The Pantheon here was built by the Emperor Haydrian in the 2nd century AD and its 1)dome is still one of the largest 2)concrete domes in the world. It's 50 yards across and its concrete is up to 20 feet thick.

     The word "concrete" comes from the Latin "concretus", meaning grown together or harden. Romanconcrete was a mixture of sand, powdered 3)limestone and stone 4)rubble, pretty well what it is today. To construct 5)arches and 6)vaults, the Romans also developed a lightening. And the higher they went the lighter they made the concrete. In this way they could roof over much larger areas than would be possible with stone.

   But one of the problems with concrete is that even after it's hardened, it still absorbs water. So the Romans added a mix of 7)volcanic 8)cinders and clay called "Potsularna". Potsularna reacts with the 9)lime in the 10)mortar and sets quickly. And most important of all, it's resistant to water.

    The Romans had invented damp proof concrete, and they used it for the foundations of buildings like the pantheon, for lining reservoirs and for constructing harbors. Concrete allowed the Romans to build big and fast, and thus 11)impose their authority on tribes still living in simple structures of wood or mud. Concrete may well have been one of the secret weapons of the Roman Empire.

古罗马的故事

    万神殿是哈第安皇帝在公元2世纪时建立的，它的圆顶至今仍是世界上最大的混凝土圆顶之一。宽有50码，混凝土厚达20英尺。

    "混凝土"一词来自拉丁语"concretus"，意思是粘合在一起或硬化。罗马的混凝土是把沙子、石灰粉和碎石掺合到一块来，和今天的做法很相似。为了建造拱门与拱顶结构，罗马人还发展了减压技术。建得越高，所用的混凝土也就越轻。他们用此办法搭建的屋顶面积比用石头搭建的要大得多。

    可混凝土的一个问题是，即使变硬以后，它仍然吸水。所以罗马人又添加进一种火山灰与黏土的混合物，称为"波修拉那"。波修那拉与灰泥中的石灰产生反应，可以快速定型。最重要的是它还防水。

    罗马人发明出防湿混凝土，用它来建造万神殿这样的建筑基础，用来围水库、起港口。混凝土使罗马人的建筑宏大、建筑速度更快，并加强了他们对依然居住在泥木搭建的简单结构里的部族的统治。混凝土也许一直是罗马帝国的秘密武器之一。

1) dome  n. 圆屋顶      

2) concrete  n. 混凝土

3) limestone n. 石灰石   

4) rubble  n. 碎石            

5) arch n. 拱门 

6) vault  n. 拱顶             

7) volcanic a. 火山的   

8) cinder  n. 灰

9) lime  n. 石灰              

10) mortar   n. 灰泥          

11) impose v. 加强