“Swallows,” exclaimed Ellen, “chimney-swifts they call them. Maybe they are the ghosts.” Mabel opened a door which disclosed a flight of steps leading to the attic, but she closed it quickly. “Don’t go up,” she cried, as something came swooping toward her. “The house has bats in its belfry.” Ellen laughed and turned toward a cupboard whose door she opened. “A discovery! A discovery!” she cried. “Come here, Mabel. See what I’ve found.” Mabel hurried over to the corner where Ellen stood examining something she held in her hand. “What is it? What is it?” Mabel inquired. “A card with a name and a date. Some one has been here this year before us. See.” Mabel took the card and read, “Compliments to the ghost.” Then followed the initials R. M., the date, and the engraved name Robert MacDonald. “What a lark!” she exclaimed. “I wonder who Robert is. He has an imagination, whoever he may be.” “Let’s write something on the back of the card and leave it,” suggested Ellen. “We’ll come back some day and see what happens.” “Done,” agreed Mabel. After consultation they decided to write: “Thanks for compliments. With hopes for a better acquaintance, The Ghost.” “That will whet Robert’s curiosity if he ever comes here again,” declared Mabel. “And it is a sort of adventure for us,” responded Ellen. “I rather hope he will come again, don’t you, Mabel?” “Yes, for it will be sort of exciting for us to follow up the affair. We must make it as mysterious as possible, and never, never let on that we have anything to do with it.” They laid the card back on the dusty shelf and left the gloomy house, laughing and excited in the possession of a secret. The summer cottages were beginning to fill up, guests were arriving at the boarding-houses, consequently there was always a crowd at the post-office when the mail was sorted. The steamboat which brought it had just steamed off when the girls reached the long flight of steps which led to the wharf. They threaded their way through the crowd which was thronging the small store. Most took advantage of the hour to do their marketing, since fresh supplies generally came on the boat, so the boxes and crates received attention until it was time for the little post-office window to be opened. Ellen and Mabel took their places in line. A young man, looking over his shoulder, stepped aside. “Take my place,” he said; “I’m not in a hurry.” He raised his hat and walked off while they moved up, and presently, loaded up with letters, papers, two bottles of milk, and a box of strawberries, they started for home. “That was a nice, polite somebody,” remarked Mabel; “I wonder who he is.” “Robert MacDonald maybe,” returned Ellen laughing. “Wouldn’t it be funny if he were? I suppose we could find out. Would you ask?” “Oh, no, don’t; it would spoil our secret. Let’s keep up the mystery for a while longer. If it should be he, we would feel sort of conscious; and if it isn’t, there is no harm done.”