Again Mary brought out her little note-book, and, with rather a look of surprise, she said: "Supposing the cannon-ball went as fast as it could go, it would take nine years to reach the sun, and the sound of the explosion would reach there in fourteen years. The cannon-ball would come along first, and five years afterward, if you were living on the sun, you would hear the sound made when the cannon was fired off. "It takes time for me to walk from the garden to the house, so it takes time for sound to travel from the earth to the sky; and sound travels only one-fifth of a mile in a second. Do you remember the thunderstorm the other day, Harry, that frightened you so?" "I shall never forget it," said Harry, trembling at the thought. "You said, 'Count slowly'; and I counted one, two, three, four, five, up to fifteen." "Then I said: 'Don't be afraid, brother; the storm is three miles away.'" "Yes, I remember," said Harry; "and I thought you were very clever, and wondered how you knew." "It was not so wonderful, after all, was it?" said Mary, laughing. "Now tell me, sister," said Harry. "Supposing I had a very long arm, and stretched it out toward the sun, and touched it with the tip of my little finger. What would happen?" "You would never know that you had burned it, for the pain of burning would be one hundred and fifty years going along your little finger, and down your giant arm nearly ninety-three millions of miles long, before it at last reached your brain. Then it would let you know that one hundred and fifty years before you had burned your little finger." Harry stretched out his little arm in the direction of the sun, and, looking at it critically, laughed at the idea of a giant arm millions of miles long. "It is too short by several inches," said his sister, reading his thoughts, and joining in the laugh. "It would take hundreds and hundreds of little arms as long as yours, would it not? Now what else do you want to know about the sun?" SIZE OF THE SUN. "If you are not very tired, sister," said Harry coaxingly, "I should like to know how large it is. Is it as large as the earth?" [Illustration]