negligible while it is changing direction! It is specifically defined 

 by the United States Coast and Geodetic Survey as the period during 

 which the current is less than one-tenth of a loiot; i.e., less than 0.169 

 feet per second. The slack water occurring nearest the time of high 

 water is called the high-water slack, and that nearest the time of low 

 water the low-water slack. The moment at which the current is zero 

 as it changes direction may be distinguished by terming it the turn 

 qf the current . 



In open waters, the direction of the current normally veers around 

 the compass and the current does not pass through intervals of slack 

 water. Such currents are called rotary, to distinguish them from the 

 reversing currents in a tidal channel. 



6. These definitions are narrower than the common usage of the 

 terms. "Tide" is commonly applied both to the rise and fall of the 

 sea and to the accompanying tidal currents. Thus the expressions 

 "head tide" and "favoring tide" designate tidal currents that retard 

 or accelerate the movement of a vessel, and the term "the ebb and 

 flow of the tide" is standard legal nomenclature. The term "ebb 

 tide" is often used to designate low water as well as the outflowing 

 tidal current. The maximum tidal stage is frequently designated as 

 "high tide" instead of "high water." Its more general meaning is, 

 however, the higher stages of the tide. Thus it is more accurate to 

 say that a channel is "navigable only at high tide," than to say that 

 it is "navigable only at high water." 



7. Lunitidal intervals. — Casual observation shows that the tides at 

 any place occur a little less than 1 hour later each succeeding day. 

 Thus if high water is at 3 p. m. today, it will be shortly before 4 

 p. m. tomorrow. Closer observation shows that the high and low 

 waters at any place follow, by about the same time interval, the pas- 

 sage of the moon across the meridian of the place. Obviously, the 

 moon must cross the plane of the meridian twice daily — once over- 

 head and once underneath. These are called respectively the upper 

 and lower meridian transits. They mark in fact the noon and mid- 

 night of the lunar day. If a clock were regulated on mean lunar 

 time, instead of mean solar time, it would show the times the high 

 and low waters at a given place at about the same hour every day, 

 but these times would vary largely from place to place. 



8. The average time interval, in solar hours and minutes, from a 

 lunar transit to the next succeeding high water at a given place, as 

 determined by an extended set of observations, is called the high- 

 water interval, (HWI) or the high-water lunitidal interval of the place. 

 Similarly the low-water interval (LWI), or the low-water lunitidal inter- 

 val is the average time, in solar hours and minutes, from a lunar transit 

 to the next succeeding low water. The high- and low-water intervals 

 usually are larger at the full and change of the moon, at about 



