Various Types of Lighthouses
The ability to roll iron into large plates revolutionized the construction of lighthouses in northern bays and sounds because it significantly reduced the cost of building a lighthouse foundation in the water to a fraction of what it had been. The screw-pile structure had revolutionized lighthouses in the bays and sounds of southern waters; however, this technology was not applicable in northern waters due to the screw-pile’s vulnerability to swift currents and ice.
The caisson construction method for lighthouses is based on the idea developed by Lawrence Potts, an English physician and inventor, who in 1845 sank a section of hollow tubing from the surface of the ocean to the sea floor. He then attached a powerful pump to the open end extending above the water and as he pumped air and water from the tube it drew up sand which allowed the tube to sink deeper into the sea bottom. The method was then employed in 1850 during the construction of bridge support towers at Rochester, New York. Workmen soon discovered that large rocks obstructed the descent of the tube so the engineer in charge reversed Pott’s process. He pumped air into the tube forcing the water out so his men could descend into the tube and remove the rocks, sand, and mud, allowing the tube to sink under its own weight into the river.
As noted, the expense of preparing a foundation at an underwater site was prohibitively expensive. By comparison a hollow rolled-iron shell could be sunk to the sea-bed in water up to thirty feet and filled with sand, rock, or concrete. Iron was selected because of its ability to resist corrosion in salt water. A lighthouse, typically of iron, was then placed on top of the caisson.
SUBMARINE SITE (PNEUMATIC CAISSONS) LIGHTHOUSES
Most of the caisson bases were simply lowered to the sea-bed and filled with concrete. However, sites where the sea-bed was uneven, unusually soft, or exposed to strong currents and waves required special preparation.
The breakwater lighthouse presented some unique challenges that were not solved until iron was introduced as a building material.
The breakwater lighthouse had to be relatively light in order to avoid stress on the foundation; the structure had to be strong in order to withstand the impact of the waves and vibrations; and the lighthouse had to be compact because of the limited space available for the structure. Frequently, the keepers quarters were in town, because breakwaters were generally too small to attach the keeper’s quarters to the tower.