Rollo's Experiments

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.

"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."

"O, but the difficulty is," said his father, "that the shadow will not come to the same places, at the same hours, on different days. It will come to the meridian line, at twelve, always,-that is, nearly to it; but it will not come to any other lines regularly,-that is, if the object, which casts the shadow, is upright."

"Will any other kind of object carry the shadow regularly?" asked Rollo.

"Yes," said his father, "an object that leans over to the north, so as to point to the North Star. If you and Jonas could put a post into the ground so as to have it point to the North Star, then you could mark, all around it, the places to which the shadow would come for every hour in the day, and afterwards it would come to the same places regularly, or nearly so. It would be near enough for your purposes; and I don't know but that it would be quite a respectable dial for you."

Rollo then asked his father why it was that a post, which pointed to the North Star, would bring a shadow any more regularly to the hour marks, than an upright one would; but he said that Rollo did not know enough, yet, to understand the explanation, even if he were to try to explain it. "Therefore," said he, "you must wait until you study astronomy before you can expect to understand it; but you can now, in the mean time, make such a dial, if you wish to do it."

Rollo did wish to do it very much. He accordingly told Jonas all that his father had said. It seemed very strange to Jonas, that a post, pointing to the North Star, should have its shadows move round any more regularly than a post in any other position. He could not imagine what the North Star could have to do with the shadows. Still, he determined to try the experiment.

A few days after this, Jonas did try the experiment. He got two narrow boards, which were once pickets belonging to a picket fence, one end of each was sharp, so that it could be driven down into the ground. Then he selected a certain part of the yard, in a corner, where the dial would be out of the way, and yet the path to the barn led along pretty near it. The reason why Jonas got two boards was this: he knew that, if he drove only one stake into the ground, and inclined it towards the North Star, it would be very likely to get started out of its proper position; but if he had two, he could drive the second one down perpendicularly from the end of the first, and then nail the two ends together; and that would keep all steady.

After having got every thing ready, the boys waited till the evening before fixing up the dial, because they could not see the North Star in the day time. But when the evening came, they went out, and began their preparations. It was a clear and pretty cold evening, and the stars were out in thousands.

"Which is the North Star?" asked Rollo.

Jonas looked about a minute or two, saying, "Let me see-where's the Dipper? O, I see a part of it; the rest is down behind the barn. It was up high the last time I saw it."

"Where is the Dipper?" said Rollo, looking eagerly in the direction to which Jonas was turned.

"Come this way," said Jonas, "so as to be out of the way of the barn, and you can see it better."

So Jonas pointed out the Dipper to Rollo, with its square body, and long, bent handle. It was at first quite difficult for Rollo to see any thing that looked at all like a dipper; as it consisted only of stars, which it required some imagination to make look like one.

"The handle reaches almost down to the ground," said Rollo.

"Down to the horizon, you mean," said Jonas.

"Is that the horizon?" said Rollo.

"Yes," said Jonas, "where the earth and sky meet. Not long ago the Dipper handle was away up there," he continued, pointing up very high.

"Does the Dipper move?" said Rollo.

"Yes, it goes round and round the North Star, all the time. All the stars that are near the North Star keep going round and round it, once every day."

"And the rest of the stars," said Rollo, "do they go round too?"

"Yes," said Jonas; "only they are so far from the North Star, that they go in larger circles, and so go down below the horizon, and are out of sight sometimes. They come up in the east, like the sun, and go over and down in the west. But they don't go over straight," he added. "They don't come right up straight; and so go directly over. They slant away, off to the south, so as to keep always just so far from the North Star."

"That's curious," said Rollo.

"I think it is," said Jonas. "And they all go together; they don't move about among themselves, at all."

"Don't they?" said Rollo.

"No," said Jonas; "only there are a few wandering stars, that keep wandering about among the others. But the rest all keep exactly in their places, and all go round together; so they are called fixed stars."

"Show me one of the wandering stars," said Rollo.

"I don't know which they are," said Jonas, "only they are pretty bright ones."

"I guess that's one," said Rollo, pointing to a pretty bright star in the east.

"Perhaps it is," said Jonas.

"I wish I knew," said Rollo.

"I'll tell you how you can find out," said Jonas.

"How?" asked Rollo.

"Why, when you go into the house, take a piece of paper, and go to the window, and make some dots upon it, for all the stars around that one. Make the dots just in the places that the stars seem to be in. Then let them all go. They will rise more and more, and go overhead, and down in the west, and to-morrow night they will come up in the east again; and then you can look at them again, and see if the bright star has changed its place at all."

Rollo said that he meant to do that; and then he said that he began to feel cold, and wanted to go in. But Jonas told him that he ought to wait and help finish the dial.

So they went to the place which Jonas had selected, and Jonas, looking up first at the North Star, made a hole in the ground, with an iron bar, in an oblique direction, so that the bar should point pretty nearly to the North Star. Then he drove in one of his stakes in the same way. He then made a hole, perpendicularly, directly under the end of this inclined stake, and drove the other stake down into that. The two upper ends of the stakes were now together.

Then Jonas stooped down, so as to bring his eye near the edge of the inclined stake, at the lower end, so that he could "sight" along the edge of it, towards the star. He had previously cut a notch in it, so that he could get his eye down far enough to look directly along the edge. At the same time, Rollo took hold of the upper end, and stood ready to move it either way, as Jonas might direct, until it should point exactly towards the North Star.

"Down," said Jonas.

Then Rollo moved it a little down.

"Down more."

Rollo moved it farther.

"Up-up a little," added Jonas. "There-that will do. Now hold the two stakes firmly together, exactly so."

Then Jonas took some nails, which he had before provided, and nailed the tops of the stakes together, Rollo holding the axe up against them, on the opposite side. This supported the end of the inclined stake firmly, so that it could not move up or down. This was all that the boys wanted to do in the evening, and so they both went in.

The next day, Jonas sawed off the ends of both stakes where they projected beyond the junction; and then Rollo said he would watch the clock all day, and mark the place where the shadow came each hour, and drive a little stake down. "Then," said he, "our dial will be done."

"But what do you suppose is the reason," said Rollo, "that we must make it point to the North Star more than to any other?"

"I don't know," said Jonas, "unless it is because the North Star is the only one that keeps always in the same place. The rest move round and round every day. Those that are far enough from the North Star to go down below the horizon, rise and set; and those that are not far enough, go round and round in circles, in the open sky. But the North Star keeps still."

"Does it?" said Rollo, turning around, and looking up to the part of the heavens where he had seen the star the evening before.

"Yes," said Jonas; "and the reason why we cannot see it now, is the bright daylight. It is up there now, just where it was last night."

"And the Dipper, too?" said Rollo.

"Yes, and the Dipper, too; only that has moved half round, I suppose, and is now away up above the North Star."

"I wish I could see it," said Rollo. And he looked as steadily and intently into the clear blue sky, as he could; but he could not possibly see the least sign of a star.

* * * * *

However, the sun shone bright, and it cast a strong shadow from the stakes which they had driven into the ground. Jonas soon went away to his work, and left Rollo to mark the hours by means of the clock.

So Rollo had to go into the house very often to see what time it was; and at last his father, who was sitting there at his writing, asked him what made him want to see the clock so much. Rollo told him the reason. So his father put down his pen, and came out to see the dial.

When he saw the two stakes, with their lower ends driven into the ground, and the upper ends nailed firmly together, he looked at them with a smile, but did not say any thing.

"Will that do?" said Rollo, looking up very eagerly into his father's face.

His father did not answer, but continued to examine the work on all sides, with a countenance expressive of curiosity and pleasure.

"It points to the North Star, exactly," added Rollo. "Jonas sighted it."

"Yes," said his father; "I think that will do; you have got quite a respectable gnomon."

"Gnomon?" said Rollo.

"Yes," said his father; "we call such a thing a gnomon. In common dials, they are made of brass; but I don't see why this won't do very well. It is rather a large gnomon."

"Is it?" said Rollo.

"Yes," said his father, "I think it is the biggest gnomon I ever saw.

"But how are you going to mark the hour lines, Rollo?" asked his father.

"Why, we are going to drive little stakes down into the ground."

"'Seems to me that you can contrive some better plan than that," said his father.

"Why?" said Rollo. "Is not that a good plan?"

"Not very good," he replied; "because you cannot be exact in driving down stakes. The beauty of a dial is its exactness. I should think that you would do better to put a board down upon the ground, and mark your lines upon that."

"O, the board would get knocked about," said Rollo.

"I dare say that Jonas would contrive some way to keep it steady."

"But he says he can't do any thing more about the dial to-day, for he must attend to his work."

"Let me see-he is putting the harnesses in order, I believe."

"Yes, sir," said Rollo.

"Well, you may tell him that after he has done the harness that he is at work upon now, he may finish his dial."

Then Rollo's father went into the house, and away went Rollo in pursuit of Jonas.

Jonas liked the plan of putting a board down very much, and in a short time he went to work to do it. He planed out a board of the right length, and then put it down upon the ground, under and between the two stakes, but nearest to the upright one. They placed it across at right angles to the line between the stakes, and of course, as the stakes were in a north and south line, the board was in an east and west line, and so the shadows were cast exactly across it.

The board being planed smooth, the edge of the shadow could be seen much more distinctly upon it, than upon the ground; so Jonas was satisfied that it would be a great deal better to draw the hour lines upon the board. After having determined upon the place where it was to go, he took it up again, and then drove down two strong but short stakes, sawed off square at the top, into the ground, one on each side; so that they should come under the two ends of the board. Then he laid the board down again upon the stakes, and nailed the ends of the board to them. The stakes had been driven in until they were just level with the surface of the ground, and so the board seemed to be lying along upon the ground too, though it was, in fact, fastened securely to the short stakes. Then the boys marked the hour lines upon the board with some black paint; and thus they had a very respectable dial. When the sun shone, Rollo could tell what o'clock it was near enough for all his purposes.

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