time of full and new moon. These tides are called spring tides. The 

 term "spring" as applied to tides has nothing to do with the season 

 of the year, but is the greater upspringing of the waters at intervals 

 of about a fortnight. Similarly the daily high waters are usually at 

 their least height, and the daUy low waters their greatest height, soon 

 after the moon is in quadrature. These tides are called neap tides. 

 On the Atlantic coast of Europe and along the British Isles the differ- 

 ence between low or high water of spring tides and low or high water 

 of neap tides may amount to several feet, and is a matter of moment 

 to navigators. On the coasts of the United States the difference 

 between spring and neap tides is not particularly noticeable, and the 

 terms "spring" and "neap" tides are not in ordinary use. In this 

 country spring tides are commonly referred to as "tides at full (or 

 new) moon" or occasionally as "moon tides." 



3. Datum planes. — The average height of all low waters at any place 

 over a sufficiently extended period of time is called n^iean low water 

 and is the official reference plane for the depths shown on navigation 

 charts, and of improved channels, in the waters of the Atlantic and 

 Gulf coasts of the United States. The average height of the lower of 

 the two daily low waters is called mean lower low water and is the 

 official reference datum in the waters of the Pacific coast of the 

 United States. In British waters the datum is usually the mean low 

 water of spring tides, or low-water springs. This reference plane is 

 also used at the Pacific entrance to the Panama Canal. The average 

 height of the sea, as determmed usually by the average of the observed 

 hourly heights over an extended period of time, is called mean sea 

 level, and is the standard datum to which elevations on land are 

 referred, 



4. Tidal ranges. — The difterence in height between high water and 

 low water at a tidal station is called the tidal range. The mean range 

 is the average of the differences between all high waters and all low 

 waters; or, as is the same thing, the difference between mean high 

 water and mean low water at the station. The diurnal range, or great 

 diurnal range, is the difference between mean higher high water and 

 mean lower low water. The extreme range is the maximum that has 

 been observed. The spring range is the difference between mean 

 high water and mean low water of spring tides, and the neap range 

 the difference between mean high water and mean low water of neap 

 tides. 



5. Tidal currents. — The tidal current setting into the bays and 

 estuaries along the coast is called the flood current. The return cur- 

 rent toward the sea is called the ebb current. The maximum velocities 

 reached during each fluctuation of the current are called the strength 

 of the flood and the strength of the ebb, or, indifferently, the strength of 

 the current. Slack water is the period during which the current is 



