"Perhaps you don't know there are mountains in the sea," said the little mermaid. "Of course, you have seen nothing but their tops. What is that little rocky ledge over yonder, where the white lighthouse stands, but the stony top of a hill rising from the bottom of the sea? And what are those pretty green islands, with their clusters of trees and grassy slopes, but the summits of hills lifted out of the water?" "Oh!" said Mary Louise, with a gasp. "You do know geography, don't you? Is it pretty, away down there under the waves?" she added wistfully. The mermaid smiled very sweetly as she answered, "Yes, it is. There are many wonderful things to see, and many strange beautiful things to hear under the sea! I will comb your hair with my magic comb," and she ran the pearly comb gently through Mary Louise's hair. "Over the sea the white ships sail, Out through the mist and the rollicking gale, While deep below the mermaids swim With their finny tails so neat and trim. So please, little magic comb, don't fail To give Mary Louise a mermaid tail." And the more she combed the longer grew the pretty curls, until, to the astonishment of Mary Louise, she found her hair trailing down to her very feet. The breeze suddenly blew it to one side, and there on the sand, instead of her two little shoes, was a mermaid's tail, with a flippy-floppy fin on the end! "Come with me," said the mermaid, and without a moment's hesitation Mary Louise followed her into the water and out beyond the breakers, swimming as easily as if she had always been a little mermaid, instead of a girl who wore tan shoes. "Where are we going?" asked Mary Louise, as the dim line of the shore disappeared and there was nothing in sight but the great, restless ocean. The mermaid did not answer, but looked about intently, as if trying to find something. "What are you looking for?" asked Mary Louise, for she was a curious little girl, and forgot one question as soon as she asked another. "Oh, there it is!" exclaimed the mermaid. "Come with me. Hold your hands out before you like this and dive down!" "But where are we going?" again asked Mary Louise as they sank lower and lower in the sea. "Oh, I forgot," answered the mermaid, turning with a smile to her little companion; "I was so busy looking for the subway entrance that I forgot your question." "Goodness!" cried Mary Louise. "I didn't know there was a subway in the sea!" "To be sure," answered the mermaid. "The track lies along the bottom of the ocean. It's not a railroad train we're going to take, but a water train that comes all the way from the Northern seas, sweeping on like a river in the sea. Wait till we get down there. You'll see how fast it goes." Mary Louise was too astonished to speak. "The Pullman cars," continued the mermaid, "are icebergs. They come from the North every summer to take a trip South." "Whew!" shivered Mary Louise. "I think we ar near one now, for I feel quite cold."