第四部分　概括大意　(20分)
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31.　The　Solar　E　
The　sun　is　the　source　of　most　of　the　heat　known　to　us.　As　a　direct　source　of　heat,　the　sun　maintains　life　upon　this　planet,　150　000　000　kilometers　distant.　It　is　also　an　indirect　source　of　heat.　Since　the　earth　is　derived　from　the　sun,　it　is　the　latter　that　we　must　trace　some　of　the　earth's　internal　heat　that　reveals　itself　through　volcanoes,　geysers　(喷泉),　and　hot　springs.

32.　Insects　and　Surface　T　
An　insect　is　not　afraid　of　gravity,　but　it　is　in　deadly　fear　of　another　force　of　nature.　This　force　is　called　surface　tension　(张力).　A　man　coming　out　of　a　bath　carries　with　him　a　film　of　water　about　one　fiftieth　of　an　inch　in　thickness.　This　weights　about　a　pound.　A　wet　mouse　has　to　carry　its　own　weight　in　water.　A　wet　fly　has　to　lift　many　times　its　own　weight,　and,　as　everyone　knows,　a　fly　once　wetted　by　water　or　any　other　liquid　is　in　a　very　serious　position　indeed.　An　insect　going　for　a　drink　is　in　as　great　danger　as　a　man　leaning　over　the　edge　of　a　cliff　in　search　of　food.　If　it　once　falls　into　the　grip　of　the　surface　tension　of　the　water　-　that　is　to　say,　gets　wet　it　is　likely　to　remain　so　until　it　drowns.　A　few　insects　contrive　to　be　unwettable;　the　majority　keep　away　from　their　drink　by　means　of　long　proboscis　(喙).

33.　T　of　Boiling　Point　of　Liquids
When　the　temperature　of　a　liquid　is　raised　enough,　the　liquid　boils.　This　means　that　bubbles　of　vapor,　containing　millions　of　molecules　form　below　the　surface.　In　order　for　such　bubbles　to　be　produced,　the　pressure　of　the　vapor　inside　them　must　be　equal　to　the　pressure　of　the　air　upon　the　surface　of　the　liquid.　If　the　air　pressure　is　greater,　the　bubble　will　collapse.　The　boiling　point　of　a　liquid,　then,　is　the　temperature　at　which　the　vapor　pressure　of　the　liquid　is　equal　to　the　atmospheric　pressure　upon　the　liquid　at　the　surface.

34.　E　and　Contraction
Solids　also　expand　with　increase　in　temperature　and　contract　when　cooled.　But　they　do　not　behave　quite　so　uniformly　(相同地)　in　this　respect　as　liquids　and　gases　do,　particularly　in　the　case　of　wide　variations,　in　temperature.　Most　solids　expend　or　contract　by　a　definite　amount　for　every　degree　of　the　temperature　that　rises　or　falls.　The　amount　by　which　on　substance　expands　and　contracts　for　one　degree　is　not　usually　the　same　as　that　for　a　different　substance.　Thus,　for　a　given　rise　in　temperature,　a　piece　of　brass　expands　at　little　more　than　a　piece　of　copper　and　much　more　than　a　piece　of　steel　of　the　same　size.

35.　A　of　Electric　Trains
Electric　trains　have　many　advantages　over　those　drawn　by　steam　engines.　There　is　no　smoke　to　soil　the　passengers'　clothes　and　the　cushions　in　the　train.　Because　they　carry　no　heavy　loads　of　coal　and　water,　these　trains　can　start　and　stop　with　less　waste　of　power.　In　a　station　they　are　silent:　there's　no　steam　to　produce　noise　to　deafen　the　passengers.