提高听力同时，你还可以了解到美国历史上的一些重要事件。请感兴趣的你一听为快吧！

<注意>hints：Underground Railroad
In the following years, each side held its beliefs more strongly. Many northerners thought slavery was wrong. Others saw it as a threat to free workers. Most white southerners considered slavery part of their way of life. Thousands of slaves escaped to the north with help from people along secret routes called the Underground Railroad. In 1860, however, one-third of the total population of slave states was not free. Most northerners did not care about slavery in the south, but they did not want slavery in the new territories. The southerners believed that these territories had the right to decide for themselves whether slavery would be allowed.
在随后的几年中，每一方怀有的信念更加强烈。许多北方人认为奴隶制是错误的。其他人把它视为一个自由工作者的威胁。大多数白皮肤南方人认为奴役是他们的生活方式的一部分。成千上万的奴隶从沿秘密路线的人的帮助下逃脱到北，所谓的地下铁道。在1860年，然而，奴隶州总人口的三分之一是不是免费的。大多数北方人不关心在南方的奴隶制，但他们不想在新界奴役。南方人认为这些领土，有权自行决定是否将被允许奴隶制。