But somehow I got my days mixed up and didn't realize the mistake until now.
可是不知道怎么回事,我把日期弄错了,而且到现在才知道这个错误。
but that was precisely the idea adopted in some of the early instruments (seismometers) for recording earthquake waves.
但是早期记录地震波的仪器(测震仪)正是采用了这种思路。