the distance as measured from the chart between the lines is equal to the distance made good by the vessel between the times of taking the bearings. Danger angle.-The utility of the danger angle in passing outlying rocks or dangers should not be forgotten. In employing the horizantal danger angle, however, charts compiled from early Russian sources, referred to in a preceding paragraph, should not be used.

Soundings. In thick weather, when near or approaching the land or danger, soundings should be taken continually and at regular intervals, and with the character of the bottom systematically recorded. By marking the soundings on tracing paper, according to the scale of the chart, along a line representing the track of the ship and then moving the paper over the chart parallel with the course until the observed soundings agree with those of the chart, the ship's position will in general be quite well determined.

Echo sounding. This method of obtaining soundings is becoming extensively used. Its advantages lie in the fact that rapid and almost instantaneous soundings can be had while the vessel is running at full speed, and the navigator thereby knows at all times the depth of water under his keel. The method of determining the ship's position as described in the preceding paragraph, is eminently applicable when echo soundings are obtained. Two types of instruments have been brought out. In one type, the returning echo is flashed on a revolving plate and the depth read by an adjacent scale, while with the other type the depths are recorded on a graph.

By means of echo soundings, a vessel can follow the 50- or 100-fathom curve with ease, and in such localities as the Pacific coast and Alaska dangerous points and capes can be rounded in thick weather with utmost safety.

Echo sounding has been developed to such an extent that is now used by the surveying vessels of the Coast and Geodetic Survey, and has practically replaced the use of the pressure tube.

Use of sounding tubes.-Although of undoubted value as a navigational instrument, the sounding tube is subject to certain defects which, operating singly or in combinations, may give results so misleading as to seriously endanger the vessel, whose safety is entirely dependent upon an accurate knowledge of the depths. There are various types of tubes in common use which are too well known to require a detailed description here. They are all based on the principle that the column of air in the tube will be compressed in proportion to the depth to which the tube is lowered in the water. The principle is sound theoretically, but in practice there are several sources of mechanical errors which effect the result in proportion to the depth of water determined. The most important sources of errors are as folows:

(a) Inherent: Those which occur as a result of permanent defects in the tube, such as the variation of the bore from a true cylinder, variation in the thickness of the cap, etc.

(b) External: Those which occur as a result of the conditions under which the sounding was taken, variations of temperature or barometric pressure from the normal, etc.

(c) Accidental: Those which effect a single sounding, due to the failure of the tube to register properly, leakage of air, loss of water from leaky valve, errors due to the presence of salt in the tube, etc.

Before undertaking the sounding necessary to make any particular landfall the vessel should be stopped for an up-and-down cast of the lead in order to test the accuracy under the prevailing conditions of the tubes which are to be used. For this purpose it is not necessary to get bottom; simply run out 60 to 80 fathoms of wire and then see how closely the tubes register that amount. A number of tubes can be sent down at one time, and it is then possible to select one or two which register most nearly correct. The accidental errors are probably the most serious of the three types, both because they are apt to be larger in amount and because it is impossible to foresee when they will occur. It should be kept in mind that tubes which have been working properly for a number of soundings may suddenly develop errors, and for this reason the mariner must beware of overconfidence in the tubes.

Sumner's method.-Among astronomic methods of fixing a ship's position the great utility of Sumner's method or one of its many modifications should be well understood, and this method should be in constant use. The Sumner line—that is, the line drawn through the two positions obtained by working the chronometer observation for longitude with two assumed latitudes, or by drawing through the position obtained with one latitude a line at right angles

to the bearing of the body as obtained from the azimuth tables-gives at times invaluable information, as the ship must be somewhere on that line, provided the chronometer is correct. If directed toward the coast, it marks the bearing of a definite point; if parallel with the coast, the distance of the latter is shown. Thus the direction of the line may often be usefully taken as a course. A sounding at the same time with the observation may often give an approximate position on the line. A very accurate position can be obtained by observing two or more stars at morning or evening twilight, at which time the horizon is well defined. The Sumner lines thus obtained will, if the bearings of the stars differ three points or more, give an excellent result. A star or planet at twilight and the sun afterward or before may be combined; also two observations of the sun with sufficient interval to admit of a considerable change of bearing. In these cases one of the lines must be moved for the run of the ship. The moon is often visible during the day, and in combination with the sun gives an excellent fix.

Position line by means of tables.-The Sumner line of position furnished ready to lay down on the chart may be derived from the tables of Simultaneous Hour Angle and Azimuth of Celestial Bodies ", published by the Hydrographic Office as Publication No. 203. By means of these tables all calculations are avoided, but they are not recommended for use within an hour of the meridian, when the Marc St. Hilaire Method of Ex-Meridian Tables should be substituted.

Position line by Marc St. Hilaire or calculated altitude method.-By this method the altitude of the celestial body is calculated for the assumed position, and the diifference between the observed and calculated altitudes is laid off toward or away from the assumed position, according to sign, along the azimuth of the observed body. The line of position is then drawn through the new point in a direction normal to the azimuth of the celestial body. This method has certain advantages, the principal one being that the solution is strong even when the body is near the meridian. Full description of this method will be found in any epitome of navigation.

Radio direction finder bearings and positions are especially valuable at night and during fog or thick weather when other observations are not obtainable. For practical navigating purposes radio waves may be regarded as traveling in a straight line from the sending station to the receiving station. Instruments for determining the bearing of this line are now available. The necessary observations may be divided into two general classes: First, where the bearing of the ship's radio call is determined by one, two, or more radio stations on shore and the resulting bearing or position is reported to the vessel (see p. 13); secondly, where the bearing of one or more known radio stations is determined on the vessel itself and plotted as a line of position or as cross bearings. Experiments show that these bearings can be determined with a probable error of less than 2°, and the accuracy of the resulting position is largely dependent on the skill and care of the observer. It must be remembered, however, that these lines are parts of great circles, and if plotted as straight lines on a Mercator chart a considerable error may result when the ship and shore station are a long distance apart. The bearings may be corrected for this distortion as explained on page 14.

Radio bearings may be combined with position lines obtained from astronomic observations and used in ways very similar to the well-known Sumner line when avoiding dangerous shoals or when making the coast.

Radiobeacons.-The United States Lighthouse Service now maintains and operates radiobeacons at a large number of lighthouses and lightships. In fog or thick weather these radiobeacons send distinctive signals and ships equipped with radio direction finders may readily obtain bearings on these beacons. For a detailed description, see page 11.

Synchronization of sound and radio signals.-At some of the stations the radio and sound signals are synchronized for distance-finding purposes. Reference should be made to the Light List to ascertain which parts of the radiobeacon signal and sound signal are coincident. When the sound signal travels through water, the number of seconds intervening between the reception of the radio signal and sound signal multiplied by 0.78 gives the distance of the vessel in miles from the station. Where the sound signal travels through air the distance in miles is obtained by dividing the number of seconds by 5.

Change in variation of the compass-The gradual change in the variation must not be forgotten in laying down positions by bearings on charts. The magnetic compasses placed on the charts for the purpose of facilitating plotting

become in time slightly in error, and in some cases, such as with small scales, or when the lines are long, the displacement of position from neglect of this change may be of importance. The compasses are replotted for every new edition if the error is appreciable. Means for determining the amount of this error are provided by printing the date of constructing the compass and the annual change in variation near its edge.

The change in the magnetic variation in passing along parts of the coast of the United States is so rapid as to materially affect the course of a vessel unless given constant attention. This is particularly the case in New England and parts of Alaska, where the lines of equal magnetic variation are close together and show rapid changes in magnetic variation from place to place, as indicated by the large differences in variation given on neighboring compass roses.

Local magnetic disturbance.-The term "local magnetic disturbance" or "local attraction" has reference only to the effects on the compass of magnetic masses external to the ship. Observation shows that such disturbance of the compass in a ship afloat is experienced only in a few places. Magnetic laws do not permit of the supposition that it is the visible land which causes such disturbance, because the effect of a magnetic force diminishes in such rapid proportion as the distance from it increases that it would require a local center of magnetic force of an amount absolutely unknown to affect a compass half a mile distant.

Such deflection of the compass is due to magnetic minerals in the bed of the sea under the ship, and when the water is shallow and the force strong the compass may be temporarily deflected when passing over such a spot, but the area of disturbance will be small, unless there are many centers near together. The law which has hitherto been found to hold good as regards local magnetic disturbance is that north of the magnetic equator the north end of the compass needle is attracted toward any center of disturbance; south of the magnetic equator it is repelled. It is very desirable that whenever an area of local magnetic disturbance is noted the position should be fixed and the facts reported as far as they can be ascertained.

USE OF OIL FOR MODIFYING THE EFFECT OF BREAKING WAVES

Many experiences of late years have shown that the utility of oil for this purpose is undoubted and the application simple. The following may serve for the guidance of seamen, whose attention is called to the fact that a very small quantity of oil skillfully applied may prevent much damage both to ships (especially of the smaller classes) and to the boats by modifying the action of breaking seas. The principal facts as to the use of oil are as follows: 1. On free waves-that is, waves in deep water-the effect is greatest. 2. In a surf, or waves breaking on a bar, where a mass of liquid is in actual motion in shallow water, the effect of the oil is uncertain, as nothing can prevent the larger waves from breaking under such circumstances, but even here it is of some service.

3. The heaviest and thickest oils are most effectual. Refined kerosene is of little use; crude petroleum is serviceable when nothing else is obtainable; but all animal and vegetable oils, and generally waste oil from the engines, have great effect.

4. A small quantity of oil suffices, if applied in such a manner as to spread to windward.

5. It is useful in a ship or boat, either when running or lying-to or in wearing.

6. No experiences are related of its use when hoisting a boat at sea or in a seaway, but it is highly probable that much time would be saved and injury to the boat avoided by its use on such occasions.

7. In cold water the oil, being thickened by the lower temperature and not being able to spread freely, will have its effect much reduced. This will vary with the description of oil used.

8. For a ship at sea the best method of application appears to be to hang over the side, in such a manner as to be in the water, small canvas bags capable of holding from 1 to 2 gallons of oil, the bags being pricked with a sail needle to facilitate leakage of the oil. The oil is also frequently distributed from canvas bags or oakum inserted in the closet bowls. The positions of these bags should vary with the circumstances. Running before the wind they should be hung on either bow-for example, from the cathead-and allowed

to tow in the water. With the wind on the quarter the effect seems to be less than in any other position, as the oil goes astern while the waves come up on the quarter. Lying-to, the weather bow, and another position farther aft seems the best places from which to hang the bags, using sufficient line to permit them to draw to windward while the ship drifts.

9. Crossing a bar with a flood tide, to pour oil overboard and allow it to float in ahead of the boat, which would follow with a bag towing astern, would appear to be the best plan. As before remarked, under these circumstances, the effect cannot be so much trusted. On a bar with the ebb tide running it would seem to be useless to try oil for the purpose of entering. 10. For boarding a wreck it is recommended to pour oil overboard to windward of her before going alongside. The effect in this must greatly depend upon the set of the current and the circumstances of the depth of water.

11. For a boat riding in bad weather from a sea anchor it is recommended to fasten the bag to an endless line rove through a block on the sea anchor, by which means the oil can be diffused well ahead of the boat and the bag readily hauled on board for refilling if necessary.

CURRENTS AT LIGHTSHIPS

For several years current observations have been made at San Francisco, Blunts Reef, Columbia River, Umatilla Reef, and Swiftsure Bank Lightships. A discussion in detail of the observations at each of the lightships will be found in Coast and Geodetic Survey Special Publication No. 121, entitled "Coastal Currents Along the Pacific Coast of the United States." In general it may be said these observations show that winds here bring about a current having a velocity 2 percent that of the wind, the direction of this wind-driven current being about 20° to the right of the wind with winds from the northeast, southeast, and northwest quadrants, and about 20° to the left of the wind with winds from the southwest quadrant. A summary of the results of these observations at each of the light vessels is given below.

SAN FRANCISCO LIGHTSHIP

Tidal current.-The tidal current here is rotary, turning clockwise, and is largely diurnal, that is, revolving in a period of about 25 hours. At strength the current has a velocity averaging less than 2 knot, setting southeastward about an hour before higher low water at San Francisco, and northwestward about 2 hours after lower low water.

Nontidal current.-During the winter months there is a nontidal current with an average velocity of about 4 knot, setting northwesterly. During the summer months this nontidal current is weaker, averaging about to knot, the direction being variable and frequently southerly. The direction and velocity of the average nontidal current is very considerably modified by the current due to the wind.

Wind current.-In general the wind current here is about 2 percent that of the wind, but currents coming with winds from the southeast quadrant are stronger than the average, while the currents coming with winds from the northwest quadrant are weaker than the average.

BLUNTS REEF LIGHTSHIP

Tidal current.-The tidal current at this light vessel is weak, having at strength a velocity of less than 4 knot. It is therefore completely masked by nontidal currents due chiefly to the wind.

Nontidal current.-The nontidal current here has a velocity averaging about 4 knot, setting southwesterly. During the winter months the wind is prevailing from the southeast and during these months the nontidal current sets northwesterly.

Wind current.-The observations bring out the fact that here the current, due to a wind of given strength, is practically the same for all directions of the wind, the velocity of the current being about 2 percent that of the wind. The direction of the wind-driven current obeys the general rule for this coast that winds from the northeast, southeast, and northwest quadrants

bring about currents the directions of which are about 20° to the right of the wind, while winds from the southwest quadrant bring about currents setting about 20° to the left of the wind.

COLUMBIA RIVER LIGHTSHIP

Tidal current.-The tidal current at Columbia River Lightship is of the rotary type with a velocity at strength of flood or ebb of 4 knot. This tidal current, however, is completely masked by nontidal currents due to river discharge and winds.

Nontidal current.—At Columbia River Lightship there is a nontidal current averaging knot with a set to the west-southwestward. This nontidal current is due largely to the discharge from the Columbia River and therefore varies with the seasons. It is greatest during the summer months, averaging then more than 2 knot. The direction of the average nontidal current is also subject to a seasonal variation, setting southwesterly in the spring and summer months and northwesterly in the fall and winter months. This variation in direction is due to the winds.

Wind current.-Along this part of the coast the wind is prevailingly from the northwest in the spring and summer months and from the east and southeast during the fall and winter months. Because of the nontidal current due to river discharge, the direction of which is west-southwest, northerly winds are accompanied by stronger currents than southerly winds. Away from the influence of the river current the general rule holds good that the volocity of the current is about 2 percent that of the wind and that winds from the northeast, southeast, and northwest quadrants bring about currents that set about 20° to the right of the wind, while the currents accompanying winds from the southwest quadrant set about 20° to the left of the wind.

UMATILLA REEF LIGHTSHIP

Tidal current.-The tidal current here is only slightly rotary, setting N. 15° W. on the flood and S. 15° E. on the ebb with a velocity at strength of about 1% knot.

Nontidal current.-The nontidal current at Umatilla Reef Lightship shows a very distinct seasonal variation. During the winter months this current sets northerly with a velocity which averages % knot, while in summer it sets southerly with a velocity of about 1% knot.

Wind current. Here the prevailing wind is from the east and southeast in winter and from the west and northwest in summer. The southerly winds bring about stronger currents than the northerly winds, the current with the southerly winds being about 3 percent of the wind, while with the northerly winds the current is about 12 percent that of the wind. The general rule for the deviation of the wind-driven current from the direction of the wind, namely, currents brought about by winds from the northeast, southeast, and northwest quadrants set about 20° to the right of the wind, while with winds from the southwest quadrant the current sets about 20° to the left of the wind, is applicable here.

SWIFTSURE BANK LIGHTSHIP

Tidal current.-At this light vessel the tidal current is distinctly rotary, turning clockwise. At strength of flood or ebb the current has a velocity of 34 knot on the average. Strength of flood sets southeasterly and comes about 12 hours after high water at Astoria; strength of ebb sets northwesterly and comes about 1 hour after low water at Astoria.

Nontidal current.-At Swiftsure Bank Lightship the nontidal current has a velocity averaging 1⁄2 knot, setting northwest. This is due largely to the drainage waters flowing seaward through the Strait of Juan de Fuca. The velocity of the nontidal current is greatest during the fall and winter months when it averages somewhat more than 2 knot, this increase being due to the effect of the wind.

Wind current. The prevailing wind here is from the east during the fall and winter months and from the west and southwest during spring and