Part 17

"It is a mark to show when it is exactly twelve o'clock."

"Let me go and see it," said Rollo, "while my sand is drying."

Rollo followed Jonas off into the barn, and when there, Jonas pointed to a small line which he had cut with his penknife upon the barn floor. It began at the foot of one of the posts, by the side of the door, and extended back into the barn exactly straight.

"Is that the noon mark?" said Rollo. He was surprised to see that a noon mark was nothing but a cut with a penknife upon a barn floor.

"Yes," said Jonas; "that is a meridian."

"A meridian!" said Rollo, looking upon it with an air of great curiosity and respect.

"Yes," said Jonas; "a line drawn exactly north and south, is called a meridian line; and that is exactly north and south."

"What do you call it a noon mark for?" said Rollo.

"Because," said Jonas, "the shadow of the edge of the door post will always be exactly upon it at noon. So that I can always tell now when it is noon, by the shadow of the post upon my noon mark, if the sun shines."

All this was very new and very curious to Rollo. He had never seen or heard of a noon mark before; and it seemed to him a very simple and beautiful way of knowing when it was noon. He asked Jonas how he found out about it, and Jonas told him that he had been reading about it in a book on astronomy.

"Your father let me have the book," said he; "and see my chalk marks for the sun's shadow."

Rollo looked, and found that Jonas had put down quite a number of chalk marks along in a line, where they had first began to mark the place where the shadow of the door reached into. Rollo and Lucy had forgotten all about their plan of making such a series of observations; but Jonas had gone on regularly, making a mark every Monday, at noon, precisely. As the sun, at that season of the year, was going round farther and farther to the south every week, it shone in farther and farther upon the floor, so that each chalk mark was farther in than the one made the week before.

In order to make his marks at the right time, Jonas wanted to know, every Monday, when it was precisely twelve o'clock, and this led him to make his noon mark, having seen the account of it in the book which Rollo's father had lent him. He learned there that the shadows of all upright objects are cast exactly north at twelve o'clock, or rather very nearly north; near enough for his purposes. Now, as the post of the barn door was upright, he knew that the shadow of it would be in the north and south line at noon. Of course, if he had a north and south line, or a meridian line, as it was called in the book, drawn upon the floor, he knew that he could tell when it was noon, by the shadow of the post coming then exactly upon that line. He explained this all to Rollo, and Rollo was very much pleased with it indeed. He determined to have a noon line somewhere in the house.

Rollo asked Jonas what was the way to draw a noon line. Jonas told him that there were several ways. One way, he said, was to observe some day, by the clock, when it was exactly noon, and then to mark, upon the barn floor, the line where the shadow of the edge of the post fell precisely at that moment. Another way was to get a compass needle, and put it down upon the floor, and then draw a north and south line precisely in the direction that the needle indicated. That would, of course, be a north and south line, because the compass needle always pointed north and south. He said that he adopted both these methods to make his noon line. First, he got a compass needle, which Rollo's father had lent him, and put that down upon the barn floor just at the foot of the door post, and observed the direction; and he also noticed when it was twelve, by the clock in the house, and he found that, when it was twelve by the clock, the shadow of the post came exactly to the line indicated by the direction of the compass needle; and so he knew that that was a correct meridian line.

JONAS'S DIAL.

That evening, Rollo told his father about his hour-glass, and also about Jonas's noon line. His father said it was very difficult to draw a meridian line.

"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."

"No matter how quick he might make it. It would take some time, wouldn't it?"

"Only a very little," said Rollo.