"It's the sun," answered Donald from the floor, where he was playing with his great St. Bernard dog, Barri. "You know it rises earlier and sets later every day now than it did a while ago. It's hotter too." "It goes higher at noon," said Frank. "In the middle of summer it is almost straight over our heads, and in the winter it seems ever so much farther to the south. I've often noticed that." "So have I," said Donald. "And in the winter the shadows are longer than they are in summer. It must be because the sun isn't so high up." "Aren't shadows funny?" said Susie. "One day when I was coming in to dinner, just for fun I tried to walk on my shadow, and I could step on my head." "I've done that lots of times," said Donald. "But it's a strange thing. Sometimes I can step clear over my head--I mean in the shadow--and then again I have to step on it." "And when you jump," said Susie, "it spoils it. The shadow always jumps too." "What kind of weather was it when you had to jump to it?" asked Uncle Robert. "I don't remember," said Donald. "Would the weather make any difference?" "I remember," said Susie, "because one time when I was jumping that way I fell down and was almost buried in the snow. "Then it was winter, wasn't it?" asked Uncle Robert. "It must have been," said Frank. [Illustration: Shadow stick.] "And since you told us that the shadows at noon tell why it is warmer in summer than in winter I've been watching them. They get shorter all the time." "How would you like to measure the shadows every day," said Uncle Robert, "and see if you can find out when they are shortest and when they are longest?" "How can we?" asked Susie. "Shadows are so queer." "Yes," said Uncle Robert, "shadows are queer, but, if we take one that doesn't jump as yours does, don't you think we can measure it?" "Of course we can," said Frank. "We can use the house. That always stands still." "The house might do," said Uncle Robert; "but wouldn't it be better to have a shadow stick?" "Where can we get one?" asked Donald. "What is it made of?" asked Frank. "It is like this," said Uncle Robert, taking paper and pencil from his pocket. "There is one long piece of board, and one short one nailed to the end--so," drawing it on the paper. "Oh, that's easy enough made," said Donald. "We can do it ourselves right here in the tool house." "Let's make it to-morrow, Don," said Frank. "It must be set up some place with the upright end turned toward the south, so that just at noon the shadow of the short piece may fall straight on the board. By drawing a line across the board at the top of the shadow and marking the date on it, we can tell how the length of the shadow changes." "Uncle," asked Donald, "when it is winter here, is it summer in some other part of the world?" "Yes," was the reply, "and now that our summer is coming, the people there are beginning to have winter."