He begins at the head of the line, and putting one word to each boy, goes regularly down, each successive pupil calculating the chances whether a word which he can accidentally spell will or will not come to him.

If he spells it, the teacher can not tell whether he is prepared or not.

That word is only one among fifty constituting the lesson.

If he misses it, the teacher can not decide that he was unprepared.

It might have been a single accidental error.
Another teacher, hearing the same lesson, requests the boys to bring their slates, and, as he dictates the words one after another, requires all to write them.