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.