Probably the first useful application of harmonic analysis was to tidal analyzing and predicting machines by Lord Kelvin, in 1876. The rise and fall of the tides, having been observed at a given port for a year or more, is represented by a curve which is then analyzed. The tidal components being known, it is possible to synthesize these for future dates, that is, to predict the tides. (50)

A tide-predicting machine of remarkable completeness and perfection has recently been constructed by the United States Coast and Geodetic Survey at Washington; Fig. 102 is a general view of the instrument. (51)

The curves being investigated often have few components, or the interest is centered in a few components; in such cases simpler forms of analyzers may be used in which the integrating is performed by a planimeter of the ordinary type. Figure 106 shows an instrument of this kind designed by Chubb. (53)