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390 TAE MaRINE REVIEW October, 1910 METHODS OF OBSERVING WAVES AT SEA HE hydrographic office is endeavoring to get new and_ carefully made observations of waves at sea and has prepared a blank for distribution among. navigators. The hydrographic office says that if 100 skillful ocean navigators would each furnish 10 sets of wave observations to the hydrographic office, the mass of in- formation thus afforded would give the office a basis for study that might prove of value to ship builders, masters, steve- dores and the traveling public. The blanks will be sent to any navigator upon request. In response to numerous requests receiyed from mariners regarding the best methods of observing the heights, lengths, and periods of sea-waves, the angle and period of rolling, and the information desirable to be recorded concerning seismic waves, the Hydro- graphic Office recommends the follow- ing: Wave Heights. These should always be taken from ouch to erest. The best 'method that is available to the mariner who is without special instrumental equip- ment is to climb the rigging or other- wise place himself at a height above the deck sufficient for him to just see the horizon over the top of the wave- crest when the ship is in the trough. In some cases it may be necessary for the observer to lower himself below the level of the upper deck and make his observations from some _ position on the side of the vessel, as for ex- ample through a port. Thus, if the deck is 15 ft. above the water line and the observer must mount 10 ft. above the deck in order that he may just see a wave-crest level with the horizon, the height of the wave is 25 ft. above the water line. If, at the time of ob- servation the vessel rides upright in the trough of the wave, the observed height will be the required height of the wave from trough to crest without any correction or allowance whatever. Three or four sets of ten observations each, taken in this way, will give an excellent determination of the aver- age height of the waves running dur- ing the period of the observations. Wave Lengths. These should always be taken from crest to crest. With the exercise of care, good determinations may be ob- tained by direct comparison" of the 'known length of the observing vessel with the length from crest to crest of the waves over which she is riding. Of course it is necessary to note the angle which the keel of the vessel used as a measure of length makes with the line of advance of the waves, in order to reduce the apparent length to the true length. Another easy method of measuring wave lengths consists in towing a log line astern of a vessel and noting the length of line when a buoy attached to the after end floats on the next wave crest abaft that on which the stern of the vessel momentarily floats. For its successful application the ob- serving vessel should be placed end- on to the waves, or else allowance made for the angle the log line makes with the direction of the ship's course. Wave Periods and Wave Lengths. Both may be determined by one process as follows: Two observers are placed as far apart as consistent and at a known distance from each other on the deck of the observing vessel, At each station a pair. of battens is erected so as to define, when used as sights, a pair of parallel lines at right angles to the ship's keel. The observers at both stations note, by watches that have been previously compared, the instant when a wave crest crosses the line of sight; they also note how long an interval elapses before the next wave crest crosses their lines. Comparing their records imey determine (1) the. time . (say. t seconds) occupied by the wave crest in passing over the length (1 ft.) be- tween their stations; (2) the time (say t: seconds) elapsing between the pas- sage of the first and the second crest across either line of sight. The latter is the "apparent period" of the waves. Suppose the ship to be advancing at a speed of V ft. per second toward the waves, her course making an angle of a degrees with that course which would place her exactly end-on to the waves; then expressing these condi- tions algebraically: pees speed of wave (ft. per sec.) Real oe Of wave (it. per sec.) == Be (=v Yenc Real length of wave (ft.) = (V: + V ty eos a). ti = 1 cos a, -- t leosa t (ts Period of wave --= Vi t b-- Vt In a ship. steaming away from the waves it is necessary to change the sign V in the foregoing equations. It is evident that the simplest con- ditions for observing the lengths and periods of waves obtain when the ob- serving vessel is placed end-on to the waves and is stationary. The true pe- riod and true speed_of the waves can then be obtained by direct observa- tion, and the lengths calculated. Very useful observations can be readily made by two observers with an ordinary watch. One observer can keep his eye on the particular chosen point where the crests of successive waves reach the vessel, while the sec- ond observer keeps his eye on the watch, as the time draws near for the crest of the wave to reach the ves- sel, notes and writes down the time when the. first "Mark!" can make this kind of observation. observer sings out With practice one observer Whenever possible the method with battens should be given the prefer- ence. The times of several sets should always be reported, though three or even two sets would give a good re- sult. Caution. Waves usually come in sets, fol- lowed by a comparative lull, during which observations may be uncertain, and great care should be taken not to observe any of the uncertain waves. Confused conditions of the sea should always be avoided for observing, and the observations should be reserved for waves that appear in somewhat regular series. Be careful to note the direction toward which the wave is moving and the course and speed of the vessel. With these data the observed periodic times can be cor- rected for the vessel's motion when the observations are received in 'the Hydrographic Office. Observers may reduce their own observations if they prefer. Rolling Observations. Battens afford the simplest correct means for observing the rolling, and for several reasons are to be pre- ferred to the indications of pendulums or clinometers. Battens can be employed whenever