The Harmonic Analysis of Tidal Observations 



311 



variations in M 2 . It seems that these can only be explained by variations 

 in the bottom configuration (sand banks, etc.) in the immediate vicinity of 

 the harbours. 



124 



123 



122 



121 



120 



331' 

 330' 

 329' 

 328' 



1880 



1890 



1900 



1910 



920 



Fig. 125. Harmonic constant M 2 for Bombay. , original value; 



corrected value (Doodson). 



4. Prediction of the Tides. Tide Tables, Tide-predicting Machines 



The knowledge of the laws governing the tides makes it possible to compute 

 in advance their occurrence for a certain harbour and thus to predict the 

 tides. Since ocean liners running according to a fixed time-table have replaced 

 the old sailing ships, there has been an ever increasing need for exact tidal 

 data, and the prediction of the tides as contained in the tide tables con- 

 stitutes one of the most important tasks of the hydrographic offices, in all 

 countries. For the prediction of the heights and times of tides in a harbour 

 one must, of course, refer to the tidal observations made previously in this 

 locality from which the basic constants of the tides characteristic of the 

 harbour must be derived. 



The detailed computations for the tides can be executed according to two 

 different methods; the so-called non-harmonic methods and the harmonic 

 methods. 

 (a) The Non-harmonic Method 



This was first developed by Lubbock (1839) (1830-50). It uses mainly 

 the time and the heights of high and low waters, and to a lesser extent the 



