horizontal　path　is　many　times　greater　than　would　result　from　the　formula　implicitly　given　by　Newton；　while；　whereas　in　land　or　marine　transport　increased　speed　is　maintained　only　by　a　disproportionate　expenditure　of　power　within　the　limits　of　experiment；　in　aerial　horizontal　transport　the　higher　speeds　are　more　economical　of　power　than　the　lower　ones。〃

This　Mr。　Maxim　is　evidently　ready　to　endorse；　stating；　in　his　own　words；　that　birds　obtain　the　greater　part　of　their　support　by　moving　forward　with　sufficient　velocity　so　as　to　be　constantly　resting　on　new　air；　the　inertia　of　which　has　not　been　disturbed。　　Mr。　Maxim's　trials　were　on　a　scale　comparable　with　all　his　mechanical　achievements。　　He　employed　for　his　experiments　a　rotating　arm；　sweeping　out　a　circle；　the　circumference　of　which　was　200　feet。　　To　the　end　of　this　arm　he　attached　a　cigar…shaped　apparatus；　driven　by　a　screw；　and　arranged　in　such　a　manner　that　aero…planes　could　be　attached　to　it　at　any　angle。　　These　planes　were　on　a　large　scale；　carrying　weights　of　from　20　lbs。　to　100　lbs。　　With　this　contrivance　he　found　that；　whatever　push　the　screw　communicated　to　the　aero…plane；　〃the　plane　would　lift　in　a　vertical　direction　from　ten　to　fifteen　times　as　much　as　the　horizontal　push　that　it　received　from　the　screw；　and　which　depended　upon　the　angle　at　which　the　plane　was　set；　and　the　speed　at　which　the　apparatus　was　travelling　through　the　air。〃　　Next；　having　determined　by　experiment　the　power　required　to　perform　artificial　flight；　Mr。　Maxim　applied　himself　to　designing　the　requisite　motor。　　〃I　constructed；〃　he　states；　〃two　sets　of　compound　engines　of　tempered　steel；　all　the　parts　being　made　very　light　and　strong；　and　a　steam　generator　of　peculiar　construction；　the　greater　part　of　the　heating　surface　consisting　of　small　and　thin　copper　tubes。　For　fuel　I　employed　naphtha。〃

This　Mr。　Maxim　wrote　in　1892；　adding　that　he　was　then　experimenting　with　a　large　machine；　having　a　spread　of　over　100　feet。　　Labour；　skill；　and　money　were　lavishly　devoted　henceforward　to　the　great　task　undertaken；　and　it　was　not　long　before　the　giant　flying　machine；　the　outcome　of　so　much　patient　experimenting；　was　completed　and　put　to　a　practical　trial。　　Its　weight　was　7；500　lbs。　　The　screw　propellers　were　nearly　18　feet　in　diameter；　each　with　two　blades；　while　the　engines　were　capable　of　being　run　up　to　360　horse　power。　　The　entire　machine　was　mounted　on　an　inner　railway　track　of　9　feet　and　an　outer　of　35　feet　gauge；　while　above　there　was　a　reversed　rail　along　which　the　machine　would　begin　to　run　so　soon　as　with　increase　of　speed　it　commenced　to　lift　itself　off　the　inner　track。

In　one　of　the　latest　experiments　it　was　found　that　when　a　speed　of　42　miles　an　hour　was　attained　all　the　wheels　were　running　on　the　upper　track；　and　revolving　in　the　opposite　direction　from　those　on　the　lower　track。　　However；　after　running　about　1；000　feet；　an　axle　tree　doubled　up；　and　immediately　afterwards　the　upper　track　broke　away；　and　the　machine；　becoming　liberated；　floated　in　the　air；　〃giving　those　on　board　a　sensation　of　being　in　a　boat。〃

The　experiment　proved　conclusively　to　the　inventor　that　a　machine　could　be　made　on　a　large　scale；　in　which　the　lifting　effect　should　be　considerably　greater　than　the　weight　of　the　machine；　and　this；　too；　when　a　steam　engine　was　the　motor。　　When；　therefore；　in　the　years　shortly　following；　the　steam　engine　was　for　the　purposes　of　aerial　locomotion　superseded　by　the　lighter　and　more　suitable　petrol　engine；　the　construction　of　a　navigable　air　ship　became　vastly　more　practicable。　　Still；　in　Sir　H。　Maxim's　opinion；　lately　expressed；　〃those　who　seek　to　navigate　the　air　by　machines　lighter　than　the　air　have　come；　practically；　to　the　end　of　their　tether；〃　while；　on　the　other　hand；　〃those　who　seek　to　navigate　the　air　with　machines　heavier　than　the　air　have　not　even　made　a　start　as　yet；　and　the　possibilities　before　them　are　very　great　indeed。〃

As　to　the　assertion　that　the　aerial　navigators　last　mentioned　〃have　not　even　made　a　start　as　yet；〃　we　can　only　say　that　Sir　H。　Maxim　speaks　with　far　too　much　modesty。　　His　own　colossal　labours　in　the　direction　of　that　mode　of　aerial　flight；　which　he　considers　to　be　alone　feasible；　are　of　the　first　importance　and　value；　and；　as　far　as　they　have　gone；　exhaustive。　　Had　his　experiments　been　simply　confined　to　his　classical　investigations　of　the　proper　form　of　the　screw　propeller　his　name　would　still　have　been　handed　down　as　a　true　pioneer　in　aeronautics。　　His　work；　however；　covers　far　wider　ground；　and　he　has；　in　a　variety　of　ways；　furnished　practical　and　reliable　data；　which　must　always　be　an　indispensable　guide　to　every　future　worker　in　the　same　field。

Professor　Langley；　in　attacking　the　same　problem；　first　studied　the　principle　and　behaviour　of　a　well…known　toythe　model　invented　by　Penaud；　which；　driven　by　the　tension　of　india…rubber；　sustains　itself　in　the　air　for　a　few　seconds。　He　constructed　over　thirty　modifications　of　this　model；　and　spent　many　months　in　trying　from　these　to　as　certain　what　he　terms　the　〃laws　of　balancing　leading　to　horizontal　flight。〃　　His　best　endeavours　at　first；　however；　showed　that　he　needed　three　or　four　feet　of　sustaining　surface　to　a　pound　of　weight；　whereas　he　calculated　that　a　bird　could　soar　with　a　surface　of　less　than　half　a　foot　to　the　pound。　　He　next　proceeded　to　steam…driven　models　in　which　for　a　time　he　found　an　insuperable　difficulty　in　keeping　down　the　weight；　which；　in　practice；　always　exceeded　his　calculation；　and　it　was　not　till　the　end　of　1893　that　he　felt　himself　prepared　for　a　fair　trial。　　At　this　time　he　had　prepared　a　model　weighing　between　nine　and　ten　pounds；　and　he　needed　only　a　suitable　launching　apparatus　to　be　used　over　water。　　The　model　would；　like　a　bird；　require　an　initial　velocity　imparted　to　it；　and　the　discovery　of　a　suitable　apparatus　gave　him　great　trouble。　　For　the　rest　the　facilities　for　launching　were　supplied　by　a　houseboat　moored　on　the　Potomac。　　Foiled　again　and　again　by　many　difficulties；　it　was　not　till　after　repeated　failures　and　the　lapse　of　many　months；　when；　as　the　Professor　himself　puts　it；　hope　was　low；　that　success　finally　came。　　It　was　in　the　early　part　of　1896　that　a　successful　flight　was　accomplished　in　the　presence　of　Dr。　Bell；　of　telephone　fame；　and　the　following　is　a　brief　epitome　of　the　account　that　this　accomplished　scientist　contributed　to　the　columns　of　Nature：

〃The　flying　machine；　built；　apparently；　almost　entirely　of　metal；　was　driven　by　an　engine　said　to　weigh；　with　fuel　and　water；　about　25　lbs。；　the　supporting　surface　from　tip　to　tip　being　12　or　14　feet。　　Starting　from　a　platform　about　20　feet　high；　the　machine　rose　at　first　directly　in　the　face　of　the　wind；　moving　with　great　steadiness；　and　subsequently　wheeling　in　large　curves　until　steam　was　exhausted；　when；　from　a　height　of　80　or　100　feet；　it　shortly　settled　down。　　The　experiment　was　then　repeated　with　similar　results。　　Its　motion　was　so　steady　that　a　glass　of　water　might　have　remained　unspilled。　　The　actual　length　of　flight　each　time；　which　lasted　for　a　minute　and　a　half；　exceeded　half　a　mile；　while　the　velocity　was　between　twenty　and　twenty…five　miles　an　hour　in　a　course　that　was　constantly　taking　it　'up　hill。'　　A　yet　more　successful　flight　was　subsequently　made。〃

But　flight　of　another　nature　was　being　courageously　attempted　at　this　time。　　Otto　Lilienthal；　of　Berlin；　in　imitation　of　the　motion　of　bi