GENERAL NAVIGATION. 13 



also useful at times to vessels running close along a coast in enabling 

 them to anticipate the effect of the tidal currents in setting them on 

 or off shore. This is especially important in fog or thick weather. 



The predicted times and heights of the high and low waters, or 

 differences bv which they may be readily obtained, are given in the 

 tide tables for all the important ports of the world. The height at 

 any intermediate time may be obtained by means of Table 2 for most 

 of the principal tidal stations of the United States given in Table 1, 

 and for the subordinate stations of Table 3 by multiplying its values 

 by the ratio of mean ranges, provided the duration of rise and fall is 

 sensibly the same at the subordinate as at the principal station. The 

 intermediate height may also be obtained by plotting the predicted 

 times and heights of high and low water and connecting the points 

 by a curve. Such knowledge is often useful in crossing a bar or 

 shallow flats. 



Planes of reference.* — The })lane of reference for soundings on 

 Hydrographic Office chai'ts made from United States Government 

 surveys and on Coast and Geodetic Survey charts of the Atlantic 

 coast of the United States is mean low water; on the Pacific coast 

 of the United States as far as the strait of Fuca, it is the mean of 

 the lower low waters; and from Puget sound to Alaska the Survey 

 has adopted the harmonic or Indian tide plane, which is roughly 

 that of the lowest low waters observed. 



On most of the British Admiralty charts the plane of reference is 

 the low water of ordinary springs; on French charts, the low water 

 of equinoctial springs. 



In the case of many charts compiled from old or various sources 

 the plane of reference may be in doubt. In such cases, or whenever 

 not stated on the chart, the assumption that the reference plane is 

 mean low water gives the largest margin of safety. 



AVhichever plane of reference may be used for a chart, it must be 

 remembered that there are times when the tide falls below it. Low 

 water is lower than mean low water about half the time, and when a 

 new or full moon occurs at perigee the low water is lower than the 

 average low water of springs. At the equinoxes the spring range is 

 also increased on the coasts of Europe, but in some other parts of the 

 world, and especially in the Tropics, such periodic low tides may 

 coincide more frequently with the solstices. 



Wind or a high barometer maj' at times cause the water to fall 

 below even a very low plane of reference. 



* The distinction between "rise" and "range" of the tide should be under- 

 stood. The former expression refers to the height attained above tlie datum 

 plane for soundings, differing with the different planes of reference : the latter, 

 to the difference of level between successive high and low waters. 



