to great depths. Islands of this kind are called atolls. How these atolls were formed is still an open question. As reefbuilding corals cannot live below 20 fathoms， it is evident that corals could not have built up the reefs from the bottom of the deep sea.
137.Man and the Ocean. -At first thought it would seem bet- ter for the life of the world if the proportion of land and water were reversed. Yet when we consider that almost barren wastes constitute many continental interiors and that plenty of rainfall is necessary to make land habitable， the utility of the great water surfaces becomes ap- parent. From the evaporation of the ocean surface comes nearly all the water which supplies man， land animals and plants.
It is not only true that all streams eventually run to the sea but it is also true that all their water comes from the sea. Other things being equal， the smaller the surface for evaporation the less the water supplied to the land. Besides supplying the land with water， the ocean has a great effect on its climate. The animals of the sea also furnish food for thousands. A large part of the earth"s population is now， and always has been， located not far from the shore of the ocean.
In early times before the advent of railways almost all commerce was carried on over the sea. Even now this is the cheaper way of transportation. Modern methods of conveyance have enabled man to live with comfort at a considerable distance from the ocean， but the dry interiors of continents still remain sparsely inhabited. All commercial nations must have an outlet to the sea and to obtain it much blood and treasure have often been spent. Man for his commerce has even broken through the barriers by which nature has separatedseas and oceans， as in the case of the Suez and Panama canals.
Summary. -The sea occupies nearly three quarters of the area of the globe. It is usually spoken of as divided into oceans， the Atlantic， Pacific， Indian， Arctic and Antarctic. Although some parts of the ocean are nearly six miles deep， the most interesting and most extensively inhabited part of it is that above the continental shelf. This shelf is thegradual slope from the edge of the different continents to a depth of about six hundred feet. Beyond this the descent becomes rapid.
On the surface of the sea are waves varying in height from a fewinches to fifty feet. These stir up the water and enable it to absorb more air， which is so necessary to the living things in the sea. Not only is the surface of the sea constantly in motion， but there are also great currents， such as the Gulf Stream， which carry the water from one latitude to another. These currents are caused almost entirely by the winds.
The tides are due primarily to the action upon each other of theearth， moon and sun. The range of tides varies from two or three feet on oceanic islands to about seventy feet at some places in the Bay of Fundy. The tides help drainage and assist vessels over bars that would otherwise be impassable.
QUESTIONS
What portion of the earth"s surface is sea？ Into what divisions is the sea usually divided？
What is the continental shelf？ Why is it of especial interest to man？
Of what is the sea water composed？ How does its composition affect animals？
If you were able to take a trip from the nearest beach over the ocean bed toanother continent， describe what you would probably find along the way.
What are waves？ What causes them？
What is the temperature of the ocean water in different parts of its surface and at different depths？
Describe the best known ocean currents. What is the cause of these？ Why is a knowledge of ocean currents important to mariners and also to those who would explain the climates of different places？
What are tides？ What causes them？ What are their effects？ How are coral reefs formed？ What are atolls？
In what ways is the ocean useful to man？
【中文阅读】
124.茫茫大海--只要拿一张世界地图或者地球仪看上一眼，上面大面积 的海洋会给我们留下深深的印象。地球表面积大约为196940000平方英里，其 中72%是海洋。大部分的陆地都处在北半球，南半球的大部分面积都是大海。 在南纬40°以下就只有很小的一部分陆地面积了，对应在北半球的话，差不多 就是费城的纬度。因此大部分世界地图都不会将南纬60°以下的部分画出来， 对应在北半球差不多是彼得格勒（圣彼得堡）的纬度。这也是在一些地图上， 赤道会偏离图的中心的原因。
125.海洋的分区--尽管地球上的海洋都是相互连通的，但是为了某些 需要，将它们分成不同的部分会更好一些。因此，纵贯地球南北的海域被分成 三大洋：大西洋、太平洋和印度洋，赤道把它们都分成了南北两半；处于极地 周围的分别被叫做北冰洋和南冰洋。有时人们也将南纬40°以外的整体海域叫 做南大洋。
这些不同大洋的边界都没有规则形状，但除了有封闭边界的南大洋和北冰 洋，其他三大洋都向北逐渐变窄。还有一些被陆地包围的海域也与它们连通， 在有些地方部分海域还会深入到内陆，比如地中海和墨西哥湾。海洋的表面本 身是水平面，但不稳定，会随着起伏不定的波涛升降起落。
126.大陆架--围绕着大陆板块的边界，或者在靠近大陆的岛屿周围，往 外延伸最远两三百英里，海底逐渐加深，这一个区域，就叫做大陆架。当海底 达到600英尺左右的深度的时候，这种逐渐加深的趋势会突然变成向着深海的 陡然沉降，达到两三英里的深度。
在大陆架上散布着许多大陆岛屿，比如不列颠群岛和东印度群岛。这一区 域，也是地球上最大的渔场，例如纽芬兰的大浅滩海域，以及冰岛和罗浮敦岛 周围的地区，渔民们世世代代在这里丰收了巨量的渔产。地球上再也找不到第 二个地方像大陆架的浅滩这样，生命的形态如此多样，生物之间的生存竞争如 此激烈。
河流会将泥沙带入海洋，它们慢慢沉淀下来，形成了沉积岩。大陆周围 也有低洼的海岸平原，这是由大陆架上的一些高地包围而成的。有理由相信， 深海的海底从未抬升到洋面成为陆地，但形成大陆板块主体部分的广袤沉积岩 层，却都曾在海底沉睡过，那时它们正是大陆架的一部分。
127.海水的成分
实验122：在一碗干净的水中放入一个用于演示的液体比重计，或者放一 根在实验35中用到过的木条，给它下沉的位置做上记号。再将比重计放入一碗 海水中，也给它下沉的位置做上记号。如果没办法取得海水，就在一品脱水中 溶解15克或者半盎司的盐，这就让水中的溶解物和海水差不多了。比较一下， 比重计在淡水中会比在海水中多下沉多少？冰块在盐水中会比在淡水中漂浮得 更明显吗？
实验123：如果能取得海水的话，取其一品脱放入锅里熬干，尝一下残留 物的味道，它的主要成分是什么？
地球表面几乎没有百分之百的天然纯净水，不过，水也是我们所知道的 最好的一种溶剂。任何一种接触到水的物质，都会或多或少地被水溶解掉一部 分。当河流汇入海洋，它们也将一路所溶解的物质转运到了海水当中。在阳光 的照耀下，海水又不断蒸发，再被空气携带到陆地上变成雨露降落下来，而之 前的溶解物便被留在了海洋当中。
因此，海洋无时无刻不在接收从陆地上转运而来的可溶物增益。非常容 易证明海水中还有盐分，我们一尝就知道。海水中还一定含有石灰，因为珊瑚 和贝壳类动物需要它来形成壳体的硬质部分。海水还含有一些有机食物成分， 不然的话，像牡蛎这样不能移动的动物便得不到食物了。它还含有空气，否则 鱼类便不能呼吸。这些都只是在探讨海水成分时我们能考虑到的主要物质，当 然，化学家们还可以在海水中发现许多种溶解于其中的其他物质。由于这些溶 解物的存在，海水也就比淡水更重，或者说具有更大的比重。1立方英尺的淡 水重量为62.5磅，而海水则为64.25磅。
128.海洋的深度--海洋已发现的最大深度已接近6英里①，位于太平洋 的强盗岛附近。大西洋的最深处位于波多黎各北部地区，深度5英里多一点。 海洋的整体平均深度大约为1.5到2英里。
尽管海底的海水压力极其巨大，但由于水本身的不可压缩性，每立方英 尺的水的重量，在海底与跟在海面差不了多少。因此，一个物体不管它处在 哪个深度，只要一旦下沉，便会直接沉到海底。在两英里的深处的压力是水 面附近处压力的300倍，我们之前也已说到，那里的压强大约为每平方英寸 15磅。