isible　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　off　the　earth　through　the　day　has　then　ascended。　　It　will　be　found　at　different　heights；　lying　in　pools　or　strata；　possibly　resembling　in　form；　could　they　be　seen；　masses　of　visible　cloud。
  The　writer　has　gathered　from　night　voyages　instructive　and　suggestive　facts　with　reference　to　the　ascent　of　air　streams；　due　to　differences　of　temperature；　particularly　over　London　and　the　suburbs；　and　it　is　conceivable　that　in　such　ascending　streams　may　lie　a　means　of　dealing　successfully　with　visitations　of　smoke　and　fog。
  One　lesson　taught　by　balloon　travel　has　been　that　fog　or　haze　will　come　or　go　in　obedience　to　temperature　variations　at　low　levels。　　Thus　thick　haze　has　lain　over　London；　more　particularly　over　the　lower　parts；　at　sundown。　　Then　through　night　hours；　as　the　temperature　of　the　lower　air　has　become　equalised；　the　haze　has　completely　disappeared；　but　only　to　reassert　itself　at　dawn。
  A　description　of　the　very　impressive　experience　of　a　night　sail　over　London　has　been　reserved；　but　should　not　be　altogether　omitted。　　Glaisher；　writing　of　the　spectacle　as　he　observed　it　nearly　forty　years　ago；　describes　London　seen　at　night　from　a　balloon　at　a　distance　as　resembling　a　vast　conflagration。　　When　actually　over　the　town；　a　main　thoroughfare　like　the　Commercial　Road　shone　up　like　a　line　of　brilliant　fire；　but；　travelling　westward；　Oxford　Street　presented　an　appearance　which　puzzled　him。　　〃Here　the　two　thickly　studded　rows　of　brilliant　lights　were　seen　on　either　side　of　the　street；　with　a　narrow；　dark　space　between；　and　this　dark　space　was　bounded；　as　it　were；　on　both　sides　by　a　bright　fringe　like　frosted　silver。〃　　Presently　he　discovered　that　　this　rich　effect　was　caused　by　the　bright　illumination　of　the　shop　lights　on　the　pavements。〃
  London；　as　seen　from　a　balloon　on　a　clear　moonlight　night　in　August　a　year　ago　（1901）；　wore　a　somewhat　altered　appearance。　　There　were　the　fairy　lamps　tracing　out　the　streets；　which；　though　dark　centred；　wore　their　silver　lining；　but　in　irregular　patches　a　whiter　light　from　electric　arc　lamps　broadened　and　brightened　and　shone　out　like　some　pyrotechnic　display　above　the　black　housetops。　　Through　the　vast　town　ran　a　blank；　black　channel；　the　river；　winding　on　into　distance；　crossed　here　and　there　by　bridges　showing　as　bright　bands；　and　with　bright　spots　occasionally　to　mark　where　lay　the　river　craft。　　But　what　was　most　striking　was　the　silence。　　Though　the　noise　of　London　traffic　as　heard　from　a　balloon　has　diminished　of　late　years　owing　to　the　better　paving；　yet　in　day　hours　the　roar　of　the　streets　is　heard　up　to　a　great　height　as　a　hard；　harsh；　grinding　din。　　But　at　night；　after　the　last　'bus　has　ceased　to　ply；　and　before　the　market　carts　begin　lumbering　in；　the　balloonist；　as　he　sails　over　the　town；　might　imagine　that　he　was　traversing　a　City　of　the　Dead。
  It　is　at　such　times　that　a　shout　through　a　speaking　trumpet　has　a　most　startling　effect；　and　more　particularly　a　blast　on　a　horn。　　In　this　case　after　an　interval　of　some　seconds　a　wild　note　will　be　flung　back　from　the　house…tops　below；　answered　and　re…answered　on　all　sides　as　it　echoes　from　roof　to　roofa　wild；　weird　uproar　that　awakes　suddenly；　and　then　dies　out　slowly　far　away。
  Experiments　with　echoes　from　a　balloon　have　proved　instructive。　　If；　when　riding　at　a　height；　say；　of　2；000　feet；　a　charge　of　gun…cotton　be　fired　electrically　100　feet　below　the　car；　the　report；　though　really　as　loud　as　a　cannon；　sounds　no　more　than　a　mere　pistol　shot；　possibly　partly　owing　to　the　greater　rarity　of　the　air；　but　chiefly　because　the　sound；　having　no　background　to　reflect　it；　simply　spends　itself　in　the　air。　　Then；　always　and　under　all　conditions　of　atmosphere　soever；　there　ensues　absolute　silence　until　the　time　for　the　echo　back　from　earth　has　fully　elapsed；　when　a　deafening　outburst　of　thunder　rises　from　below；　rolling　on　often　for　more　than　half　a　minute。　　Two　noteworthy　facts；　at　least；　the　writer　has　established　from　a　very　large　number　of　trials：　first；　that　the　theory　of　aerial　echoes　thrown　back　from　empty　space；　which　physicists　have　held　to　exist　constantly；　and　to　be　part　of　the　cause　of　thunder；　will　have　to　be　abandoned；　and；　secondly；　that　from　some　cause　yet　to　be　fully　explained　the　echo　back　from　the　earth　is　always　behind　its　time。
  But　balloons　have　revealed　further　suggestive　facts　with　regard　to　sound；　and　more　particularly　with　regard　to　the　varying　acoustic　properties　of　the　air。　　It　is　a　familiar　experience　how　distant　sounds　will　come　and　go；　rising　and　falling；　often　being　wafted　over　extraordinary　distances；　and　again　failing　altogether；　or　sometimes　being　lost　at　near　range；　but　appearing　in　strength　further　away。　　A　free　balloon；　moving　in　the　profound　silence　of　the　upper　air；　becomes　an　admirable　sound　observatory。　　It　may　be　clearly　detected　that　in　certain　conditions　of　atmosphere；　at　least；　there　are　what　may　be　conceived　to　be　aerial　sound　channels；　through　which　sounds　are　；momentarily　conveyed　with　abnormal　intensity。　　This　phenomenon　does　but　serve　to　give　an　intelligible　presentment　of　the　unseen　conditions　existing　in　the　realm　of　air。
  It　would　be　reasonable　to　suppose　that　were　an　eye　so　constituted　as　to　be　able　to　see；　say；　cumulus　masses　of　warmer　air；　strata　mottled　with　traces　of　other　gases；　and　beds　of　invisible　matter　in　suspension；　one　might　suppose　that　what　we　deem　the　clearest　sky　would　then　appear　flecked　with　forms　as　many　and　various　as　the　clouds　that　adorn　our　summer　heavens。
  But　there　is　matter　in　suspension　in　the　atmosphere　which　is　very　far　from　invisible；　and　which　in　the　case　of　large　towns　is　very　commonly　lying　in　thick　strata　overhead；　stopping　back　the　sunlight；　and　forming　the　nucleus　round　which　noisome　fogs　may　form。　　Experimenting　with　suitable　apparatus；　the　writer　has　found　on　a　still　afternoon　in　May；　at　2；000　feet　above　Kingston　in　Surrey；　that　the　air　was　charged　far　more　heavily　with　dust　than　that　of　the　London　streets　the　next　day；　and；　again；　at　half　a　mile　above　the　city　in　the　month　of　August　last　dust；　much　of　it　being　of　a　gross　and　even　fibrous　nature；　was　far　more　abundant　than　on　grass　enclosures　in　the　town　during　the　forenoon　of　the　day　following。
  An　attempt　has　been　made　to　include　England　in　a　series　of　international　balloon　ascents　arranged　expressly　for　the　purpose　of　taking　simultaneous　observations　at　a　large　number　of　stations　over　Europe；　by　which　means　it　is　hoped　that　much　fresh　knowledge　will　be　forthcoming　with　respect　to　the　constitution　of　the　atmosphere　up　to　the　highest　levels　accessible　by　balloons　manned　and　unmanned。　　It　is　very　much　to　be　regretted　that　in　the　case　of　England　the　attempt　here　spoken　of　has　rested　entirely　on　private　enterprise。　　First　and　foremost　in　personal　liberality　and　the　work　of　organisation　must　be　mentioned　Mr。　P。　Y。　Alexander；　whose　zeal　in　the　progress　of　aeronautics　is　second　to　none　in　this　country。　　Twice　through　his　efforts　England　has　been　represented　in　the　important　work　for　which　Continental　nations　have　no　difficulty　in　obtaining　public　grants。　　The　first　occasion　was　on　November　8th；　1900；　when　the　writer　was　privileged　to　occupy　a　seat　in　the　balloon　furnished　by　Mr。　Alexander；　and　equipped　with　the　most　modern　type　of　instruments。　　It　was　a　stormy　and　fast　voyage　from　the　Crystal　Palace　to　Halstead；　in　Essex；　48　miles　in　40　minutes。　　Simultaneously　with　this；　Mr。　Alexander　dismissed　an　unmanned　balloon　from　Bath；　which　ascended　8；000　feet；　and　landed　at　Cricklade。　　Other　balloons　which　took　part　in　the　combined　experiment　were　two　from　Paris；　three　from　Chalais　Meudon；　three　from　Strasburg；　two　from　Vienna；　two　from　Berlin；　and　two　from　St。　Petersburg。
  The　section　of　our　countrymen　specially　interested　in　aeronauticsa　growing　communityis　represented　by　the　Aeronautical　Society；　formed　in　1865；　with　the　Duke　of　Argyll　for　president；　and　for　thirty　years　under　the　most　energetic　management　of　Mr。　F。　W。　Brearey；　succeeding　whom　as　hon。　secs。　　have　been　Major　Baden…Powell　and　Mr。　Eric　S。　Bruce。　　Mr。　Brearey　was　one　of　the　most　successful　inventors　of　flying　models。　　Mr。　Chanute；　speaking　as　Preside