File:Nahant-1A-FCT-Plan.jpg

English: This set of floor plans, from 1944-45, comes from the Report of Completed Works of the U.S. Army Corps of Engineers. It describes the more northerly of the two Coast Artillery fire control towers on Swallow Cave Rd. in Nahant, MA, known as Site 1A.

This tower is fairly typical of some 60 or more of these towers that were built along the Atlantic coast from Maine to Rhode Island during WW2 for the purpose of detecting seaborne and airborne targets and directing coast artillery and antiaircraft fire upon them. They ranged in height from 3 to 10 stories, depending on the heights of the sites at which they were built. Made of poured concrete, they often had unoccupied lower floors below their occupied observation levels. Staircases ran up to their lowest observation level, and wooden ladders then were used to climb to higher observation levels.

This tower held three observation posts. The 6th Floor housed an observation post for the command of the northern guns defending Boston Harbor. The 7th Floor held a base end station (B) and a spotting station (S) for Battery 206, a pair of 6-inch guns on the nearby shore. The 8th Floor was to have held a similar pair of stations for the battery of 16-inch guns at Fort Dawes in Deer Island, but those guns were never installed. Also on the top floor was a sort of concrete "crow's nest" observation post that protruded through the roof and was covered by a trapdoor. An antiaircraft spotter could climb into the this concrete crow's nest with his head and shoulders above the roof line, as part of Boston's WW2 air defenses.

This tower had electric lights, phones, and radio communications, and a time interval bell that was used for coordinating fire control information. Although this tower has a simple, square appearance, some versions of these towers in New England had round or partly octagonal plans. In general, the sort of octagonal pad shown here on the top floor mounted a depression position finder, a massive telescope and rangefinder. The pipe stands shown on Floors 6 and 7 usually held azimuth scopes, which were less complex telescopes that determined bearings to a target but not its range from the tower.

The center of the 8th Floor concrete mounting pad was usually a precisely surveyed point with a geodetic marker embedded in the tower's roof directly overhead. This was done to enable accurate triangulation between towers as part of the fire control process. The other observing instruments on lower floors of the tower were usually lined up directly beneath the 8th Floor mounting pad and the rooftop marker, so they shared the same latitude and longitude.