1797, he was appointed engineer to the newly formed Northern Lighthouse Board, the lighthouse authority for Scotland and the Isle of Man. His greatest achievement was the construction of the Bell Rock Lighthouse in 1810, one of the most impressive feats of engineering of the age. This structure was based upon the design of the earlier Eddystone Lighthouse by John Smeaton, but with several improved features, such as the incorporation of rotating lights, alternating between red and white. Stevenson worked for the Northern Lighthouse Board for nearly fifty years during which time he designed and oversaw the construction and later improvement of numerous lighthouses. He innovated in the choice of light sources, mountings, reflector design, the use of Fresnel lenses, and in rotation and shuttering systems providing lighthouses with individual signatures allowing them to be identified by seafarers. He also invented the movable jib and the balance crane as a necessary part for lighthouse construction. Stevenson established a great dynasty of engineers specializing in lighthouse construction - his descendants were responsible for most of the lighthouse construction in Scotland for a century. Three of Robert's sons followed in his path: David, Alan, and Thomas. Two of David's children, David Alan and Charles Alexander also became distinguished lighthouse engineers in their own right. Alexander Mitchell designed the first screw-pile lighthouse - his lighthouse was built on piles that were screwed into the sandy or muddy seabed. Construction of his design began in 1838 at the mouth of the Thames and was known as the Maplin Sands lighthouse, and first lit in 1841. However, though its construction began later, the Wyre Light in Fleetwood, Lancashire, was the first to be lit (in 1840). Lighthouses in America The first lighthouse in America was the Boston Light, built in 1716 at Boston Harbor. Lighthouses were soon built along the marshy coast lines from Delaware to North Carolina, where navigation was difficult and treacherous. These were generally made of wood, as it was readily available. Due to the fire hazard, masonry towers were increasingly built - the oldest standing masonry tower was Sandy Hook Lighthouse, built in 1764 in New Jersey. Screw-pile lighthouses were used in the Chesapeake Bay and along the Carolina coast in the United States. The first screw pile light in the United States was Brandywine Shoal in the Delaware Bay. They became especially popular after the Civil War when the Lighthouse