Under ordinary circumstances on board ship the nearer one standing on deck is to the water the more limited will the horizon be, and likewise the higher from the water, as in the "crow's nest," the bigger the horizon. Standing on deck, seventeen feet from the surface of the water, the horizon, on a clear day, would, roughly speaking, be three miles. The same rule practically applies in periscope work, which, as it can extend seventeen feet in the air, will give a three-mile horizon vision. Therefore, according as is the position on the horizon rays, so will the distance away of the object ship be gauged. The course, speed, and distance off being calculated, the U has then to manoeuvre get into position for firing, taking care to avoid the actual track of the approaching ship and to get nearly "broadside on" to her. Hence there is nothing worries them and upsets their calculations more than when the ship alters her course frequently--this manoeuvre being called zigzagging--and which accounts largely for the high percentage of misses registered. The more often the alteration of course takes place, the bigger and more uneven the zigzag, the better for the ship. Daylight is most suitable for torpedo firing, but there is nothing to prevent its being accurate enough during moonlight, even so far as minor work is concerned. As the torpedo can be discharged from the surface as well as when submerged, the precaution of having every light on the ship extinguished is a necessary one. It is remarkable how few collisions at sea have taken place during the war, even though the ships are running without any navigation lights and even during fog no whistle is sounded. The periscope being in those days a "familiar object," it is believed that some of the later U-boats have another device in the shape of a small hatch opening upward which can be opened by the observer on the submarine, enabling him to look around the surface of the water. Ordinary navigation on the U-boat is done on usual lines, i.e., sextant and compass and bearings from landmarks by day and observation by stars and of lighthouses by night, but the reckoning of distances by dead reckoning, that is, on the log, has to be dispensed with. The compass used is a gyroscopic one, so there is no deviation due to magnetic influence, and the courses are all therefore "true courses." There were no signs of any soundings being taken while submerged, but of course they were frequently taken from the surface while approaching land. While on the surface, if the sea was at all rough, the U rolled considerably and the officers, in oilskins, were lashed to the nearest stanchion to prevent being washed overboard. On the for'ard deck on either side of the bow was a small anchor about the same size as used on a 500-ton ship.