然而罗马的那些大桥原来并不是让人行走，而是用来引水的。在古希腊，你要是想洗澡，就不得不用水罐子到河边或井里打水，或者就直接把小溪当浴缸。不过在属罗马的城里，许多房子都装上了自来水，也有公共澡堂，供人们在这盛满清澈溪水的漂亮室内游泳池里尽情地沐浴。所有的水都是由长长的高架渠和顶端开槽的石桥引进城的。这些高架渠绵延乡间数英里，引山泉入城。
当高架渠途径山谷时，并没有爬上爬下，而是直跨山谷——真像一座高架桥。罗马人不大会造水管，所以要是让高架渠顺着山势上下的话，水就会在底部产生倒流。最有名的高架渠要算法国境内尼姆城附近加尔河上著名的“加德桥高架渠”遗迹。
罗马帝国衰败后，桥梁建筑也衰落了。整个黑暗时期几乎就没怎么造桥。接下来到公元12世纪，出现了一个奇怪的现象。欧洲所有的桥梁都回归祭司管理。而这个时期的祭司都是信仰基督教的神职人员。他们成立了一个社团组织，叫做“桥梁兄弟会”。
一开始，桥梁兄弟会只在河岔渡口开设几家小客栈，供行人歇脚。但不久他们就在这些地方动手造桥。他们通常会把桥中间的路面修得很窄，一次只过单人匹马。这样一来，强盗和兵丁就很难快速过桥抢劫行人。当然，四轮马车碰到这种桥和这样的路就麻烦大了。这些桥大多在两头建有高大的石塔作碉堡加强守卫，可以及时阻止强盗团伙，甚至阻挡敌军过桥。
中世纪最著名的桥也许要算那座泰晤士河上的伦敦古桥了。桥上还建有房子，有些达四五层楼高，但桥墩不太结实，所以就老是要修缮。桥的某些部位都塌过好多次了。你还记得《伦敦桥在坍塌》那首歌吗？唱的就是伦敦桥经过多次维修，从1209年维持到1831年，终于被拆，并在此建立了新伦敦桥。
正像你所知道的那样，中世纪之后便是文艺复兴时期，在此期间，人们造了许多名桥。如果篇幅够的话，我会做些介绍——像世界上最值得拍照的威尼斯叹息桥、佛罗伦萨的韦基奥桥、至今还被称作新桥的巴黎最古老的纳夫桥以及巴黎的皇家桥和玛丽桥。所有这些桥都是石桥。
现代桥梁建筑始于1830年左右的铁路建设。那时先造的铁桥，接着造了钢桥，最后是钢筋混凝土桥。钢筋混凝土桥是在混凝土里掺杂铁块，使桥更坚固。近年来人们建造了许多漂亮的钢筋混凝土桥。总的说来，它们都是拱桥——有的是单拱，有的是多拱。在美国它们是最受欢迎的路桥。
铁桥和钢桥通常都是桁架桥。事实上，桁架桥基本上都是现代的。亚洲和南美洲最早的一批桥都是吊桥。它们悬挂在用绳索或藤蔓做的缆绳上，摇摇晃晃的。有些至今还在使用。在过吊桥时，人总会忍不住地指望能平安无事地走过去。除了摇晃之外，它们其实是很结实的。但再结实我也不想试着骑大象过桥，也不愿开小车过桥。
现代吊桥都是吊在钢缆上的。大部分都很大，造价好几百万美金。其中最著名的一座就是纽约东河上的布鲁克林桥。尽管后来又造了一些更大的桥，但这座现代吊桥的始祖仍然被认为是最耐看的吊桥之一。成群的大象在上面跑都没有问题。事实上，桥上每天都是车水马龙。
下次有机会出门旅游时，你可要睁大眼睛多看看桥啊。世界上许多漂亮的桥都在美国。有些旅行者在旅途中玩桥牌游戏。在游戏中，吊桥算20分，悬臂桥15分，拱桥10分，桁架桥5分，独木桥2分。有时候你过了桥却没看出是哪种桥。只看到桥上的栏杆和桥面的只能得1分。谁先看出桥种类的，谁得分。还是把最后这一点介绍完吧。并不是所有的桥都好看，但是难看的桥可能总比其他难看的建筑物要少；而且只要你认真去找，即使是难看的桥，通常在背后都有一个有趣的故事。在英国的巴恩斯特伯尔有座桥，可算是最难看的桥之一了。它有许多大小不一的拱，拱的大小不是建筑师设计的，而是根据市民所捐钱的多少决定的。
THE SCRAPERS OF THE SKY
摩天大厦
HOW high is “up”？For a mountain，“up”may be several miles.For an airplane，“up”may be even higher than the highest mountain.For a building，“up”is a little over one thousand feet.Not nearly so high as the mountain top，not nearly as high as the airplane can fly，but the one thousand feet “up”of the building seems to me even more wonderful than mountain heights or airplane flights.
Very tall buildings，as you know，are called sky-scrapers.They are an American invention and all but a few of them are in the United States.Most American cities have sky-scrapers but the place where they make the finest showing is New York City.New York has almost two hundred sky-scrapers sticking up into the air like the bristles on a monstrous tooth brush.From a distance they are fairy towers，dream buildings，almost unbelievable.And when you have ridden up to the top of one of the very high sky-scrapers；they become even more unbelievable.
Gothic cathedrals have tall towers and lofty spires，but next to the tall sky-scrapers the Gothic cathedrals would not look high at all.How can men，you wonder，possibly build so far above the ground？But there you are，one hundred and two stories above the ground，and people on the streets below look like moving black specks.You pinch yourself and it hurts，so you must be awake and not dreaming.The building is real，after all.What a long time it must have taken to build it！
You are wrong.It takes a very short time to build a sky-scraper.The Gothic cathedrals，you remember，took hundreds of years to build.The Empire State Building in New York City took less than one year.And yet it has one hundred and two stories.Magic！
Here is more magic.A modern sky-scraper is built from auto trucks！It rises in the air according to schedule.Each steel girder，each piece of stone，each section of pipe，rolls into the building on a truck at just the right time.If the wrong piece got there first，it couldn’t be used at once.There would be no place to put it.It would get in the way.Traffic would be blocked on the street.The whole building would have to wait.So the material is taken out of the trucks and hoisted into place at once，instead of being piled up on the ground to wait its turn.
You can easily see，then，that a very important part of building a sky-scraper has to be done beforehand.The plans of the architect and engineer have to be very carefully made and checked.All the material has to be ordered and made ready for use so that it will arrive at just the right time—neither too soon nor too late.And that is one reason why the whole huge，wonderfulstructure can be put together so quickly and without even blocking the streets around it.
A sky-scraper is made differently from the older kinds of buildings.It has a steel framework.Each story is a kind of steel cage，and on the cage are fastened the outside walls.These walls don’t help at all to hold up the building.The steel cages do all the holding up.The walls are like the walls of a tent—to protect the inside from the weather，not to help support the weight.You would be surprised if you found the outside walls of your house were not resting on the ground，but the outside walls of a sky-scraper do not rest on the ground.They hang on the steel cages.Sometimes you can even see a crack between the sidewalk and the walls of a sky-scraper where the walls aren’t even touching the ground！
Of course no one would want to walk up to the top floor of a skyscraper.It would take too long.If you tried it，you would find you were so tired out when you reached the top that you probably couldn’t walk down again.So sky-scrapers wouldn’t be any good without elevators.A big sky-scraper has many elevators run by electricity.There are locals and expresses，as on a railroad，so that you can reach the top stories quickly without stopping at each floor all the way up.The elevators on the latest sky-scrapers are arranged so that a passenger never has to wait for one for more than a minute.