on the heavens-第25节

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n。　　　Now　in　its　own　place　every　body　endowed　with　both　weight　and　lightness　has　weightwhereas　earth　has　weight　everywhere…but　they　only　have　lightness　among　bodies　to　whose　surface　they　rise。　Hence　when　a　support　is　withdrawn　such　a　body　moves　downward　until　it　reaches　the　body　next　below　it；　air　to　the　place　of　water　and　water　to　that　of　earth。　But　if　the　fire　above　air　is　removed；　it　will　not　move　upward　to　the　place　of　fire；　except　by　constraint；　and　in　that　way　water　also　may　be　drawn　up；　when　the　upward　movement　of　air　which　has　had　a　common　surface　with　it　is　swift　enough　to　overpower　the　downward　impulse　of　the　water。　Nor　does　water　move　upward　to　the　place　of　air；　except　in　the　manner　just　described。　Earth　is　not　so　affected　at　all；　because　a　common　surface　is　not　possible　to　it。　Hence　water　is　drawn　up　into　the　vessel　to　which　fire　is　applied；　but　not　earth。　As　earth　fails　to　move　upward；　so　fire　fails　to　move　downward　when　air　is　withdrawn　from　beneath　it：　for　fire　has　no　weight　even　in　its　own　place；　as　earth　has　no　lightness。　The　other　two　move　downward　when　the　body　beneath　is　withdrawn　because；　while　the　absolutely　heavy　is　that　which　sinks　to　the　bottom　of　all　things；　the　relatively　heavy　sinks　to　its　own　place　or　to　the　surface　of　the　body　in　which　it　rises；　since　it　is　similar　in　matter　to　it。　　　It　is　plain　that　one　must　suppose　as　many　distinct　species　of　matter　as　there　are　bodies。　For　if；　first；　there　is　a　single　matter　of　all　things；　as；　for　instance；　the　void　or　the　plenum　or　extension　or　the　triangles；　either　all　things　will　move　upward　or　all　things　will　move　downward；　and　the　second　motion　will　be　abolished。　And　so；　either　there　will　be　no　absolutely　light　body；　if　superiority　of　weight　is　due　to　superior　size　or　number　of　the　constituent　bodies　or　to　the　fullness　of　the　body：　but　the　contrary　is　a　matter　of　observation；　and　it　has　been　shown　that　the　downward　and　upward　movements　are　equally　constant　and　universal：　or；　if　the　matter　in　question　is　the　void　or　something　similar；　which　moves　uniformly　upward；　there　will　be　nothing　to　move　uniformly　downward。　Further；　it　will　follow　that　the　intermediate　bodies　move　downward　in　some　cases　quicker　than　earth：　for　air　in　sufficiently　large　quantity　will　contain　a　larger　number　of　triangles　or　solids　or　particles。　It　is；　however；　manifest　that　no　portion　of　air　whatever　moves　downward。　And　the　same　reasoning　applies　to　lightness；　if　that　is　supposed　to　depend　on　superiority　of　quantity　of　matter。　But　if；　secondly；　the　kinds　of　matter　are　two；　it　will　be　difficult　to　make　the　intermediate　bodies　behave　as　air　and　water　behave。　Suppose；　for　example；　that　the　two　asserted　are　void　and　plenum。　Fire；　then；　as　moving　upward；　will　be　void；　earth；　as　moving　downward；　plenum；　and　in　air；　it　will　be　said；　fire　preponderates；　in　water；　earth。　There　will　then　be　a　quantity　of　water　containing　more　fire　than　a　little　air；　and　a　large　amount　of　air　will　contain　more　earth　than　a　little　water：　consequently　we　shall　have　to　say　that　air　in　a　certain　quantity　moves　downward　more　quickly　than　a　little　water。　But　such　a　thing　has　never　been　observed　anywhere。　Necessarily；　then；　as　fire　goes　up　because　it　has　something；　e。g。　void；　which　other　things　do　not　have；　and　earth　goes　downward　because　it　has　plenum；　so　air　goes　to　its　own　place　above　water　because　it　has　something　else；　and　water　goes　downward　because　of　some　special　kind　of　body。　But　if　the　two　bodies　are　one　matter；　or　two　matters　both　present　in　each；　there　will　be　a　certain　quantity　of　each　at　which　water　will　excel　a　little　air　in　the　upward　movement　and　air　excel　water　in　the　downward　movement；　as　we　have　already　often　said。

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　　The　shape　of　bodies　will　not　account　for　their　moving　upward　or　downward　in　general；　though　it　will　account　for　their　moving　faster　or　slower。　The　reasons　for　this　are　not　difficult　to　see。　For　the　problem　thus　raised　is　why　a　flat　piece　of　iron　or　lead　floats　upon　water；　while　smaller　and　less　heavy　things；　so　long　as　they　are　round　or　long…a　needle；　for　instance…sink　down；　and　sometimes　a　thing　floats　because　it　is　small；　as　with　gold　dust　and　the　various　earthy　and　dusty　materials　which　throng　the　air。　With　regard　to　these　questions；　it　is　wrong　to　accept　the　explanation　offered　by　Democritus。　He　says　that　the　warm　bodies　moving　up　out　of　the　water　hold　up　heavy　bodies　which　are　broad；　while　the　narrow　ones　fall　through；　because　the　bodies　which　offer　this　resistance　are　not　numerous。　But　this　would　be　even　more　likely　to　happen　in　air…an　objection　which　he　himself　raises。　His　reply　to　the　objection　is　feeble。　In　the　air；　he　says；　the　'drive'　（meaning　by　drive　the　movement　of　the　upward　moving　bodies）　is　not　uniform　in　direction。　But　since　some　continua　are　easily　divided　and　others　less　easily；　and　things　which　produce　division　differ　similarly　in　the　case　with　which　they　produce　it；　the　explanation　must　be　found　in　this　fact。　It　is　the　easily　bounded；　in　proportion　as　it　is　easily　bounded；　which　is　easily　divided；　and　air　is　more　so　than　water；　water　than　earth。　Further；　the　smaller　the　quantity　in　each　kind；　the　more　easily　it　is　divided　and　disrupted。　Thus　the　reason　why　broad　things　keep　their　place　is　because　they　cover　so　wide　a　surface　and　the　greater　quantity　is　less　easily　disrupted。　Bodies　of　the　opposite　shape　sink　down　because　they　occupy　so　little　of　the　surface；　which　is　therefore　easily　parted。　And　these　considerations　apply　with　far　greater　force　to　air；　since　it　is　so　much　more　easily　divided　than　water。　But　since　there　are　two　factors；　the　force　responsible　for　the　downward　motion　of　the　heavy　body　and　the　disruption…resisting　force　of　the　continuous　surface；　there　must　be　some　ratio　between　the　two。　For　in　proportion　as　the　force　applied　by　the　heavy　thing　towards　disruption　and　division　exceeds　that　which　resides　in　the　continuum；　the　quicker　will　it　force　its　way　down；　only　if　the　force　of　the　heavy　thing　is　the　weaker；　will　it　ride　upon　the　surface。　　　We　have　now　finished　our　examination　of　the　heavy　and　the　light　and　of　the　phenomena　connected　with　them。


　　　　　　　　　　　　　　　　　　　　　　　　　　　　THE　END　。