flying machines-第27章

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that　no　one　can　be　compelled　to　complete　a　contract

which　in　itself　is　impossible　to　perform。　For　instance；

a　contract　to　row　a　boat　across　the　Atlantic　in　two

weeks；　for　a　consideration；　could　never　be　enforced　because

it　is　within　judicial　knowledge　that　such　an　undertaking

is　beyond　human　power。　Again；　contracts　formed

for　the　doing　of　acts　contrary　to　nature　are　never

enforcible；　and　here　is　where　our　difficulty　comes　in。　Is

it　possible　to　build　a　machine　or　species　of　craft　which

will　transport　a　person　or　goods　through　the　air？　The

courts　know　that　balloons　are　practical；　that　is；　they

know　that　a　bag　filled　with　gas　has　a　lifting　power　and

can　move　through　the　air　at　an　appreciable　height。

Therefore；　a　contract　to　transport　a　person　in　such　manner

is　a　good　contract；　and　the　conditions　being　favorable

could　undoubtedly　be　enforced。　But　the　passengers'

right　of　action　for　injury　would　be　very　limited。



No　Redress　for　Purchasers。



〃In　the　case　of　giving　warranties　on　aeroplanes；　we

have　yet　to　see　just　what　a　court　is　going　to　say。　It　is

easy　enough　for　a　manufacturer　to　guarantee　to　build　a

machine　of　certain　dimensions　and　according　to　certain

specifications；　but　when　he　inserts　a　clause　in　the　contract　to

the　effect　that　the　machine　will　raise　itself　from

the　surface　of　the　earth；　defy　the　laws　of　gravity；　and

soar　in　the　heavens　at　the　will　of　the　aviator；　he　is　to

say　the　least　contracting　to　perform　a　miracle。



〃Until　aeroplanes　have　been　made　and　accepted　as

practical；　no　court　will　force　a　manufacturer　to　turn　out

a　machine　guaranteed　to　fly。　So　purchasers　can　well

remember　that　if　their　machines　refuse　to　fly　they　have

no　redress　against　the　maker；　for　he　can　always　say；

'The　industry　is　still　in　its　experimental　stage。'　In

contracting　for　an　engine　no　builder　will　guarantee　that

the　particular　engine　will　successfully　operate　the　aeroplane。

In　fact　he　could　never　be　forced　to　live　up　to

such　an　agreement；　should　he　agree　to　a　stipulation　of

that　sort。　The　best　any　engine　maker　will　guarantee

is　to　build　an　engine　according　to　specifications。〃







CHAPTER　XX。



SOARING　FLIGHT。



By　Octave　Chanute。



＇5＇There　is　a　wonderful　performance　daily　exhibited　in

southern　climes　and　occasionally　seen　in　northerly

latitudes　in　summer；　which　has　never　been　thoroughly

explained。　It　is　the　soaring　or　sailing　flight　of　certain

varieties　of　large　birds　who　transport　themselves　on　rigid；

unflapping　wings　in　any　desired　direction；　who　in　winds

of　6　to　20　miles　per　hour；　circle；　rise；　advance；　return　and

remain　aloft　for　hours　without　a　beat　of　wing；　save　for

getting　under　way　or　convenience　in　various　maneuvers。

They　appear　to　obtain　from　the　wind　alone　all　the　necessary

energy；　even　to　advancing　dead　against　that　wind。

This　feat　is　so　much　opposed　to　our　general　ideas　of

physics　that　those　who　have　not　seen　it　sometimes　deny

its　actuality；　and　those　who　have　only　occasionally

witnessed　it　subsequently　doubt　the　evidence　of　their　own

eyes。　Others；　who　have　seen　the　exceptional　performances；

speculate　on　various　explanations；　but　the　majority

give　it　up　as　a　sort　of　〃negative　gravity。〃



＇5＇　Aeronautics。



Soaring　Power　of　Birds。



The　writer　of　this　paper　published　in　the　〃Aeronautical

Annual〃　for　1896　and　1897　an　article　upon　the　sailing

flight　of　birds；　in　which　he　gave　a　list　of　the　authors　who

had　described　such　flight　or　had　advanced　theories　for

its　explanation；　and　he　passed　these　in　review。　He　also

described　his　own　observations　and　submitted　some　computations

to　account　for　the　observed　facts。　These　computations

were　correct　as　far　as　they　went；　but　they　were

scanty。　It　was；　for　instance；　shown　convincingly　by

analysis　that　a　gull　weighing　2。188　pounds；　with　a　total

supporting　surface　of　2。015　square　feet；　a　maximum　body

cross…section　of　0。126　square　feet　and　a　maximum　cross…

section　of　wing　edges　of　0。098　square　feet；　patrolling　on

rigid　wings　（soaring）　on　the　weather　side　of　a　steamer

and　maintaining　an　upward　angle　or　attitude　of　5　degrees

to　7　degrees　above　the　horizon；　in　a　wind　blowing　12。78

miles　an　hour；　which　was　deflected　upward　10　degrees

to　20　degrees　by　the　side　of　the　steamer　（these　all　being

carefully　observed　facts）；　was　perfectly　sustained　at　its

own　〃relative　speed〃　of　17。88　miles　per　hour　and　extracted

from　the　upward　trend　of　the　wind　sufficient　energy

to　overcome　all　the　resistances；　this　energy

amounting　to　6。44　foot…pounds　per　second。



Great　Power　of　Gulls。



It　was　shown　that　the　same　bird　in　flapping　flight　in

calm　air；　with　an　attitude　or　incidence　of　3　degrees　to　5

degrees　above　the　horizon　and　a　speed　of　20。4　miles　an

hour　was　well　sustained　and　expended　5。88　foot…pounds

per　second；　this　being　at　the　rate　of　204　pounds　sustained

per　horsepower。　It　was　stated　also　that　a　gull　in　its　observed

maneuvers；　rising　up　from　a　pile　head　on　unflapping

wings；　then　plunging　forward　against　the　wind　and

subsequently　rising　higher　than　his　starting　point；　must

either　time　his　ascents　and　descents　exactly　with　the

variations　in　wind　velocities；　or　must　meet　a　wind　billow

rotating　on　a　horizontal　axis　and　come　to　a　poise　on　its

crest；　thus　availing　of　an　ascending　trend。



But　the　observations　failed　to　demonstrate　that　the

variations　of　the　wind　gusts　and　the　movements　of　the

bird　were　absolutely　synchronous；　and　it　was　conjectured

that　the　peculiar　shape　of　the　soaring　wing　of　certain

birds；　as　differentiated　from　the　flapping　wing；　might；

when　experimented　upon；　hereafter　account　for　the　performance。



Mystery　to　be　Explained。



These　computations；　however　satisfactory　they　were

for　the　speed　of　winds　observed；　failed　to　account　for　the

observed　spiral　soaring　of　buzzards　in　very　light　winds

and　the　writer　was　compelled　to　confess：　〃Now；　this

spiral　soaring　in　steady　breezes　of　5　to　10　miles　per　hour

which　are　apparently　horizontal；　and　through　which　the

bird　maintains　an　average　speed　of　about　20　miles　an

hour；　is　the　mystery　to　be　explained。　It　is　not　accounted

for；　quantitatively；　by　any　of　the　theories　which　have

been　advanced；　and　it　is　the　one　performance　which　has

led　some　observers　to　claim　that　it　was　done　through

'aspiration。'　i；　e。；　that　a　bird　acted　upon　by　a　current；

actually　drew　forward　into　that　current　against　its　exact

direction　of　motion。〃



Buzzards　Soar　in　Dead　Calm。



A　still　greater　mystery　was　propounded　by　the　few

observers　who　asserted　that　they　had　seen　buzzards　soaring

in　a　dead　calm；　maintaining　their　elevation　and　their

speed。　Among　these　observers　was　Mr。　E。　C。　Huffaker；

at　one　time　assistant　experimenter　for　Professor　Langley。

The　writer　believed　and　said　then　that　he　must　in　some

way　have　been　mistaken；　yet；　to　satisfy　himself；　he　paid

several　visits　to　Mr。　Huffaker；　in　Eastern　Tennessee　and

took　along　his　anemometer。　He　saw　quite　a　number　of

buzzards　sailing　at　a　height　of　75　to　100　feet　in　breezes

measuring　5　or　6　miles　an　hour　at　the　surface　of　the

ground；　and　once　he　saw　one　buzzard　soaring　apparently

in　a　dead　calm。



The　writer　was　fairly　baffled。　The　bird　was　not　simply

gliding；　utilizing　gravity　or　acquired　momentum；　he　was

actually　circling　horizontally　in　defiance　of　physics　and

mathematics。　It　took　two　years　and　a　whole　series　of

further　observations　to　bring　those　two　sciences　into

accord　with　the　facts。



Results　of　Close　Observations。



Curiously　enough　the　key　to　the　performance　of　circling

in　a　light　wind　or　a　dead　calm　was　not　found

through　the　usual　way　of　gathering　human　knowledge；

i。　e。；　through　observations　and　experiment。　These　had

failed　because　I　did　not　know　what　to　look　for。　The

mystery　was；　in　fact；　solved　by　an　eclectic　process　of

conjecture　and　computation；　but　once　these　computations

indicated　what　observations　should　be　made；　the　results

gave　at　once　the　reasons