We've probably had four major periods in American history of there's one other picture I wanted to put up.

在美国历史上,我们大约经历了四个时期,在我想贴的另一张图片上

耶鲁公开课 - 美国内战与重建课程节选

We have probably four great reform periods in American history.

我们美国历史上约有四个大改革时期

耶鲁公开课 - 美国内战与重建课程节选