新东方背诵50篇——托福听力专用(21)

时候，他发现了石油。他的油井从此每天生产20桶原油。

瑞克成功的消息将石油勘探者们吸引到现场。截止到19世纪60年代早期，这些冒险者为寻找"黑色的金子"钻探遍了整个宾西法尼亚西部。这项繁荣的事业在刺激性和粗犷的西部气氛上可与1848年的加州淘金热相媲美，而且它为勘探者带来了远超过淘金潮的财富。

原油能被提炼成许多产品。多年以来煤油一直是主要的一种产品。它在杂货店中出售由人挨户推销。19世纪八十九十年代炼油者们懂得了生产其它石油产品，如蜡和润滑油。那时石油还没有被用来制造汽油或采暖装置用油。

Raising Oysters 饲养牡蛎

2009-09-30 来源: 【大 中 小】 我要投稿 下载音频

In the past oysters were raised in much the same way as dirt farmers raised tomatoes - by transplanting them.

First, farmers selected the oyster bed, cleared the bottom of old shells and other debris, then scattered clean shells about. Next, they "planted" fertilized oyster eggs, which

within two or three weeks hatched into larvae. The larvae drifted until they attached themselves to the clean shells on the bottom. There they remained and in time grew into baby oysters called seed or spat. The spat grew larger by drawing in seawater from which they derived microscopic particles of food. Before long, farmers gathered the baby oysters, transplanted them in other waters to speed up their growth, then

transplanted them once more into another body of water to fatten them up. Until recently the supply of wild oysters and those crudely farmed were more than enough to satisfy people's needs.But today the delectable seafood is no longer available in abundance. The problem has become so serious that someoyster beds have vanished entirely.

Fortunately, as far back as the early 1900's marine biologists realized that if new measures were not taken, oysters would become extinct or at best a luxury food. So they set up well-equipped hatcheries and went to work. But they did not have the proper equipment or the skill to handle the eggs. They did not know when, what, and how to feed the larvae. And they knew little about the predators that attack and eat baby oysters by the millions. They failed, but they doggedly kept at it. Finally, in the 1940's a significant breakthrough was made. The marine biologists

discovered that by raising the temperature of the water, they could induce oysters to spawn not only in the summer but also in the fall, winter, and