“O no, father,” said Rollo; “Jonas has drawn one, and he told me how, and it was a very easy way.”
“Yes,” said his father, “it is easy to draw something which you can call a noon mark; but it is a very difficult and delicate operation to do it with any considerable degree of exactness.”
“I think that Jonas’s is exact,” said Rollo.
“It probably may be as exact as he could make it with his means and instruments; but there are a great many sources of error which he could not possibly have avoided.”
“What?” asked Rollo.
“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.”