In the early 19th century, the Great Lakes formed the country's most important transportation system, and mariners depended heavily on lighthouses to warn of navigation hazards and mark major land features. At this light, a fourth order Fresnel lens designed by French physicist Augustin Fresnel was mounted at the top of the tower, where a single oil lamp encased in brass projected a warning beam visible up to 13 nautical miles away. Mariners identified it by its characteristic signal of two red and four white flashes, which told them they were seeing the Presque Isle light, commonly known as the flashlight. Throughout the Great Lakes, more than 100 lighthouses stood from Lake Superior east to the headwaters of the St. Lawrence, and early ship masters relied on them as a fundamental aid to navigation. The keeper had to climb the tower at least every 4 hours each night to check the oil supply, clean the lens, and reset the lens rotating mechanism. Responsibility for maintaining the light rests with the U.S. Coast Guard. Modern shipboard electronics have greatly reduced the need for lighthouses, though they remain significant geographic points of reference for small boats. The lighthouse was electrified in the 1920s, and today its light is cast by a 250 watt bulb inside a 300mm plastic lens, displaying a fixed white light on a three-second on, three-second off sequence, with an electronic light sensor serving as the keeper of today.