30-Mile Point Lighthouse was built in 1875 and first lit on April 27, 1876, to protect ships on Lake Ontario from a shallow sandbar extending from 30-Mile Point, near which a number of historic shipwrecks had occurred before the lighthouse was constructed. The United States Lighthouse Service operated it until July 1, 1939, when the United States Coast Guard took over administration of America's lighthouses. Its third-order fresnel lens magnified a lard oil or kerosene flame so it could be seen up to 16 miles away on a clear night, and the one-ton, six-foot-tall lens was slowly rotated by counterweights and cables so the light flashed periodically. In the early 1930s, municipal electricity reached 30-Mile Point, and the tower's kerosene burner was replaced by a 500-watt lightbulb while the fresnel lens continued to magnify the light. The keeper's most important duty was to ensure the light remained lit; the keeper and assistant alternated 24-hour shifts, checked the light every three hours, made a weather log every four hours, and also wound the lens's clockwork mechanism, filled the oil reservoir, trimmed the wick, and polished the lens. As the northernmost point of Lake Ontario's south shore, 30-Mile Point was an important landmark for navigation, and like all lighthouses, it had a distinctive characteristic by which seamen could identify it, including the light's color, the frequency of its flash, and any fog signal.