The evolution of the lighthouse tower
Lighthouses come in many shapes and sizes. This evolution has taken centuries and was influenced by technological change. Twelve lighthouses were built in the United States before the Constitution transferred lighthouse control from the states to the federal government. No two were constructed from the same set of plans and all were built from local materials. Therefore, it is no surprise that no two were alike. Examination of the one which remains one which remains, Sandy Hook, New Jersey, and the evidence of those which have not survived, reveals that these did share some common features.
These early lighthouses were constructed of wood or stone. Those built of wood eventually fell victim to fire. The stone towers were built simply by piling one stone on top of another. These were held together by mortar. Since the walls contained no additional support, such as reinforcing rods, they had to be tapered as they rose. This enabled the base to support the ever-increasing weight prevented the tower from becoming unstable. Therefore, the higher they wished to build the light, the thicker the tower had to be at the base.
Given the flat nature of East Coast geography, where all pre-federal lights were built, there was a need to build a lighthouse as tall as possible. Since colonial lights peaked at about 90 feet, we may assume that this was the maximum practical high for a tower built during this era. It is surprising how little we know about lighthouses of this era. We have almost no construction details. We do have the Sandy Hook tower to study, but there are still many unanswered questions.
Lighthouses built between 1789 and 1820 deserve close attention. In 1789 the federal government took over responsibility from the states and in 1820, the Fifth Auditor of the Treasury became responsible for lighthouses. At first, President George Washington took an active interest in lighthouses. It was not too long before the President had to delegate this responsibility. First, the responsibility passed to the Commissioner of the Revenue and then it went to the Secretary of the Treasury. For quite some time, Albert Gallatin, the Secretary from 1801 until 1814, played an active role in lighthouse administration. Hardly a year went by when one or two new lighthouses were not appropriated.
During the later days of this era, cut stones were used for the first time. This permitted the construction of taller and stronger towers because the weight could be more evenly distributed. Of the first forty towers constructed, only a few survive today. These include the abandoned tower at Cape Henry, Virginia; Portland Head, Maine; and Georgetown Harbor, South Carolina. Most of the remaining lights built during the 1789-1820 era were no longer standing during the U.S. Civil War
A villain has emerged from the Camelot-like history of the early days of lighthouse construction, the Fifth Auditor of the Treasury. From 1820 through 1852, he was responsible for lighthouse administration. Having become too large to remain a direct responsibility of the Secretary, it was assigned to this subordinate. The Fifth Auditor of the Treasury was a financial zealot who proudly returned funds appropriated for the construction and repair of lighthouses to the Treasury unspent. He was a lighthouse novice when assigned the task and did little to improve his knowledge of lighthouse technology during his 32-year tenure. This period, which began in 1820, might well be labeled the era of "the lowest bidder." The lighthouses built during this period were inferior structures constantly in need of replacement. Furthermore, the lighting system used was grossly inferior to, but far less expensive than, those employed in Europe.
Not surprisingly, there are few examples of the Fifth Auditor lighthouses which have survived today. The evolution of the Matinicus Rock Light, however, is a good example of the fate of most constructed during the tenure of the Fifth Auditor. Three twin lighthouses were constructed at this site, the first in 1827, the second in 1848, and the third ten years later. The first two sets of towers were far too short and too close together. Adding to the Fifth Auditors problems was the United States addition of territories where the massive stone tower, had little value. For example, of the 40 or so brick towers built in the South, at least 25 either blew over or sank into the soft ground. In 1852, a special committee, appointed by Congress, conducted an investigation into the lighthouse system and concluded that it was grossly inadequate. A new era in American lighthouse construction was ushered in and the next few decades were the most dynamic period in American lighthouse construction. Although more attention has been given to technologys influence upon lighthouse construction during this period, the administrative change from the Fifth Auditor to the newly-created Lighthouse Board was more influential. The advances in technology had already taken place, they only needed to be applied to lighthouse construction.
The wave-swept lighthouse probably conforms to our mental image of a classical lighthouse more than any other two. The three most famous lights are Minots Ledge, Massachusetts; St. Georges Reef, California; and Mile Rock, California. The first of these is probably the greatest engineering achievement of lighthouse construction given the difficulty of the task and the technology available.
The first attempt to put a lighthouse on Minots Ledge was during the closing days of the Fifth Auditor. This iron tower on stilts was destroyed by a storm a little over one year after completion. The legs held secure to the ledge in which they had been embedded; but they broke off. The second Minots Ledge structure was completed in 1860. A good part of its foundation is underwater. Work on preparing the ledge on which it sits took 3 years before the first course of stones were laid. The granite blocks are dove-tailed together and bonded vertically by bolts. The first 40 feet of the tower is solid stone. This tower is 97 feet tall and has been topped by a wave at least twice.
The protected, screw-pile lighthouse was introduced into the United States in 1850 during the closing days of the Fifth Auditors reign. A protected, screw-pile lighthouse was typically a lightweight, wooden tower on iron stilts, the legs of which are tipped with cork-screw-like flanges. These legs are turned into the soft ground of protected waters, such as bays and sounds. This new type of lighthouse was dependent upon the development of wrought-iron columns for the legs and cast-iron for the screw-like flanges. This technology permitted the construction of lighthouses on sites too soft to support the weight of a heavy tower. The first screw-pile lighthouse had been built in England. This technology was introduced in America at Brandywine Shoals in the Delaware Bay in 1850. Within a few decades, perhaps as many as 100 protected screw-pile lighthouses were built throughout the United States, principally in the Carolina sounds and Chesapeake Bay. But they could also be found in the Gulf of Mexico and one was even built in the Great Lakes at Maumee Bay, but survived only a short time. This type of structure was particularly suited to slow moving, shallow water which was not subject to freezing. The principal enemies of this type of structure were fast-flowing water, ice, and fire. The screw-pile lighthouse design for exposed sites evolved two years after its less complex cousin, the type built for protected waters.
The exposed, screw-pile lighthouse was designed for and used in the Florida coral reefs. This structure varied in two important ways from the style designed for the protected bays and sounds. First, the lighthouse structure was a tall, skeleton iron tower and not a squat wooden structure. The screw-pile lighthouse in the bays and sounds did not need to project their lights more than a few miles so the height of the lens was not a major concern. But this was not for the Florida reef lights. These structures were to be major coastal lighthouses with first order lenses weighing a number of tons. Therefore, they had to be tall structures. The second major difference was in the screw flange. Large, iron-foot plates were added above the screw tip in order to diffuse the pressure caused the weight of the tower. Six screw-pile lighthouses for exposed sites were constructed in the Florida reefs, three before the Civil War and three afterward. Also, one light was built in the Gulf of Mexico at Ship Shoal, Louisiana, in 1858. Those built before the war are much more simple in appearance than those built two decades later.
The new Lighthouse Board of 1852 began the construction of brick towers of increasing height. By 1859 nine brick towers over 160 feet had been built. Six more were constructed after the Civil War. These towers are conical shape, except for Cape Romain, South Carolina, which is six-sided. None of the towers which they replaced topped 100 feet. One would suspect that some engineering or technological breakthrough had occurred in the 1850s which allowed over a 60-foot increase in the height of a coastal tower. This does not seem to be the case. Apparently, the walls of these towers are solid brick without reinforcements. This construction technique had existed long before the 1850s. There are a number of possible answers as to why the towers grew in the 1850s. The Fresnel lens was replacing the much less efficient reflector system; the new Lighthouse Board was willing to spend more on the construction of towers than the Fifth Auditor; and advancements in the science of mathematics allowed engineers to more accurately calculate stress factors. All but one of the 15 towers remain standing, although erosion makes the fate of a number of the others precarious. All 15 towers were built along the Atlantic Coast, as far north as Fire Island, New York, and as far south to Dry Tortugas, Florida The tallest of these, Cape Hatteras at 193 feet, is the tallest lighthouse constructed in the United States.
The use of cast iron in lighthouse construction probably began in the early 1840s. In 1844, a cast iron tower was built on Long Island Head in Boston Harbor. The 1848 tower at Brandvwine Shoals, the first screw pile lighthouse in the United States, had a cast iron tower. Also, the three Florida reef lights and the first Minots Ledge lighthouse. all built in the early 1850s, incorporated much cast iron in their construction. The advantages of cast iron were that it was light when compared to stone and brick, it was inexpensive, it was strong, it was water tight and it had a slow rate of deterioration. A number of cast iron towers were lined with brick for additional stability and insulation. By the close of the 19th century, cast iron towers proliferated throughout the United States. The tallest cast iron tower was the Cape Henry light built in 1881; it is 165 feet and is still standing.
Crib foundation construction was used extensively on the Great Lakes. Wooden cribs were constructed ashore, towed to the site, and filled with stone. Once the crib had settled to the bottom, it was capped with concrete or other masonry. Frequently it was necessary to level the structure by adding weight to one side or another. From an engineering viewpoint, the two most significant crib lighthouses in the Great Lakes are Spectacle Reef and Standard Rocks. The first was completed in 1874 and the second 8 years later. Spectacle Reef is 10 miles from the nearest land and Standard Rocks is 23. Each required a number of years to complete.
Cofferdam construction was used where it was desired to build the foundation on a dry site and where it was not necessary to penetrate the seabed to any great depth. This method could only be used in very shallow water. The wooden walls of the cofferdam were constructed ashore, taken to the site, assembled into a box in the water, bolted together and sealed, and the water pumped. Workmen then entered this open-topped structure and prepared the foundation for the lighthouse.
Three lighthouses were constructed upon man-made foundations built upon underwater ledges, stone by stone without the use of a crib. They were Stratford Shoal, New York; Race Rock, New York; and New London Ledge, Connecticut. These sites were exposed to strong currents which these foundations had to absorb. These lighthouses are characterized by a typical light tower, which might be found at any shore site, placed on top of a massive stone foundation designed to absorb the fury of the waves. However, the tower itself was not designed to sustain the force of waves crashing against it, as is the case for the wave-swept structure. Each of these foundations was very expensive to build. This fact, coupled with the development of the submarine site lighthouse, probably accounts for the fact that only three of this type were built.
Cast iron 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 its vulnerability to swift currents and ice. The traditional method of preparing a foundation on an underwater site in northern waters was by laying and interlocking a bed of large stones weighing from three to five tons each. But this method was time consuming and expensive. The 10,000 ton foundation at Race Rock, New York, took five years to prepare and cost a quarter of a million of 1875 dollars. And the lighthouse still needed to be built. By comparison, a hollow cast-iron shell could be sunk to the seabed in water up to 30 feet and filled with sand, rock, or concrete. Cast iron was selected because of its ability to resist corrosion in salt water. A lighthouse, typically of cast iron, was then placed on top of the caisson although other materials were also used for the tower. By the 1870s these caisson lighthouses had proliferated through the northern waters of the nation. Approximately 50 caisson lighthouses were built.
Most cast-iron caisson bases were simply lowered to the seabed and filled with concrete. However, sites where the seabed was uneven, unusually soft, or exposed to strong currents and waves, required special preparation. For these lights, known as submarine site lighthouses, a caisson containing a double bottom was sunk in position. Once stood upright, air was pumped into the caisson, forcing the seawater out through the bottom. Workmen then entered the top of the caisson through an air-lock system and prepared the foundation. The bottom rim of the caisson acted as a cutting edge which settled into the seabed as workmen excavated sand and mud from inside the caisson. Water was kept from seeping under the edge and into the work chamber by air pressure. The excavated soil was hauled out or sucked out through an airtight shaft. The workmen might sink the cast iron caisson structure as much as 33 feet below the seabed.
The first submarine, or pneumatic site lighthouse, was constructed at Fourteen Foot Bank, Delaware, in 1887. Eleven submarine site lighthouses were built. The most exposed is Sabine Bank, Louisiana, which is 15 miles off the shore in the Gulf of Mexico.
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 keepers quarters to the tower. The majority of breakwater lighthouses were constructed in the Great Lakes.
The introduction of reinforced concrete once again changed the direction of lighthouse construction. This material was in many ways superior to iron and steel. It was cheaper and required much less maintenance. Also, it was extremely strong. Many lighthouses built in places susceptible to earthquakes were made of reinforced concrete. Therefore, most major concrete towers are found on the West Coast.
The first reinforced concrete tower was built at Point Areas, California, in 1910. It is 115 feet, one of the two tallest towers on the West Coast. The tallest reinforced concrete tower is Navassa in the West Indies. This tower is 150 feet. The Brandywine Shoal Lighthouse, Delaware (1914), was the first combination of a reinforced concrete tower on a caisson base. A series of reinforced concrete towers of art-deco design were constructed in Alaska during the 1920s and 1930s. One of these, Scotch Cap, was destroyed by a tidal wave in 1946; five lives were lost. The newest reinforced concrete tower is Oak Island, North Carolina. This 169-foot tower was completed in 1958. This silo-style tower was erected by using a Swedish-developed moving slip-form method. Concrete was poured and once that section dried, the form was moved up and another section was poured. The color is integrated into the concrete. The lantern room is aluminum.
Aluminum had been introduced into lighthouse construction following World War II, primarily in the lantern room area. The Charleston tower completed in 1960 was the first structure where aluminum was extensively used in the construction of the tower. The skeleton of this 140-foot tower is made of high-strength steel and the panels are aluminum. The tower is designed to withstand winds up to 160 mph and it is the only lighthouse with an elevator.
World War II technology permitted lighthouses to be built at locations previously served by lightships. These offshore light structures are based upon technology developed in the oil industry. Their legs are driven 170 feet into the seabed. The towers are designed to withstand 65-foot seas and 125-mile-per-hour winds. Seven towers of this type have been built.
These were the major evolutions in lighthouse towers.
Information acquired from the The USCG Historians' Office Home Page article entitled "Lighthouses: Then and Now"
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