FirstletusregardtheactualpathofaballooninspacewhenbeingmanoeuvredinthewaywereadofinMr。Glaisher’sownaccounts。Thispartisinmostcasesapproximatelyindicatedinthatmostattractivevolumeofhisentitled，\"TravelsintheAir，\"bydiagramsgivingasectionalpresentmentofhismoreimportantvoyages;butalittlecommonplaceconsiderationmaytaketheplaceofdiagrams。
Ithasbeencommontoassertthataballoonpoisedinspaceisthemostdelicatebalanceconceivable。Itsintrinsicweightmustbeexactlyequaltotheweightoftheairitdisplaces，andsincethedensityoftheairdecreasesaccordingtoafixedlaw，amounting，approximately，toadifferenceinbarometricreadingof0。1inchforevery90feet，itfollows，theoretically，thatifaballoonispoisedat1，000feetabovesealevel，thenitwouldnotbeinequilibriumatanyotherheight，solongasitsweightandvolumeremainthesame。Ifitwere50feethigheritmustcommencedescending，and，iflower，thenitmustascendtillitreachesitstruelevel;and，morethanthat，intheeventofeithersuchexcursionmereimpetuswouldcarryitbeyondthislevel，aboutwhichitwouldoscillateforashorttime，afterthemannerofthependulum。
Thisissubstantiallytrue，butitmustbetakeninconnectionwithotherfactswhichhaveafargreaterinfluenceonaballoon’spositionormotion。
Forinstance，inthevolumejustreferredtoitisstatedbyM。
GastonTissandierthatononeoccasionwhenalofthethrewoverboardachickenbone，and，immediatelyconsultingabarometer，hadtoadmiton\"clearestevidencethatthebonehadcausedariseoffromtwentytothirtyyards，sodelicatelyisaballoonequipoisedintheair。\"Here，withoutpausingtocalculatewhetherthedischargeofanounceorsowouldsufficetocausealargeballoontoascendthroughninetyfeet，itmaybepointedoutthattherecordcannotbetrustworthy，fromthemerefactthatafreeballoonisfrommomenttomomentbeingsubjectedtootherpotentinfluences，whichnecessarilyaffectitspositioninspace。Indaytimethesun’sinfluenceisanall—importantfactor，andwhethershiningbrightlyorpartiallyhiddenbyclouds，aslightdifferenceinobscurationwillhaveareadyandmarkedeffectontheballoon’saltitude。Again，aballoonintransitmaypassalmostmomentarilyfromawarmerlayerofairtoacolder，orviceversa，theplaneofdemarcationbetweenthetwobeingverydefiniteandabrupt，andinthiscasealtitudeisatonceaffected;or，yetagain，therearethedescendingandascendingcurrents，metwithconstantlyandunexpectedly，whichhavetobereckonedwith。