Using the examples from the lecture, explain two developments that allowed ancient Roman cities to expand.
请用课堂上的案例解释有助于古罗马城市扩张的两项发明。
Listen to part of a lecture in a history class.
请听历史课上的部分内容。

So, most cities of the ancient world tended to be small, often limited to the banks of a river.
古代的大多数城市往往规模都比较小，通常都会受到河岸的限制。
They had very little means to expand.
扩张的手段非常少。
These old cities couldn't really cross natural barriers like rivers or be located very far from water sources.
这些古代的城市无法真正跨越像河流这样的自然屏障，也无法远离水源而立。
But Roman cities, on the other hand, grew much larger.
但是另一方面，罗马城市的规模却发展得很大。
How did this happen? Well, for one thing, the Romans had more advanced technology.
这是如何做到的呢？首先，罗马人技术更先进。
Let's look at a couple ?of Roman developments that allowed its cities to expand.
让我们来看看有助于罗马城市扩张的几项发明吧。
One development that'll allow Roman cities to grow was their advanced building materials.
一项就是他们先进的建筑材料。
The Romans developed a special kind of concrete, a building material that would harden under water.
罗马人发明了一种特殊的混凝土，这种建筑材料在水下会硬化。
And this concrete made new kinds of structures possible.
这种混凝土使得新的建筑类型变为可能。
Take their bridges for example. Because of this special concrete, they could build better bridges,
就以他们的桥梁为例吧。多亏了这种特殊的混凝土，罗马人能够建造出更好的桥梁，
bridges that could go across wide rivers, bridges that were big enough to transport equipment and materials with wagons and carts.
这些桥梁能够跨越很宽的河面，桥面也有足够的空间通行运着设备和材料的车辆。
So with these strong bridges, Roman cities could grow on both sides of the river,
借助着这些坚固的桥梁，罗马的城市可以在河的两岸都进行扩张，
creating larger cities than would have been possible otherwise.
建立原本不可能出现的大城市。
Another development that helped Roman cities expand was an improved way to move fresh clean water.
有助于罗马城市扩张的另一项发明就是改良的运送新鲜净水的方法。
People need access to fresh water and the Romans created an especially effective way to bring it to them.
人们需要获取新鲜干净的水源，而罗马人开发出了一种尤其有效的方法把水送到人们面前。
They built structures called aqueducts.
他们建造了一种称为高架渠的物体。
Now, aqueducts are series of open channels, waterways that stretch from water sources high in the mountains to cities.
高架渠是由一系列户外的渠道组成的，这些水渠从高山上的水源一路延伸至城中。
They were carefully planned and built so the steady drop in altitude provides the steady flow of water to cities.
这些水渠的安置和建造非常谨慎，所以高处稳定的水源为城市提供了稳定的用水。
These aqueducts could move tremendous amount of water over great distances and even bring fresh water to places far from rivers.
这些水渠能够远距离地运输大量水源，甚至还能给远离河流的地方带来新鲜的净水。
Because of this, people could have clean water for drinking and bathing without being located near a river.
多亏了这些水渠，人们才能在不必非得定居在河流附近的情况下，也有可以饮用和洗漱的净水。
So cities were able to grow larger in new locations.
因此，城市才能在新的地点不断扩张。