250 Principal Features of Tidal Phenomena 



association with the "age" of the moon were also known to Caesar. Plinius 

 wrote about the sun and the moon being the cause of these phenomena. 

 In general, however, the grandeur of this natural phenomenon was looked 

 upon as something sinister and overwhelming by those who had never before 

 experienced it. An example of this feeling, which has become famous, is the 

 description by Curtius Rufus of the mighty tides which were encountered 

 by the fleet of Alexander the Great in the estuary of the Indus River. 



It is to be assumed that the sea-faring peoples living on the open coasts 

 were familiar with the tides ever since. Ancient literature lends some support 

 to this assumption. Then, many centuries passed by from which there is no 

 record of any scientific observations of the tides. Only in the sixteenth century 

 were the first attempts made to explain the tidal phenomenon. These were 

 based upon the relations between the sun and the moon with their diurnal, 

 monthly and annual inequalities, which were originally derived to serve the 

 needs of navigation. Important changes in the views as to the causes of the 

 tides were brought about in the Renaissance. After preliminary studies by 

 Kepler, Galileo and others, it was Newton who, in 1687, discovered the 

 correlation between the movements of the moon and sun and the tides. 

 In his book Philisophiae naturalis principia mathematica, he published the 

 laws of gravity which still form the base for the theory of the tides. 



2. Tidal Observations, Gauges 



The methodical analysis of the tides at a coastal locality requires regular 

 observations and recordings of variations of the sea level. Observations of 

 this kind have been gathered for centuries in numerous locations, because it 

 is so essential for coastal navigation. The rise and fall of the sea surface can 

 be easily determined on poles and harbour dams, docks and the like by means 

 of a vertical board (tide staff) divided into feet and tenths of feet. Such 

 observations are, of course, inaccurate, because the water surface is in continu- 

 ous motion as a consequence of the ever present ocean waves. If one wishes 

 to increase the accuracy and obtain perfectly correct measurements, the level 

 should be determined at a point where the sea has free access, but the canal 

 connecting the gauge with the open ocean should be made so narrow as to 

 dampen short disturbances of the sea surface, like waves, etc. For this purpose, 

 a tape gauge is used (Fig. 106). The rise and fall of the surface due to waves 

 is then largely eliminated. For its description we quote Sverdrup (1942, 

 p. 360). 



The principle of the tape gauge may be adapted for obtaining a continu- 

 ous automatic record of the tide level. In the standard automatic gauge used 

 by the U.S. Coast and Geodetic Survey (Rude, 1928), the float, which is 

 suspended in a well, is attached to a wire that turns a pulley mounted on the 

 threaded rod. As the pulley turns, a carriage with a pencil moves back and 

 forth along the threaded rod that is mounted at right angles to a clockwork- 



