英语听力文摘 水塔物理学原理
水塔，一般居民区里蓄水作用，有些还是水厂生产工艺的一个重要组成部分，用于保持和调节给水管网中的水量和水压。主要由水柜、基础和连接两者的支筒或支架组成。在工业与民用建筑中，水塔是一种比较常见而又特殊的建筑物。它的施工需要特别精心和讲究技艺，如果施工质量不好，轻则造成永久性渗漏水，重则报废不能使用。
Water towers take advantage of the force of gravity to provide pressure for the water they contain. Every vertical foot adds point-four-three pounds per square inch to the water pressure. Towns usually keep their water pressure between fifty and one-hundred pounds per square inch, so a simple equation tells them how high to build the tower. And since a typical tower contains a full day’s worth of water, the force of gravity can maintain the hydrostatic pressure of the water system even when the power goes out.
水塔其实是利用重力给蓄水提供压力，每增加一英尺高度，水压增加0.43磅/平方英寸。城镇所需水压在50-100磅/平方英寸之间，简单换算一下就知道水塔需要建多高。普通水塔储蓄一整天的用水，由此，即便在停电的情况下，重力作用仍能保持供水系统的静水压力。
Actually they serve an important day-to-day purpose as well. A city’s water usage varies throughout the day, usually peaking in the early morning when many people are showering and washing. Water usage during this time can be four or five times higher than during other parts of the day. To maintain water pressure during peak hours of the day, a city could invest in a very powerful pump, but this would be expensive and wasteful, since its capacity would go unused for most of the day.
事实上，水塔在日常用途中也起着十分重要的作用。城市一天用水量变化不断，通常情况下，早晨人们洗漱的时候达到高峰值。这个时段的耗水量是一天中其他时段的4到5倍。为了在高峰时段维持水压，城市需要投资购买大功率的水泵，然而水泵在其他时段却派不上用场，因此会浪费的昂贵品。
Let me guess，instead, the city draws on the water tower’s supply of water during these hours of high demand.
我猜想，在高需求时段，城市利用的是水塔的供水。
That's right, the water tower and pumps work together to keep the water pressure stable throughout the day.
的确如此，水塔和水泵协力保持水压在一天中持续稳定。
NOTES
1.pounds per square inch:磅/平方英寸，缩写为PSI，英制压力单位，欧美等国家习惯使用psi作单位，在很多进口的压力容器安全阀上采用这种标示。
在中国，压力用“公斤”描述(而不是“斤”)，体单位是“kg/cm^2”,一公斤压力就是一公斤的力作用在一个平方厘米上。而在国外常用的单位是“Psi”，具体单位是“lb/in2”, 就是“磅/平方英寸”，这个单位就像华氏温标(F )。此外，还有Pa(帕斯卡，一牛顿作用在一平方米上)，KPa，Mpa，Bar，毫米水柱，毫米汞柱等压力单位。
1兆帕(MPa)=145磅/英寸2(psi)=10.2公斤/平方厘米=10巴(bar)=9.8大气压(atm)
2.hydrostatic pressure：静水压力。
