NOTES RELATIVE TO THE USE OF CHARTS OF THE PHILIPPINE ISLANDS ISSUED BY THE COAST AND GEODETIC SURVEY.

There are four kinds of publications issued by the Coast and Geodetic Survey as guides to navigation in the Philippine Islands; the Charts, the Sailing Directions, the Tide Tables, and the Notices to Mariners.

Agencies for the sale and distribution of these publications have been established at several ports in the Islands, at Hongkong, and at Newcastle, New South Wales. They can also be obtained from the offices of the Coast and Geodetic Survey, in Washington, D. C., or in Manila, P. I. Catalogues containing full descriptions and prices of charts and other publications may be had free of charge on personal application to the office or any of its agencies, or by letter addressed to the office.

Charts. When issued from the office the artificial aids to navigation, such as light-houses, beacons, buoys, etc., have been corrected from information received to date of issue, and such other corrections as can be made by hand have also been effected. The date of issue is stamped in the lower left-hand corner, and later corrections to be applied may be found by examining the Notices to Mariners published subsequently.

These charts are compiled from various sources, but in every case the latest reliable information available has been embodied in the publication. New editions are published when warranted by the amount and importance of new material, the old editions being then canceled. The date. of the edition is given in the title.

All notes on the charts should be carefully read, as they may call attention to important dangers or aids to navigation that can not be clearly represented on the chart.

The charts of the Islands may be classified as follows:

1. General chart of the Philippine Islands, on mercator projection; mean scale, Toooo (about 22 nautical miles to the inch). The scale of this chart is too small to make it suitable for general use in navigation. 2. General charts on mercator projection; scale in middle latitude, 10000 (about 5 nautical miles to the inch). The entire Archipelago is now covered by this series, Nos. 4710 to 4724, inclusive. These are intended for coastwise navigation, and show the configuration of the shore, the positions of islands, rocks, shoals, light-houses, and other natural and artificial landmarks.

3. Coast charts, on polyconic projection; scale, 100000 (1 nautical mile equals 0.73 inch). These give the coast in greater detail than the general charts and are intended for those navigators calling at coastwise ports where a near approach to the coast is necessary. In some cases, harbor plans on large scales, generally from 10000 to 20000, are published on the same chart, doing away with the necessity of separate detached harbor charts. This series is intended to cover the entire Archipelago, but few sheets have thus far been published. Others will be issued as rapidly as material from new surveys or other information accumulates.

4. Harbor charts, on polyconic projection, on large scales, to meet the needs of local navigation. Plans of harbors are in many cases grouped geographically and published on one sheet.

Accuracy of charts.-The accuracy of a chart must depend upon the accuracy of the survey on which it is based. The early surveys were far from complete and in many cases were unreliable, and until they are replaced by more accurate surveys, these charts must be used with caution. In this respect the source from which the information has been compiled and its date are good guides. In many instances the charts have been compiled from early surveys as a base, with additions and corrections from comparatively recent surveys.

The fullness or scantiness of the soundings is an important means of estimating the completeness of a chart, bearing in mind that all the soundings obtained during the survey are not shown on the chart. When the soundings are sparse or unevenly distributed it is an indication that the survey was not in great detail. Large blank spaces among soundings mean that no soundings have been obtained in these spots. When the surrounding soundings are deep it may be assumed that in the blanks the water is also deep; but when they are shallow, or when it can be seen from the rest of the chart that reefs or banks exist, such blank spaces should be regarded with suspicion. This is especially the case in coral regions and off rocky coasts and it should be remembered that in waters where rocks abound, it is always possible that a survey, however detailed, may have failed to find every small patch. Isolated soundings, shoaler than the surrounding depths, should be avoided, as there is always the possibility that the shoalest spot may not have been found.

A wide berth should therefore be given to every rocky shore or patch, and this rule should be invariably followed, that instead of considering a coast to be clear unless it is shown to be foul the contrary should be assumed. It should not be forgotten that in waters where sand and mud bottoms prevail changes are constantly occurring to a greater or less extent, and important ones may have taken place since the date of the last survey. In localities which are noted for frequent and radical changes, such as the entrance to the Cagayan and other rivers, notes are sometimes

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Danger curves.-The curves of depths will often be found useful in giving greater prominence to outlying dangers. In some cases it is advisable to trace out with a colored pencil the curve next greater than the draft of the vessel using the chart and regard this as a "danger curve" not to be crossed without precaution.

Caution in using small-scale charts. In approaching land or dangerous banks, regard must always be had to the scale of the chart used. A small error in laying down a position means only yards on a large-scale chart, whereas on a small scale the same amount of displacement means large fractions of a mile. For this reason bearings or angles to near objects are to be preferred to distant objects, although the latter may be more prominent, as a small error in bearing, or in laying it down on the charts, has a greater effect in misplacing the position the longer the line to be drawn. The largest-scale chart available should always be used in approaching the coast and entering harbors.

Buoys.-Reliance should not be placed on buoys always maintaining their exact position. They should be regarded as warnings and not as infallible navigating marks, and when possible a vessel should always be navigated by bearings or angles of fixed objects on shore and not by buoys.

Lights. The distances of visibility of lights on the charts are calculated for a height of an observer's eye of 15 feet. In making a light its intensity as well as its height should also be considered.

Plane of reference for soundings on charts.-The soundings on charts from new surveys are referred to the mean of the lower low waters. The water will at times fall below this plane due to extreme tides, wind, or a very high barometer. As an effect of the monsoons the mean tide level varies during the year. On the west coast of Luzon it is lowest in February and highest in September; in Manila Harbor this difference amounts to about 1 foot.

Fixing positions.-The most accurate method of fixing a ship's position relative to the shore is by sextant angles between three or more well-defined objects on the charts and plotting the same by means of a protractor or station pointer. The objects chosen should be at a moderate distance and well identified, the best condition being three objects in nearly a straight line at right angles to the line of sight, or with the central object nearer the observer than the other two. This method when practicable is preferable to cross bearings taken by the compass, or to bow and beam bearings. It should be remembered that there is always a chance for error in fixing the position by two compass bearings, and a third bearing should always be taken as a check. One or more sextant angles used with a true bearing by compass also gives a good fix when a protractor or station pointer is not available.

A vertical sextant angle of an object whose height and position is known may be used to find its distance; and this with a compass bearing

of the object will give the position of the vessel. Tables are published for the use of this method, but in the absence of any tables the following rule may be used: The height of the object in feet multiplied by constant .565, and this sum divided by the angular height in minutes will give the approximate distance from the object in nautical miles. By observing the angle "on and off the limb" any index error the sextant may have will be eliminated. The above rule should not be used when the base of the observed object is below the horizon of the observer.

Currents. The current arrows on charts show only the most usual or the mean direction of a tidal stream or current; it must not be assumed that the direction will not vary from that indicated by the arrow. The velocity of a current varies with circumstances and the rate given on a chart is merely the mean of those found during the survey, possibly from very few observations.

Tide rips, caused by currents setting against the wind or over irregular bottom, or by the meeting of currents setting in different directions, frequently produce the effect of breakers and have sometimes been reported as such, with the result that shoals have been indicated on the chart in their vicinity when none exist.

In navigating along coasts where the tidal range is considerable especial caution is necessary, as there are indrafts to all bights and bays, although the general direction of the current is parallel to the coast. Countercurrents and eddies may occur near the shore, especially in straits, bights, and near points, and a knowledge of them is useful in order that they may be taken advantage of or avoided. Along most shores, when not much affected by bays, tidal streams, etc., the current turns soon after high and low water.

Tides. A knowledge of the times of high and low water, together with the rise and fall of the tide, is of considerable importance where the depth at low water is near to or less than the draft of the vessel, and is frequently of service in estimating in advance the direction and velocity of a tidal current at a given time and place. Tables 2 A and 2 B of the tide tables give the methods of determining the height of the tide at any intermediate time between high and low water, for reduction of soundings, or calculating the depths in shoal channels.

Tide tables. These tables, published by the Coast and Geodetic Survey, give full predictions for Manila (Pasig River entrance), and in Table 3 are given the data for ascertaining the time of high and low water, and the range at numerous other ports in the Philippine Islands. Sailing Directions for the Philippine Islands.-These are at present published in seven sections, covering the entire Archipelago. The information contained in them is compiled from various sources, including the personal observations of the compilers in the field. They are undergoing constant revision due to new information from late surveys and other

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The Sailing Directions are distributed in pamphlet form, upon application to the office or any of its agencies, in person or by letter. The index chart in the front of each pamphlet gives the scope of the various sections.

Notices to Mariners.-These are published at intervals throughout the year as information is received. They are printed on one side of the paper only in order that they may be cut up and inserted in their proper places in the Sailing Directions. They are distributed free of charge and may be had on application to the office or any of its agencies.