It was up-hill work for the British to guard thousands of ships over millions of miles against the hidden foe, who sometimes struck without being seen at all. A ship is a small thing on millions of square miles. A slinking submarine is very much lower and harder to see on the surface. A periscope is far harder still. The ordinary periscope is simply a tube, a few inches in diameter, with a mirror in the upper end reflecting the outside view on the corresponding mirror at the lower end, where the captain watches his chance for a shot. No wonder the Germans got on well for so long. It was over two years before British merchantmen were armed. There was a shortage of guns; and the neutral American Government would not allow any armed merchantmen into their ports, though many and many a life was lost because a vessel was unarmed. But, bit by bit, the merchantmen were forced to arm or die like sheep before the German wolves; and once they had a gun they soon learnt how to use it. One gun over the stern was all that most ships had. It was mounted astern because the best chance of escape was to turn away and go full speed, zig-zagging every which way as you went, firing at the chasing submarine; This made vessels harder for submarines to hit, not only on account of the zig-zags, but because the ship, going the same way as the torpedo, made fast and short shots harder to get; also because the backwash of the screw helped to put torpedoes off their course; and finally because the target was itself firing back at the submarine. Even so, however, it was often touch-and-go; and very few people ever enjoyed the fun of being fired at as much as that little Canadian girl of six, who, seeing a torpedo shimmering past the ship's side, called out, "Oh, Mummy, look at the pretty fish!" Once a fast torpedo was hit and exploded by a shell from the vessel its submarine was chasing. But this was a perfect fluke.