VOA慢速英语:How Earth Is Cracked Like a Giant Eggshell

in motion在开动中, 在运转中

例句：Friction is a form of resistance for an object in motion.
      摩擦力是一种阻止物体相对运动的阻力。

2.Only since the nineteen-sixties have scientists begun to understand that the Earth is a great, living structure.

Only倒装

    在句首的only如果后面跟了副词、介词短语或从句等状语，则主句用倒装结构。如only后跟
    的是主语，就不用倒装结构了。例如：
    only when the war was over in 1918 was he able to get back to work．
    only in this way can you learn english well．
    only after a bitter struggle was the aim achieved．
    only the teachers are allowed to use the room．（不倒装）
    only five men were hurt in the accident．（不倒装）

    注意，only和所修饰的状语如果不在句首，则不倒装。

    如：

    the boy was helped out only when his father arrived in a hurry．
    only then did he realize the importance of english
    只有在那时候他才意识到英语的重要性。
    only then ,是过一小会儿的意思，一般用在句首，引导倒装句。
    例句only then did he realize the importance of english．
    解析：在句首的only如果后面跟了副词、介词短语或从句等状语，则主句用倒装结构。如only后跟的是主语，就不用倒装结构了。
    再比如
    only when the war was over in 1918 was he able to get back to work．(倒装句）
    only five men were hurt in the accident．（不倒装）