“Oh, I see it now,” said Tom. “The card divides the chimney into two. The air, heated by the candle flame, rises up one side of the card, and the cold air goes down the other side to supply its place, drawing the smoke with it. The candle does not go out now, because it gets a constant stream of fresh air.” “Are the balloons, which are large enough to carry people, fire-balloons?” asked Dolly. “No, my dear,” replied Uncle George. “They are filled with a gas that is lighter—very much lighter—than the air. They rise up easily, and can carry quite a heavy load.” Questions and Exercises. 1. Take an empty bottle and heat it gently. Now turn it upside down, and place the neck of it in cold water. Why does the water rise in the bottle? 2. How is a _draught_ caused? 3. Why does a fire burn more brightly on a cold, frosty day, than on a warm summer’s day? IX.—THE GULL. Frank, Tom, and Dolly had never seen the sea. Long ago Uncle George had told them that he would take them, and at last the time came for them to go. It was afternoon when they reached the little sea-side village where they were to spend a few days. The children were filled with wonder when they saw the great, restless ocean. They watched the waves breaking into white surf, and gathered sea-weeds and shells on the sandy beach. By and by they all sat down to rest and to watch the gulls, for there were many here-about. “We have seen birds like these before, have we not?” said Uncle George. “Yes,” said Tom, “we see them at home in the fields at spring-time. But are these the same kind of gulls?” “Yes, Tom, they are. There are several kinds of gulls. Here we have just two kinds. The _common gull_ and the _tern_. You can tell the one from the other even in the distance by their mode of flying. “Do you see that one flying rapidly over the sea? Every now and then he swoops down to the water and skims along, almost touching it with the tips of his long, pointed wings. Do you know any other bird that flies like that?” [Illustration: Gull flying.] “The swallow flies over the lake at home something like that,” said Frank. “That is so,” said Uncle George. “The tern is very like the swallow both in shape and in his manner of flight. He differs, however, in size and colour. He is often called the ‘sea-swallow,’ and is the smallest of the gull family. “Your swallows at home catch flies and other insects as they skim through the air. The ‘sea-swallows’ live on fish. Every time they dart down they seize and swallow a little fish. But it is done so quickly that you cannot see it.