[Illustration: THE PLANET MARS.] "I know it is old," replied his sister, "because the older a planet is, the smaller are the seas and lakes and the amount of water on its surface. As the planet gets older and older, the water disappears, until not a drop is left. But there are wonderful canals all over Mars, and if there were boats up there, you could go all over Mars by means of these canals. When Mr. Lowell looked at Mars through his fine telescope, he not only saw the canals, but round spots where the canals meet." "Perhaps the spots are landing-places where the captains take new passengers aboard," said Harry earnestly. "Perhaps, Harry," said his sister, laughing; "that is, if there are any people on Mars, and captains and boats. How you would enjoy going in a yacht up and down these canals, seeing the lovely flowers and scenery on Mars, for I am sure it must be a very beautiful little world. "It is not quite as bright on Mars as it is here, since it is farther away from the sun and only gets one-half as much light and heat. The year is also nearly twice as long and lasts six hundred and eighty-seven days, instead of only three hundred and sixty-five. Therefore, the summer season is nearly twice as long, but not nearly as warm as here." "Then the winter must be twice as long and much colder than here," Harry said. "I do not think I should like that. But perhaps the canals freeze over in the winter time, and there may be fine skating up there?" [Illustration: CANALS OF MARS (LOWELL).] "No, the canals disappear altogether during the winter time," replied Mary; "or, rather, we cannot see them until they reappear again as faint dark lines in the spring-time. They get wider and wider until the summer season, then they get narrow again and disappear. Some of them are double, but the double lines we see may mean only grass and ferns on each side of a large canal fifty miles wide. When the canals double, the little round spots at the junctions of the canals darken. Perhaps these spots are like little islands in a desert, and they are covered with grass during the summer time." "I should like to live on one of those little islands," said Harry. "Wouldn't you, Nellie?" "If I could take my dollie with me," she replied, as she gazed at it tenderly. "And we might go for little boat-rides all around the islands. Do you think there are any little girls on Mars who have beautiful dollies like mine?" "I really do not know," replied Mary; "but if there are any people living on Mars, I do know they are not like us. We could not live there, as there is not enough air for us to breathe. We would gasp just as that poor fish did the other day, when Uncle Robert hauled it up out of the lake and threw it into the boat. We must have air, and plenty of it, if we want to live." "So we could not live on Mars, could we, sister?" said Harry.