Note that in a closed universe you could start a journey off in one direction and, if allowed enough time, ultimately return to your starting point; in an infinite universe, you would never return.
We also installed a chemical switch that allowed the electron transfer process to proceed in the opposite direction.
我们同样按照了一个化学开关，它可以允许电子交换过程以相反的方向进行。
3
It controlled a trap door between the boxes and allowed only fast-moving (ie, hot) molecules through in one direction and only slow-moving (ie, cold) ones through in the other.