William Ferrel, a mathematician who analyzed the motions of tides for the U.S. Coast and Geodetic Survey, described this machine in 1880. It was completed in 1882 and remained in use for 25 years. The Survey referred to it as the
"Maxima and Minima Tide-Predicting Machine." A tide predictor treats the motion of the tides as a series of components deduced from observed data. Using a series of pulleys, it calculates values representing the motion of the following year’s tides. The times and heights of the high and low tides are read from dials on the face of the instrument.
At the back of the machine is a pulley corresponding to each component of the tide. The pulleys are set on cranks that carry them around eccentrically; the distance of each pulley from its center of motion is proportional to the
amplitude of the corresponding component. Each crank has an index attached to it, which moves on a graduated circle, so that it may be set at the proper
initial position. A fine chain passes over and under each pulley in succession
and communicates the net effect of the combined motion of the pulleys to the
interior of the machine. From there, it is communicated to the front face of the
machine. The main dial on the front has three hands, a long hand, a short hand, and a double or oscillating needle. The long hand moves relative to the short hand at the ratio of the motion of the mean sun to that of the mean moon. The long double hand oscillates about the vertical as a (missing) crank on the left side of the machine is turned, driving the mechanism. When the lunar hand crosses the upper part of the oscillating needle, a high tide is signified. When it crosses the lower part of the needle, a low tide is signified. The time of the tide is indicated by the position of the long or solar hand and the height on vertical scales to the left of the main dial, with one scale for high tides and another for low. The front also has a thermometer to the right of the main dial and four smaller dials with pointers. The instrument is made of wood,
metal, and glass.
The first tide predictor was designed in 1876 by William Thomson, a
physicist in Glasgow. It was composed of two separate instruments. The harmonic analyzer broke down the components of the observed tide data. The tide predictor then used the numbers generated by the harmonic analysis to print out a continuous curve that represented the tide on a long roll of paper. From that curve, the high and low tides could be deduced. Ferrel’s machine computed the sum of the harmonic series itself rather than relying on a separate harmonic analyzer, and calculated the tides from up to 17 observed pieces of data rather than only 10. The tide prediction could be read off the face of the instrument rather than calculated from a curve
Ref: "Harmonic Analyzer" and "Tide Predictor" in Robert Bud and Deborah Warner, eds., Instruments of Science. An Historical Encyclopedia (New York & London, 1998), pp. 304-305 and 621-623.
William Ferrel, "Description of a Maxima and Minima Tide-Predicting Machine," Report of the Superintendent of the U.S. Coast and Geodetic Survey (June, 1883), Appendix No. 10.
