To be invisible, a ship must reflect as much light and the same shade of light as do its surroundings. If it is seen against the background of the sea, it must be of a bluish or a greenish tint, but a submarine lies so low in the water that any object seen at a distance is silhouetted against the sky, and so the ship must have a coat of paint that will reflect the same colors as does the sky. Now, the sky may be of almost any color of the rainbow, depending upon the position of the sun and the amount of vapor or dust in the air. Fortunately in the North Sea and the waters about the British Isles, where most of the submarine attacks took place, the weather is hazy most of the time, and the ship had to be painted of such a color that it would reflect the same light as that reflected by a hazy sky. With a background of haze and more or less haze between the ship and the periscope of the U-boat, it was not a very difficult matter to paint a ship so that it would be invisible six or seven miles away. One shade of gray was used to conceal a ship in the North Sea and an entirely different shade was used for the brighter skies of the Mediterranean. [Illustration: (C) International Film A Giant Gun Concealed Among Trees Behind the French Lines] In this way, the artists made it possible for ships to sail in safety much nearer the pursuer who was trying to find them, and by just so much they reduced his powers of destruction. But still the odds were too heavy against the merchantman. Something must be done for him when he found himself within the seven-mile danger-zone. Here again the artists came to the rescue. [Illustration: (C) Committee on Public Information Observing the Enemy from a Papier-Mâché Replica of a Dead Horse] Before merchant ships were armed, a submarine would not waste a torpedo on them, but would pound them into submission with shell. Even after ships were provided with guns, submarines mounted heavier guns and unless a ship was speedy enough to show a clean pair of heels, the pursuing U-boat would stand off out of range of the ship's guns and pour a deadly fire into it. But the ships, too, mounted larger guns and the submarines had to fall back upon their torpedoes. GETTING THE RANGE FOR THE TORPEDO