but when the dawn came he saw the lone tower standing dark against the pink and primrose of the east, and about its base the sullen swirl of black water, and he heard the wonderful roar of it. so he hung off and on, all that day and for six days besides. and when he had watched seven days he knew something. for you are certain to know something if you give for seven days your whole thought to it, even though it be only the first declension, or the nine-times table, or the dates of the norman kings.