Among the many terrible things used for the first time in the great World War, the airplanes and submarines attracted the most notice. On February 4, 1915, the German government proclaimed a war zone about the British Isles and declared its intention of sinking without warning any enemy merchant ships found within this zone. On May 1, 1915, the Lusitania was sunk. On February 1, 1917, Germany began her "ruthless submarine warfare". Immediately, friendly relations were broken off and on April 6, 1917, the United States declared war. At first it seemed as if the Germans would win. Then it was found that the submarines could be seen from airplanes directly above ; also very swift tropedo boats, destroyers, were able to drive them away. As a matter of fact, not one U. S. transport was lost on its way to Europe, and but three on the way home. The submarine is the weakest, most helpless of fighting craft. It cannot fight under the rules of warfare laid down by international law. Its only safety lies in swiftness and surprise, All the great nations now have submarines. They are here to stay. A submarine may travel on the surface or under the water. It has a system of engines for surface running and for charging storage batteries. These storage batteries are the motive power when submerged. Notice the tall periscopes. In each one is a vertical system of lenses and prisms by which the observer down below is able to see on every side. There is also a sound detector which indicates the approach and motion of a ship.
Keystone View Company, “Submarines in the foreground and battleships and torpedo boats in the background, San Diego Bay, California,” Monash Collections Online, accessed September 21, 2020, http://repository.monash.edu/items/show/25527