“Could not we get Jonas to drive a tall stake down?” said Lucy.
“Yes,” said Rollo; “I suppose so.”
The children went out into the garden to find a good smooth place, and while they were walking about there, Rollo’s mother came out, and they told her the whole story. She seemed quite interested in the plan, and told them of a better way than any that they had thought of.
“You see,” said she, “that the height of the stake or pole that makes the shadow is not material; for the shadow of a small one will vary just as much, in proportion to its length, as that of a long one will. So, instead of taking a wooden stake, out of doors, you might take a large pin, and drive it down a little way into the window sill, in the house. Then you can mark the shadow with a pen, very exactly.”
“So we can,” said Lucy, clapping her hands.
“And you might put a piece of white paper, or a card down first,” continued Rollo’s mother, “and drive the pin through that, and then mark the places where the end of the shadow comes every day, directly on the card, with a fine pen. Thus you could be a great deal more exact than you can in chalking upon a barn floor.”
Rollo asked his mother if she would not be kind enough to help them fix their apparatus; but she said she would give them particular directions, though she should prefer letting them do the whole themselves, and then, if they met with any difficulties, they might come and report them to her, and she would tell them how to surmount them. So she recommended to them to go and find a blank card, or piece of white pasteboard, or of stiff white paper, as big as a common card. “Then,” said she, “choose some window where the sun shines in at noon, and put the card down upon the sill, and drive the pin down through it. But you must not drive the pin through the middle of the card, for the shadow will always be off to the north of the pin, and therefore the pin may be pretty near the south end of the card. Then the shadow will be more likely to come wholly upon the card, even when it is longest. You had better place the card in such a position, too, that its sides shall lie in the direction of north and south. Then the shadow at noon will lie along exactly in the middle of it. You must get a large and stout pin, too; and drive it in firmly, a little way, with a small hammer. It will be well, too, to drive another smaller pin into the other end of the card, so as to keep it fixed in its north and south position.”
“How can we know when it is north and south, exactly?” said Lucy.
“You cannot do it exactly,” said Rollo’s mother; “but you can get it pretty near. One way is to borrow father’s little compass, and adjust it by that. Another way is to see when it is exactly twelve o’clock by the clock, and then the shadow of the pin will of itself be about north.
“Then you might move the north end of the card until the shadow is brought exactly into the middle of the card, and then put the other pin in, and fix it in that place. Then if you make a mark along where the shadow comes, that mark will be a north and south line, and you can mark the place where the shadow of the pin’s head crosses that line, when it crosses it every day at noon.”