"Why, in the first place, the clock is not exact. It is near enough to answer all the purposes of a family; but it may often be a minute or more out of the way. Then besides, while Jonas is going from the clock out to the barn, the shadow is slowly moving on, all the time; so that he cannot tell exactly where the shadow was, when it was precisely twelve by the clock. "Then again, it is not always exactly noon when the shadow comes to the north and south line. It varies a little at different seasons of the year, though it is so near that we say, in general terms, that at noon all shadows of upright objects point to the north. Still, it is not _precisely_ true, except on a very few days in the year. Then, again, the post of the barn door is not exactly upright." "I thought they always made door posts exactly upright," said Rollo. "They do make them as nearly upright as they can, with the common carpenters' instruments; but they are not _exact_. To set a post of any kind, with great precision, perpendicular to the horizon, would require very expensive mathematical instruments, and very laborious and nice observations. Then, again, if the clock had been exact, and the post perfectly upright, Jonas could not have marked the place of the shadow exactly. The shadow has not an exact and well-defined edge; and then, even while he was marking at one end, the shadow would be moving along at the other end, and so his noon mark would not be exactly straight." "Why, father, he could make the mark right along quick." "No matter how quick he might make it. It would take some time, wouldn't it?" "Only a _very little_," said Rollo. "And do you suppose the sun would stand still, even during that little time, so as to let the shadow remain stationary? "However," continued his father, "I don't say this to disparage Jonas's noon mark. I dare say, it is accurate enough for his purposes. He only wants to know from it when it is time for him to come in to dinner, or something like that. I only want you to understand what exactness is, and to see, a little, how difficult it is to attain to any considerable degree of it, in such cases. So thus, it seems, that Jonas has got a sort of a dial?" "Why, it only tells him what o'clock it is at one hour in the day," said Rollo. "But I think he might make it do for all the afternoon and forenoon." "How?" inquired his father. "Why, all he has got to do is to watch some day when it is nine o'clock, and ten o'clock, and so on, every hour; and then make a line where the shadow comes every hour, just as he did for twelve o'clock. Then he will have marks for every hour in the day, and when the shadow comes along to these marks, one after another, he will know what time it is."