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SHIDY. 
the five tropic ranges, the tropic high and low water ine¬ 
qualities, and the percentage of reversals which may occur 
in the sequence of tides. Criteria for the classification of 
tides are also obtained. Taken all together, it seems that this 
constitutes one of the greatest advances in tidal work since 
the introduction of harmonic analysis. 
As astronomy seeks to unravel the mysteries of the solar 
and sidereal systems in order that the various changes in ap¬ 
parent position of the sun, moon, planets, and stars may be 
accurately foretold, thus enabling mankind to make use of 
these far-distant bodies for many important uses upon both 
land and sea, so all tidal work has, in a similar way, the pre¬ 
diction of tides as an ultimate end in view. The tides play 
such an important part in most harbors that all the great 
nations of the earth now publish tables containing predicted 
tides for the use of mariners. Probably rude prediction tables 
for the time of high or low water were made in quite early 
times, although the oldest one which still exists, so far as I 
am aware, is a tide table for London bridge, the manuscript 
of which is in the British Museum, and said to be the work 
of John Wallingford, abbot of St. Albans, who died in 1213. 
Probably the first step toward making tidal predictions con¬ 
sisted in the rude observations of early mariners that high or 
low water, at a given port, occurred whenever the moon oc¬ 
cupied certain positions in the heavens. More than 2,000 
years ago men were thus enabled to note a connection be¬ 
tween the moon and the tides. 
Many processes for predicting tides have been devised, 
some of them very elaborate, as, for instance, Ferrel’s method 
of predicting the Boston tides, which required a whole month 
of time for a good computer to make the predictions for one 
year at a single port. Contrasted with this there were pro¬ 
cesses so rude that in a single day one could make predic¬ 
tions for a year at any port. But with the introduction of 
the harmonic analysis came tide-predicting machines, for 
the uniform speed of each component of the tide, which is 
the same for all places, and their fixed amplitudes for a 
