Tide-Predicting Machine No. 2, also known as Old Brass Brains, was a special-purpose mechanical computer that uses gears, pulleys, chains, and other mechanical components to compute the height and time of high and low tides for specific locations. The machine can perform tide calculations much faster than a person could do with pencil and paper. The United States Coast and Geodetic Survey put the machine into operation in 1910. It was used until 1965, when it was replaced by an electronic computer.
Early U.S. tide-prediction efforts Tides are the rise and fall of sea levels caused by the combined effects of gravitational forces exerted by the Moon, Sun, and rotation of the Earth. In 1867 the United States Coast Survey started printing annual tide tables to support safe and effective maritime, coastal, and defense activities. Before long, these tables showed the times and heights of high and low tides to the nearest minute and tenth of a foot, respectively. Tables were printed for a year at a time and distributed prior to the start of the year. The prediction of tides is very challenging as it depends on multiple factors–including the alignment of the Sun and Moon, the shape of the coastline, and near-shore bathymetry. Tide theories attempt to account for these factors but lead to complex calculations. Originally, calculations were performed by hand, which was very labor-intensive and error-prone. The burden became even larger when the United States Coast and Geodetic Survey (as the U.S. Coast Survey was renamed in 1878) started using the more accurate harmonic method for predictions of tides in 1884. To significantly reduce the work required to predict tides, in 1881 William Ferrel of the Coast and Geodetic Survey designed a tide-predicting machine. Fauth & Co. Instrument Makers built Tide-Predicting Machine No. 1 and delivered it in 1882. The Survey started using the machine routinely in 1883.
History and mechanism In 1895 the Coast and Geodetic Survey grew concerned because Tide-Predicting Machine No. 1 had developed considerable wear from almost constant use over 12 years. The office decided to construct a new machine that was faster, more accurate, and more reliable. This became Tide-Predicting Machine No. 2. Rolin Harris and E. G. Fischer of the Coast and Geodetic Survey led the effort. The design team studied previous British and U.S. tide-predicting machines and incorporated their best attributes in the design of the new machine. The machine, also known as “Old Brass Brains”, used an intricate arrangement of gears, pulleys, slides, and other components. The design of the new machine was approved in 1895, and construction began in 1896. Tide-Predicting Machine No. 2 was the first tide-predicting machine to incorporate both a paper graph of the tides–the approach used by earlier British machines–and dials and scales that showed the tide height and corresponding date and time–used by Tide-Predicting Machine No. 1. The dials and scales made it much easier for an operator to precisely determine the height and time of high and low tides. The paper graph, referred to as a tide curve, was very useful as a record of the computation that could be checked later to confirm the calculations were performed correctly. A hand crank turned by the operator provides the power for the machine's mechanical calculations. Battery-powered electrical circuits are used to mark the start of hours and days on the paper graph and to stop the machine when high and low tides were reached so the operator can note the height and time.
Much consideration was given to the mechanical characteristics of the components to ensure reliability and accuracy. For instance, some components that were hard to replace were designed with a 50-year lifetime. Also, the summation chains were moved across gears under tension for a year of work days before being installed in the machine to ensure they were sufficiently flexible and their length would remain constant. Other work in the Coast and Geodetic Survey took precedence over construction of the new machine, and a reduction in staff levels precluded all work on the new machine for three years. As a result, Tide-Predicting Machine No. 2 was not functional until 1910. It was first applied to predicting values for the 1912 and 1913 tide tables. Then the machine was disassembled, polished, plated, lacquered, and reassembled in time to provide predictions for the 1914 tide tables. Comparisons of the accuracy of the mechanical predictions of tides compared to hand calculations for two challenging locations demonstrated errors in heights of 0.72 inches (1.83 cm) or less. Old Brass Brains is 10.8 feet (3.3 m) long, 6.2 feet (1.9 m) high, 2 feet (0.6 m) wide and weighs approximately 2,500 pounds (1,134 kg).
Use To compute tides for a coastal location, the operator has to configure the machine for that location. This is done by adjusting physical settings on the machine based on up to 37 factors. Those factors are determined empirically by harmonic analysis of a time series of tides at the location, and represent the influence of the moon, sun, depth of bay, offshore islands, etc. Once computed the factors for a location can be applied to past and future years. and are shared widely so anyone can perform tide calculations.
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