The catalogue of charts issued by the Coast and Geodetic Survey is furnished to all naval vessels by the Hydrographic Office and may be obtained by all other mariners directly from the Coast and Geodetic Survey.

TIDES.—A knowledge of the times of high and low water and of the amount of vertical rise and fall of the tide is of great importance in the case of vessels entering or leaving port, especially when the low water is less than or near their draft. Such knowledge is 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.

Planes of reference. The planes of reference for soundings on United States Hydrographic Office charts, when known, is stated on the chart. On charts made from Hydrographic Office surveys the soundings are usually reduced to either mean low water, mean low-water springs, or the mean of the lower low waters. On charts based on those of other nations the datum is that of the original authority.

On United States Coast and Geodetic Survey Charts of the Atlantic coast of the United States and Puerto Rico the plane of reference for soundings is the mean of all low waters; for the Pacific coast of the United States and Alaska and for the Hawaiian and Philippine Islands it is the mean of the lower low waters. For the Atlantic coast of the Canal Zone, Panama, the plane of reference for soundings is mean low water and for the Pacific coast of the same it is low-water springs.

The plane of reference for British Admiralty charts, based on British surveys, is, in waters where the daily inequality is small, the level of mean lowwater springs, and in places where the daily inequality is considerable the level of Indian spring low water. Where British Admiralty charts are founded on the charts of other nations the datum is that used by the original authority. The plane of reference adopted by the various other chart-producing countries varies greatly, but is usually lower than mean low water. A marked exception is found in the charts of the Baltic Sea and the Black Sea, where, due to the small tidal effect, the datum adopted is the mean level of the respective seas.

"International low water," a plane of reference recommended by the International Hydrographic Conference of 1919 but adopted on only a few charts, is fixed by taking one-half the range between mean lower low water and mean higher high water and multiplying this half-range by 1.5.

All planes of reference should be considered as approximate, as an exact determination requires a longer series of tidal observations than is usually practicable. Whenever not stated on the chart, the assumption that the reference plane is mean low water and not low water ordinary springs gives a larger margin of safety. There are times when the tide may fall below any plane of reference that is used on the chart.

Mean sea level. The depths at half tide are practically the same for all tides, whether neaps or springs. Half tide, therefore, corresponds with mean sea level. This makes a very exact plane of reference, easily found, to which it would be well to refer all high and low waters.

If required to take special soundings for the chart at a place where there is no tidal bench mark, mean sea level should be found and the plane for reductions established at the proper distance below it, as ascertained by the Tide Tables, or by observations, or in some cases, if the time be short, by estimation, the data used being made a part of the record.

Planes of reference for heights.-The plane to which heights on the Hydrographic Office charts are referred is usually the plane used by the original survey upon which the charts are based. There is little uniformity in this datum plane among the charts published by the various maritime countries, though mean sea level or some plane of high water is more usual. In the case of lights, however, the figures given on the Hydrographic Office charts, as also in its other publications, have been corrected, in all areas where the tidal range is apreciable, to read above some plane of high water.

Tidal currents.-In navigating along coasts where the tidal range is considerable, special caution is necessary. It should be remembered that there are generally indrafts and corresponding outdrafts abreast of all large bays and bights, although the current may generally run nearly parallel with the shore outside of the entrances.

The turn of the tidal current offshore seldom coincides with the time of high and low water along the shore. In some channels the tidal current may overrun the turn of the vertical movement of the tide by three hours, so that at high and low water by the shore the current is running at its greatest velocity. The effect of the tidal wave in causing currents may be illustrated by two

cases:

(1) Where there is a small tidal basin connected with the sea by a large opening.

(2) Where there is a large tidal basin connected with the sea by a small opening.

In the first case the velocity of the current in the opening will have its maximum value when the height of the tide within is changing most rapidly; i. e., at a time about midway between high and low water. The water in the basin keeps at approximately the same level as the water outside. The flood current corresponds with the rising and the ebb current with the falling of the tide.

In the second case the velocity of the current in the opening will have its maximum value when it is high water or low water without, for then there is the greatest head of water for producing motion. The flood current in such cases generally begins about three hours after low water and the ebb current about three hours after high water, slack water thus occurring about midway between the tides.

Along most shores not much affected by bays, tidal rivers, etc., the current usually turns soon after high water and low water.

The swiftest current in straight portions of tidal rivers is usually in the middle of the river, but in curved portions the most rapid current is toward the outer edge of the curve, and here the deepest water will generally be found. The pilot rule for best water is to follow the outer edge of the curves or the ebb tide reaches.

Countercurrents and eddies may occur near the shores of straits, especially in bights and near points. A knowledge of them is useful in order that they may be used or avoided.

A swift current often occurs in a narrow passage connecting two large bodies of water, owing to their considerable difference of level at the same instant. The several passages between Vineyard Sound and Buzzards Bay are cases in point.

Tide rips are generally made by a rapid current setting over an irregular bottom, as at the edges of banks where the change of depth is considerable, but current rips sometimes occur on the high seas.

Tide Tables, published annually by the United States Coast and Geodetic Survey, give the predicted times and heights of the high and the low waters for every day in the year at a number of the principal ports of the world, and from these, by means of tidal differences and ratios, at a very large number of subordinate ports. The tables for the Atlantic and the Pacific coast ports of the United States are also published separately. These tables predict the times of high and low water, and not the times of turning of the current or of slack water, which may be quite different.

The distinction between "rise" and "range" of the tide should be understood. The former expression refers to the height attained above the datum plane of sounding, differing with the different planes of reference; the latter, to the difference of level between successive high and low waters.

Full explanations and directions for using the tide tables are given in that book. Current Tables are published annually by the United States Coast and Geodetic Survey in the form of two pamphlets. The tables for the Atlantic coast of North America give the predicted times of slack water for each day of the year for a number of stations from the Bay of Fundy to Florida, together with time differences for slack water at subordinate stations and other current information for the area. The tables for the Pacific coast of North America give similar information for a number of stations from Lower California to Alaska.

AIDS TO NAVIGATION-Lights.-Charts and Light Lists give the visibility of lights for a height of 15 feet of the observer's eye, and the effect of a greater or less height of the observer's eye can be obtained by the tables of distance published in all of the Light Lists.

A light is sooner sighted aloft, as there the observer's range of vision is increased, and an approximate bearing of the light may be obtained by noting while aloft a star, over or nearly over the light, and later observing the bearing of the star with the compass.

Powerful lights often loom far beyond the limit of visibility of the actual rays of the light, and this must not be confounded with the true range. Refraction also often causes a light to be seen farther than under normal conditions.

On first sighting a light, by at once lowering the eye several feet and noting whether the light is made to dip, it may be determined whether the vessel is on the circle of visibility corresponding with the usual height of the eye or unexpectedly near the light.

When expecting to sight a light in thick weather, its power and color should always be considered. Haze obscures a weak or a colored light and decreases the chance of sighting it.

The distance from a light can not be estimated either by its brilliance or by its dimness, but only by its change of bearing.

When a light is sighted, it should be identified at once by checking its characteristics as given in the Light List, particularly when approaching a well-lighted coast, where lights with similar characteristics are sometimes found close together.

The power of a light can be estimated by its candlepower as given in the Light List, and in some cases by noting how much its visibility in clear weather falls short of the range corresponding to its height. For example, a light elevated 120 feet above high water and only visible 9 miles in clear weather must be of small candlepower, for if of sufficient candlepower its height would give it a visibility of over 16 miles.

Buoys.-Buoys do not always maintain exact positions; therefore they should always be regarded as warnings and not as fixed navigational marks, espe442705 - 42 - 2

cially during the winter months or when moored in exposed waters. A vessel's position should, when possible, be plotted not by buoys but by bearings or by angles of fixed objects on shore.

Light buoys can not always be relied on, because the light may become extinguished, or, if periodic, the apparatus may fail to operate.

Whistle and Bell buoys are sounded by the action of the sea; therefore in calm weather they are less effective, and at times may not sound.

NAVIGATION.—Piloting, in the modern sense of the word, is the art of conducting a vessel in channels and harbors and along coasts where landmarks and aids to navigation are available for fixing the position and where the depth of water and dangers to navigation require a constant watch and frequent changes of course. Piloting requires the greatest experience and nicest judgment of any form of navigation. An error in position on the high seas may be rectified by later observations, but an error in position while piloting often results in disaster. Therefore the mariner should endeavor to be proficient in this important branch, mindful that a modern well-found vessel is usually safe on the high seas and in danger when approaching land and making harbor.

In planning to enter a strange port the mariner should give careful previous study to the chart, sailing directions, and tide tables, and should select the most suitable marks for use, providing substitutes in case those selected cannot be recognized with absolute certainty. Ranges should be noted if available and the lines drawn on the charts both for leading through the deepest water in channels and for guarding against particular dangers; for the latter purpose safety bearings should in all cases be laid down when no suitable ranges offer. The courses to be steered in entering should be laid down and distances marked thereon.

If intending to use the sextant and danger angle in passing dangers, and especially in passing between dangers, the danger circle should be plotted and regular courses planned, rather than to run haphazard by the indication of the angle alone, with possible trouble from bad steering at critical points. It should be remembered that channel buoys seen from a distance are diflicult to identify, because their color is sometimes not easily distinguished, and they may appear equally distant from the observer even when at widely varying distances.

The vessel's position should be fixed at all times, even when entering ports considered safe and easy of access, and should be constantly checked, using for the purpose those marks whose identity has been established beyond doubt.

The vessel should ordinarily steer exact courses and follow an exact line as planned from the chart, changing course at precise points; and, where the distance run on the same course is considerable, the position should be checked at frequent intervals. This procedure is desirable even when it may seem unnecessary for safety, because if running by the eye alone, and the vessel's exact position be immediately required, as in a fog or sudden squall, a fix at that particular moment may be difficult to obtain.

This habit of running exact courses with precise changes of course will also be found most useful when it is desired to enter port or pass through inclosed waters by means of the buoys alone, as during a fog; here safety demands that the buoys be made successively, which requires, if the fog be dense, very accurate courses and careful attention to the speed of the vessel and the set of the current; failure to make a buoy when expected leaves, as a rule, no safe alternative but to anchor at once, with perhaps a consequent serious loss of time.

Changes of course should in general be made by exact amounts, naming the new course or the amount of the change desired, rather than by ordering the rudder to be put over and then steadying when on the desired course, with the danger of the attention being diverted and of forgetting that the vessel is still swinging. The helmsman, knowing what is desired and the amount of change to be made, is able to act more intelligently and to steer a good course, which is essential in narrow channels.

In passing between dangers where there are no suitable ranges, as, for instance, between two islands or an island and the main shore when the conformations of the shore line are very similar, with dangers extending from both, a mid-channel course may be steered by the eye alone with great accuracy, as the eye is able to estimate very closely the line that lies midway between.

In piloting among coral reefs or banks a time should be chosen when the sun will be astern, conning the vessel from aloft or from an elevated position forward, for the line of demarcation between the deep water and the edges of the shoals is indicated with surprising clearness.

Coast piloting involves the same principles as piloting in a harbor or channel, and similarly requires that the vessel's position be continuously determined as landmarks are passed.

The routes should be planned for normal conditions of weather, with suitable variations in case of fogs or bad weather or for making points at night.

On well-surveyed coasts there is a great advantage in coasting near the land, keeping the marks and the soundings, and thereby knowing at all times the vessel's position, rather than keeping offshore and losing the marks, with the necessity of again making the land from a doubtful position, with perhaps the added inconvenience of fog or bad weather.

The danger circle for either the horizontal or the vertical danger angles should be plotted wherever the method can be usefully employed and the angles should be marked on the chart. This practice will save many miles in rounding dangerous points, with no sacrifice of safety. Where available, ranges should also be marked on the chart, either to lead clear of dangers or to check the deviation of the compass.

In making a coastwise trip against a strong offshore or head wind, it may be desirable, with trustworthy charts, to skirt the shore as closely as possible in order to avoid the heavier seas and adverse currents that prevail farther out. In some cases, with small vessels, a passage can be made only in this way. The important saving of fuel and of time thus effected by skillful coast piloting makes this subject one of prime importance to the navigator. Many vessels attempting to save time or distance approach dangers too closely and get into trouble, so a mariner should always remember that the safety of the vessel is the first consideration.

In case of regular runs over the same route the courses and distance should be entered in a notebook and laid down on the chart where they will be available for ready reference.

The officer of the watch should keep a continuous record of the progress of the vessel, entering in the navigator's notebook and ship's log book the time and patent log reading of all changes of course and the bearings of objects, especially when abeam. In this way reckoning is regularly kept without the presence or particular order of the captain or navigating officer. The fresh and exact position thus available is useful at all times, and is particularly valuable at night or in case of a sudden squall or fog.