aeroplanes-第23章

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BAROMETERS。These　instruments　are　used　for

registering　heights。　A　barometer　is　a　device　for

measuring　the　weight　or　pressure　of　the　air。

The　air　is　supposed　to　extend　to　a　height　of　40

miles　from　the　surface　of　the　sea。　A　column　of

air　one　inch　square；　and　forty　miles　high；　weighs

the　same　as　a　column　of　mercury　one　inch　square

and　30　inches　high。



Such　a　column　of　air；　or　of　mercury；　weighs

14　3/4　pounds。　If　the　air　column　should　be

weighed　at　the　top　of　the　mountain；　that　part

above　would　weigh　less　than　if　measured　at　the

sea　level；　hence；　as　we　ascend　or　descend　the　pressure

becomes　less　or　more；　dependent　on　the　altitude。



Mercury　is　also　used　to　indicate　temperature；

but　this　is　brought　about　by　the　expansive　quality

of　the　mercury；　and　not　by　its　weight。



_Fig。　74。　Aneroid　Barometer。_



ANEROID　BAROMETER。The　term　Aneroid　barometer

is　frequently　used　in　connection　with　air…

ship　experiments。　The　word　aneroid　means　not

wet；　or　not　a　fluid；　like　mercury；　so　that；　while

aneroid　barometers　are　being　made　which　do　use

mercury；　they　are　generally　made　without。



One　such　form　is　illustrated　in　Fig。　74；　which

represents　a　cylindrical　shell　A；　which　has　at　each

end　a　head　of　concentrically　formed　corrugations。

These　heads　are　securely　fixed　to　the　ends　of　the

shell　A。　Within；　one　of　the　disk　heads　has　a

short　stem　C；　which　is　attached　to　the　short　end

of　a　lever　D；　this　lever　being　pivoted　at　E。　The

outer　end　of　this　lever　is　hinged　to　the　short　end

of　another　lever　F；　and　so　by　compounding　the

levers；　it　will　be　seen　that　a　very　slight　movement

of　the　head　B　will　cause　a　considerable　movement

in　the　long　end　of　the　lever　F。



This　end　of　the　lever　F　connects　with　one　limb

of　a　bell…crank　lever　G；　and　its　other　limb　has　a

toothed　rack　connection　with　a　gear　H；　which

turns　the　shaft　to　which　the　pointer　I　is　attached。



Air　is　withdrawn　from　the　interior　of　the　shell；

so　that　any　change　in　the　pressure；　or　weight　of

the　atmosphere；　is　at　once　felt　by　the　disk　heads；

and　the　finger　turns　to　indicate　the　amount　of

pressure。



HYDROPLANES。Hydro　means　water；　hence　the

term　hydroplane　has　been　given　to　machines

which　have　suitable　pontoons　or　boats；　so　they

may　alight　or　initiate　flight　from　water。



There　is　no　particular　form　which　has　been

adopted　to　attach　to　aeroplanes；　the　object　generally

being　to　so　make　them　that　they　will　sustain

the　greatest　amount　of　weight　with　the　least

submergence；　and　also　offer　the　least　resistance

while　the　motor　is　drawing　the　machine　along　the

surface　of　the　water；　preparatory　to　launching　it。



SUSTAINING　WEIGHT　OF　PONTOONS。A　pontoon

having　within　nothing　but　air；　is　merely　a　measuring

device　which　determines　the　difference　between

the　weight　of　water　and　the　amount　placed

on　the　pontoon。　Water　weighs　62　1/2　pounds　per

cubic　foot。　Ordinary　wood；　an　average　of　32

pounds；　and　steel　500　pounds。



It　is；　therefore；　an　easy　matter　to　determine

how　much　of　solid　matter　will　be　sustained　by　a

pontoon　of　a　given　size；　or　what　the　dimensions

of　a　pontoon　should　be　to　hold　up　an　aeroplane

which　weighs；　with　the　pilot；　say；　1100　pounds。



As　we　must　calculate　for　a　sufficient　excess　to

prevent　the　pontoons　from　being　too　much　immersed；

and　also　allow　a　sufficient　difference　in

weight　so　that　they　will　keep　on　the　surface　when

the　aeroplane　strikes　the　surface　in　alighting；　we

will　take　the　figure　of　1500　pounds　to　make　the

calculations　from。



If　this　figure　is　divided　by　62　1/2　we　shall　find

the　cubical　contents　of　the　pontoons；　not　considering；

of　course；　the　weight　of　the　material　of　which

they　are　composed。　This　calculation　shows　that

we　must　have　24　cubic　feet　in　the　pontoons。



As　there　should　be　two　main　pontoons；　and　a

smaller　one　for　the　rear；　each　of　the　main　ones

might　have　ten　cubic　feet；　and　the　smaller　one

four　cubic　feet。



SHAPES　OF　THE　PONTOONS。We　are　now　ready

to　design　the　shapes。　Fig。　75　shows　three　general

types；　A　being　made　rectangular　in　form；

with　a　tapering　forward　end；　so　constructed　as　to

ride　up　on　the　water。



The　type　B　has　a　rounded　under　body；　the　forward

end　being　also　skiff…shaped　to　decrease　as

much　as　possible　the　resistance　of　the　water　impact。



_Fig。　75。　Hydroplane　Floats。_



The　third　type　C　is　made　in　the　form　of　a

closed　boat；　with　both　ends　pointed；　and　the　bottom

rounded；　or　provided　with　a　keel。　Or；　as　in

some　cases　the　body　may　be　made　triangular　in

cross　section　so　that　as　it　is　submerged　its　sustaining

weight　will　increase　at　a　greater　degree

as　it　is　pressed　down　than　its　vertical　measurement

indicates。



All　this；　however；　is　a　matter　left　to　the　judgment

of　the　designer；　and　is；　in　a　great　degree；

dependent　on　the　character　of　the　craft　to　which

it　is　to　be　applied。









CHAPTER　XII



EXPERIMENTAL　WORK　IN　FLYING





THE　novice　about　to　take　his　first　trial　trip　in

an　automobile　will　soon　learn　that　the　great　task

in　his　mind　is　to　properly　start　the　machine。　He

is　conscious　of　one　thing；　that　it　will　be　an　easy

matter　to　stop　it　by　cutting　off　the　fuel　supply

and　applying　the　brakes。



CERTAIN　CONDITIONS　IN　FLYING。In　an　aeroplane

conditions　are　reversed。　Shutting　off　the

fuel　supply　and　applying　the　brakes　only　bring

on　the　main　difficulty。　He　must　learn　to　stop　the

machine　after　all　this　is　done；　and　this　is　the

great　test　of　flying。　It　is　not　the　launching；

the　ability　to　get　into　the　air；　but　the　landing；　that

gives　the　pupil　his　first　shock。



Man　is　so　accustomed　to　the　little　swirls　of　air

all　about　him；　that　he　does　not　appreciate　what

they　mean　to　a　machine　which　is　once　free　to

glide　along　in　the　little　currents　which　are　so　unnoticeable

to　him　as　a　pedestrian。



The　contour　of　the　earth；　the　fences；　trees；　little

elevations　and　other　natural　surroundings；　all

have　their　effect　on　a　slight　moving　air　current；

and　these　inequalities　affect　the　air　and　disturb

it　to　a　still　greater　extent　as　the　wind　increases。

Even　in　a　still　air；　with　the　sun　shining；　there　are

air　eddies；　caused　by　the　uneven　heating　of　the

air　in　space。



HEAT　IN　AIR。Heat　is　transmitted　through　the

air　by　what　is　called　convection；　that　is；　the　particles

of　the　air　transmit　it　from　one　point　to　the

next。　If　a　room　is　closed　up　tight；　and　a　little

aperture　provided　so　as　to　let　in　a　streak　of　sunlight；

it　will　give　some　idea　of　the　unrest　of　the

atmosphere。　This　may　be　exhibited　by　smoke

along　the　line　of　the　sun's　rays；　which　indicates

that　the　particles　of　air　are　constantly　in　motion；

although　there　may　be　absolutely　nothing　in　the

room　to　disturb　it。



MOTION　WHEN　IN　FLIGHT。If　you　can　imagine

a　small　airship　floating　in　that　space；　you　can

readily　conceive　that　it　will　be　hurled　hither　and

thither　by　the　motion　which　is　thus　apparent　to

the　eye。



This　motion　is　greatly　accentuated　by　the　surface

of　the　earth；　independently　of　its　uneven　contour。

If　a　ball　is　thrown　through　the　air；　its

dynamic　force　is　measured　by　its　impact。　So

with　light；　and　heat。　In　the　space　between　the

planets　it　is　very　cold。　The　sunlight；　or　the　rays

from　the　sun　are　there；　just　the　same　as　on　the

earth。



Unless　the　rays　come　into　contact　with　something；

they　produce　no　effect。　When　the　beams

from　the　sun　come　into　contact　with　the　atmosphere

a　dynamic　force　is　exerted；　just　the　same

as　when　the　ball　struck　an　object。　When　the　rays

reach　the　earth；　reflection　takes　place；　and　these

reflected　beams　act　on　the　air　under　different　conditions。



CHANGING　ATMOSPHERE。If　the　air　is　full　of

moisture；　as　it　may　be　at　some　places；　while

comparatively　dry　at　other　points；　the　reflection

throughout　the　moist　area　is　much　greater　than　in

the　dry　places；　hence　evaporation　will　take　place

and　whenever　a　liquid　vaporizes　it　means　heat。



On