May 2l.--I am sitting alone in the cabin writing. It is very late. I hear the steps of the mate as he paces the deck. The calm still holds us in its fearful clasp. Great God! What is to be the end of it all? There has been a break in the monotony of our existence to-day. Uncle John got into a hot discussion with Captain Raymond at the dinner table about the efficacy of the wonderful explosive compound. The captain seemed doubtful. Uncle John was for the instant angry. "I'll show you, then," he said, and he rushed into the cabin where his boxes are stored, and came out shortly with two tin cans, each holding something less than a pint. He unscrewed the top of one disclosing a brownish powder. "Take care," said the captain, who seemed needlessly cautious, and almost fearful. "Why, I thought you said it was useless," said Uncle John with a laugh, "and yet you are afraid of it. Look here." He lighted a match and held it close to the powder. A dark smoke arose that instantly extinguished the little flame, and floated off, leaving a queer smell behind. That was all. "Perfectly harmless, captain," continued uncle, who had now recovered his usual good nature. "Perfectly harmless unless you wet it. Then look out." The cook had made a sort of dumpling for dinner, and a great lot of it remained. Uncle John took a mess of this dough, for it was little else, squeezed it until it was quite dry and molded it into a ball. "Come with me," he said, "and, Arthur, bring a plate of that dough with you." He took the cans and we followed him to the deck. There he carefully covered the ball of dough with the powder, and, going to the rail, threw it as far as he could out over the placid sea. As the ball struck the water there was a loud explosion and the spray was thrown high into the air. The crew, who had been hanging over the port rail forward, turned and rushed over to see what was up. Uncle John made another ball and threw it with like result. "Oh, houly torpeter!" growled one of the men, and they turned back to their former places. Uncle John, now evidently anxious to give us thorough proof of the value of his compound, was for throwing more balls, when the boatswain, rolling aft, touched his hat, and said to the captain: "Please, sur, there's a big shark as has showed his fin hoff the port bow, and if so be that the doctor'll wait a bit with his torpeters, we'll show 'im some fun a-catchin' of it." "All right, bo'sun," said the captain, and we all went over to the port rail. "There he is," said the captain, pointing to a sharp, black thing, that, rising just above the water, was cutting quietly through it. "That is his fin, and there's a big shark under it or I'm much mistaken."