UNIT 5 Humans and natureAll nature is but art，unknown to thee.整个自然都是艺术，这是你所不知的。—PopeNature is not governed except by obeying her.自然不可驾驭，除非顺从她。—BaconDeviation from nature is deviation from happiness.对自然的背离就是对欢乐的背离。 —Samuel JohnsonOur motto，as you know，is Live according to Nature.如你所知，我们的格言是“遵照自然去生活”。—SenecaThe intimacy between man and Nature began with the birth of man on the earth ， and becomes each century more intelligent and farreaching.To Nature，therefore，we turn as to the oldest aim and most influential teacher of our race ；from one point of view ，once our taskmaster，now our servant ；from another point of view，our constant friend，instructor and inspirer.Have you ever noticed that ，in the summer ，when your sweat dries it will leave marks on your Tshirt? The marks are tiny bits of salt.If you go to the seaside ， you will know that seawater does the same to your clothes ， because it also has a lot of salt in it.Water evaporates ( 蒸 发 ) from the surface of the sea but the salt remains.Have you ever thought about whether the seas will keep getting saltier? The answer is no.The oceans have stayed at about 3.5 percent salt content for hundreds of millions of years.It maintains a constant level of salt in different ways.The outer hard crust of Earth is made up of a dozen or so hard plates that drift (漂流) on extremely hot，soft rock like floating islands on a sea.The heat within Earth is not the same everywhere and the plates move due to the heat.When an ocean plate bumps into a land plate ， the less dense land plate floats over the ocean one.The ocean floor gets pushed under ， and its salty rocks，along with shells and bones，are lost deep within Earth.So，that's why the seas are salty but don't get any saltier.[探究发现]1．Find out what are the marks on your Tshirt?Tiny bits of salt.2．Find out if the salt in the seas can evaporate?No . 3．Find out if the heat within Earth is the same?No.
