the unseen world and other essays-第3节

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s。　All　the　planets　raise　tides　upon　the　surface　of　the　sun　and　the　periodicity　of　sun…spots　（or　solar　cyclones）　depends　upon　this　fact。　These　tidal　waves　act　as　a　drag　or　brake　upon　the　rotation　of　the　sun；　somewhat　diminishing　its　rapidity。　But；　in　conformity　with　a　principle　of　mechanics　well　known　to　astronomers；　though　not　familiar　to　the　general　reader；　all　the　motion　of　rotation　thus　lost　by　the　sun　is　added　to　the　planets　in　the　shape　of　annual　motion　of　revolution；　and　thus　their　orbits　all　tend　to　enlarge；they　all　tend　to　recede　somewhat　from　the　sun。　But　this　state　of　things；　though　long…enduring　enough；　is　after　all　only　temporary；　and　will　at　any　rate　come　to　an　end　when　the　sun　and　planets　have　become　solid。　Meanwhile　another　set　of　circumstances　is　all　the　time　tending　to　bring　the　planets　nearer　to　the　sun；　and　in　the　long　run　must　gain　the　mastery。　The　space　through　which　the　planets　move　is　filled　with　a　kind　of　matter　which　serves　as　a　medium　for　the　transmission　of　heat　and　light；　and　this　kind　of　matter；　though　different　in　some　respects　from　ordinary　ponderable　matter；　is　yet　like　it　in　exerting　friction。　This　friction　is　almost　infinitely　little；　yet　it　has　a　wellnigh　infinite　length　of　time　to　work　in；　and　during　all　this　wellnigh　infinite　length　of　time　it　is　slowly　eating　up　the　momentum　of　the　planets　and　diminishing　their　ability　to　maintain　their　distances　from　the　sun。　Hence　in　course　of　time　the　planets　will　all　fall　into　the　sun；　one　after　another；　so　that　the　solar　system　will　end；　as　it　began；　by　consisting　of　a　single　mass　of　matter。

But　this　is　by　no　means　the　end　of　the　story。　When　two　bodies　rush　together；　each　parts　with　some　of　its　energy　of　motion；　and　this　lost　energy　of　motion　reappears　as　heat。　In　the　concussion　of　two　cosmical　bodies；　like　the　sun　and　the　earth；　an　enormous　quantity　of　motion　is　thus　converted　into　heat。　Now　heat；　when　not　allowed　to　radiate；　or　when　generated　faster　than　it　can　be　radiated；　is　transformed　into　motion　of　expansion。　Hence　the　shock　of　sun　and　planet　would　at　once　result　in　the　vaporization　of　both　bodies；　and　there　can　be　no　doubt　that　by　the　time　the　sun　has　absorbed　the　outermost　of　his　attendant　planets；　he　will　have　resumed　something　like　his　original　nebulous　condition。　He　will　have　been　dilated　into　a　huge　mass　of　vapour；　and　will　have　become　fit　for　a　new　process　of　contraction　and　for　a　new　production　of　life…bearing　planets。

We　are　now；　however；　confronted　by　an　interesting　but　difficult　question。　Throughout　all　this　grand　past　and　future　career　of　the　solar　system　which　we　have　just　briefly　traced；　we　have　been　witnessing　a　most　prodigal　dissipation　of　energy　in　the　shape　of　radiant　heat。　At　the　outset　we　had　an　enormous　quantity　of　what　is　called　〃energy　of　position；〃　that　is；　the　outer　parts　of　our　primitive　nebula　had　a　very　long　distance　through　which　to　travel　towards　one　another　in　the　slow　process　of　concentration；　and　this　distance　was　the　measure　of　the　quantity　of　work　possible　to　our　system。　As　the　particles　of　our　nebula　drew　nearer　and　nearer　together；　the　energy　of　position　continually　lost　reappeared　continually　as　heat；　of　which　the　greater　part　was　radiated　off；　but　of　which　a　certain　amount　was　retained。　All　the　gigantic　amount　of　work　achieved　in　the　geologic　development　of　our　earth　and　its　companion　planets；　and　in　the　development　of　life　wherever　life　may　exist　in　our　system；　has　been　the　product　of　this　retained　heat。　At　the　present　day　the　same　wasteful　process　is　going　on。　Each　moment　the　sun's　particles　are　losing　energy　of　position　as　they　draw　closer　and　closer　together；　and　the　heat　into　which　this　lost　energy　is　metamorphosed　is　poured　out　most　prodigally　in　every　direction。　Let　us　consider　for　a　moment　how　little　of　it　gets　used　in　our　system。　The　earth's　orbit　is　a　nearly　circular　figure　more　than　five　hundred　million　miles　in　circumference；　while　only　eight　thousand　miles　of　this　path　are　at　any　one　time　occupied　by　the　earth's　mass。　Through　these　eight　thousand　miles　the　sun's　radiated　energy　is　doing　work；　but　through　the　remainder　of　the　five　hundred　million　it　is　idle　and　wasted。　But　the　case　is　far　more　striking　when　we　reflect　that　it　is　not　in　the　plane　of　the　earth's　orbit　only　that　the　sun's　radiance　is　being　poured　out。　It　is　not　an　affair　of　a　circle；　but　of　a　sphere。　In　order　to　utilize　all　the　solar　rays；　we　should　need　to　have　an　immense　number　of　earths　arranged　so　as　to　touch　each　other；　forming　a　hollow　sphere　around　the　sun；　with　the　present　radius　of　the　earth's　orbit。　We　may　well　believe　Professor　Tyndall；　therefore；　when　he　tells　us　that　all　the　solar　radiance　we　receive　is　less　than　a　two…billionth　part　of　what　is　sent　flying　through　the　desert　regions　of　space。　Some　of　the　immense　residue　of　course　hits　other　planets　stationed　in　the　way　of　it；　and　is　utilized　upon　their　surfaces；　but　the　planets；　all　put　together；　stop　so　little　of　the　total　quantity　that　our　startling　illustration　is　not　materially　altered　by　taking　them　into　the　account。　Now　this　two…billionth　part　of　the　solar　radiance　poured　out　from　moment　to　moment　suffices　to　blow　every　wind；　to　raise　every　cloud；　to　drive　every　engine；　to　build　up　the　tissue　of　every　plant；　to　sustain　the　activity　of　every　animal；　including　man；　upon　the　surface　of　our　vast　and　stately　globe。　Considering　the　wondrous　richness　and　variety　of　the　terrestrial　life　wrought　out　by　the　few　sunbeams　which　we　catch　in　our　career　through　space；　we　may　well　pause　overwhelmed　and　stupefied　at　the　thought　of　the　incalculable　possibilities　of　existence　which　are　thrown　away　with　the　potent　actinism　that　darts　unceasingly　into　the　unfathomed　abysms　of　immensity。　Where　it　goes　to　or　what　becomes　of　it；　no　one　of　us　can　surmise。

Now　when；　in　the　remote　future；　our　sun　is　reduced　to　vapour　by　the　impact　of　the　several　planets　upon　his　surface；　the　resulting　nebulous　mass　must　be　a　very　insignificant　affair　compared　with　the　nebulous　mass　with　which　we　started。　In　order　to　make　a　second　nebula　equal　in　size　and　potential　energy　to　the　first　one；　all　the　energy　of　position　at　first　existing　should　have　been　retained　in　some　form　or　other。　But　nearly　all　of　it　has　been　lost；　and　only　an　insignificant　fraction　remains　with　which　to　endow　a　new　system。　In　order　to　reproduce；　in　future　ages；　anything　like　that　cosmical　development　which　is　now　going　on　in　the　solar　system；　aid　must　be　sought　from　without。　We　must　endeavour　to　frame　some　valid　hypothesis　as　to　the　relation　of　our　solar　system　to　other　systems。

Thus　far　our　view　has　been　confined　to　the　career　of　a　single　star；our　sun；with　the　tiny；　easily…cooling　balls　which　it　has　cast　off　in　the　course　of　its　development。　Thus　far；　too；　our　inferences　have　been　very　secure；　for　we　have　been　dealing　with　a　circumscribed　group　of　phenomena；　the　beginning　and　end　of　which　have　been　brought　pretty　well　within　the　compass　of　our　imagination。　It　is　quite　another　thing　to　deal　with　the　actual　or　probable　career　of　the　stars　in　general；　inasmuch　as　we　do　not　even　know　how　many　stars　there　are；　which　form　parts　of　a　common　system；　or　what。　are　their　precise　dynamic　relations　to　one　another。　Nevertheless　we　have　knowledge　of　a　few　facts　which　may　support　some　cautious　inferences。　All　the　stars　which　we　can　see　are　undoubtedly　bound　together　by　relations　of　gravitation。　No　doubt　our　sun　attracts　all　the　other　stars　within　our　ken；　and　is　reciprocally　attracted　by　them。　The　stars；　too；　lie　mostly　in　or　around　one　great　plane；　as　is　the　case　with　the　members　of　the　solar　system。　Moreover；　the　stars　are　shown　by　the　spectroscope　to　consist　of　chemical　elements　identical　with　those　which　are　found　in　the　solar　system。　Such　facts　as　these　make　it　probable　that　the　career　of　other　stars；　when　adequately　inquired　into；　would　be　found　to　be　like　that　of　our　own　sun。　Observation　daily　enhances　this　probability；　for　our　study　of　the　sidereal　universe　is　continually　showing　us　stars　in　all　stages　of　development。　We　find　irregular　nebulae；　for　example；　we　find　spiral　and　spheroidal　nebulae；　we　find　stars　which　have　got　beyond　the　nebulous　stage；　but　are　still　at　a　whiter　heat　than　our　sun；　and　we　also　find　many　stars　which　yield　the　same　sort　of　spectrum　as　our　sun。　The　inference　seems　forced　upon　us　that　the　same　process　of　concentration　which　has　gone　on　in　the　case　of　our　solar　nebula　has　been　going　on　in　the　case　of　other　nebulae。　The　history　of　the　sun　is　but　a　type　of　the　history　of　stars　in　general。　And　when　we　consider　that　all　other　visible　stars　and　nebulae　are　cooling　and　contracting　bodies；　like　our　sun；　to　what　other　conclusion　could　we　very　well　come？　When　we　look　at　Sirius；　for　instance；　we　do　not　see　him　surrounded　by　planets；　for　at　such　a　distance　no　planet　could　be　visible；　even　Sirius　himself；　though　fourteen　times　larger　than　our　sun；　appearing　only　as　a　〃twinkling　little　star。〃　But　a　comparative　survey　of　the　heavens　assures　us　that　Sirius　can　hardly　have　arrived　at　his　present　stage　of　concentration　without　detaching；　planet…forming　rings；　for　there　is　no　reason　for　supposing　that　mechanical　laws　out　there　are　at　all　different　from　what　they　are　in　our　own　system。　And　the　same　kind　of　inference　must　apply　to　all　the