"I have repeated all that it is necessary for you to know now," added the commander, as he took an orange from his pocket, and proceeded to show the difference between a great circle and any other. "You learned that all circles, whether great or small, even if no bigger than a nickel, contain three hundred and sixty degrees; and that every degree is divided into sixty parts called miles--geographical miles. These miles are the navigator's knots." "Except the knots, the rest all comes back to me," said the lady. "But I don't understand the difference between a knot and a statute mile, as you call it." "I can't say that I know much of anything about it," added Dr. Hawkes. "A degree contains sixty-nine and a quarter miles, though the fraction varies with different authorities. Now, if you will divide 69.25 statute miles by 60," continued the captain, performing the operation on the back of an envelope, "the result will be 1.154 statute miles to a knot. The sixteen knots of this ship would therefore be nearly eighteen statute miles an hour." "I think I understand it now, Captain Ringgold," said Mrs. Belgrave; and the others said the same. "If we go as fast as that, we shall soon overtake the Maud," suggested Mrs. Woolridge. "Not to-day, madam," replied the commander. "When shall we catch up with her?" "I told Captain Scott not to try to make more than ten knots an hour, which is very good sailing for a steamer of her size. She left at midnight, and is therefore twelve hours, or one hundred and twenty miles ahead of us." "Knots or statute miles?" asked Mrs. Belgrave. "Knots always at sea. We rarely make any use of statute miles. I have directed Mr. Shafter to make his best speed, so that we sail six knots faster than the Maud. Gaining six knots an hour, it will take us twenty hours to overhaul the Maud," said the captain. "I shall expect to see her about eight o'clock to-morrow morning, when we shall be off Magrowa Point." Captain Ringgold invited all the party to his cabin, where the chart of the Mediterranean Sea was spread out on the table. He pointed out Alboran Light to them, with a ring thirty miles in diameter drawn around it. On the northern edge of this circle was a cross, which was connected with Europa Point by a red line. "What is the ring for?" asked Dr. Hawkes. "It indicates the distance from all points at which the light can be seen," replied the commander. "I suppose that red line shows the course we are sailing," added Uncle Moses. "But why is your course alone marked on the chart?" "I marked that myself with a red pencil; it was not printed on the paper, as you seem to suppose. Captain Scott made just such a line on his chart," the captain explained. "But it is not straight," the surgeon objected. "If you should keep on this course you would hit the African coast at Columbi Island," as he read the name from the chart.