The periscope was merely a tube in which there were arranged mirrors so that anything reflected in the first mirror, the one above the surface of the water, was again reflected till it showed in a mirror at the bottom of the tube, within the hull of the vessel, where its commander could observe it safely. A crew of about twenty-five men was necessary to operate one of these crafts, and theirs was an unpleasant duty, first because of the danger that accompanied each submergence of their vessel; second because of the discomforts aboard. The explosive engines which drove the craft, whether burning oil or the lighter refinements such as gasoline, gave off gases that caused headaches and throbbing across the forehead; and it was almost impossible to heat the interior of the craft. Though merchantmen had gone to the bottom as victims of German submarines before the proclamation of a "war zone" was issued they were individual cases; the first instance of a merchant ship being sunk as a result of the new policy of the German admiralty was the sinking of the British steamer _Cambark_ on the 20th of February, 1915. This ship was bound for Liverpool, from Huelva, Spain. While off the north coast of Wales, on the morning of the 20th, the periscope of a hostile submarine was sighted only 200 yards ahead. The engines of the steamship were immediately reversed, but she had no time to make off, for a torpedo caught her amidships and she started to sink immediately. Her crew managed to get off in small boats, but all of their personal belongings were lost.