485 When transmitted by semaphore, the commander in chief or the ship having a P. D. L. dispatch, hoists the third repeater(1) At the yardarm as a call to the adjacent ship. The third repeater remains at the yardarm during the transmission of the dispatch.

(2) Hauls down the third repeater when the dispatch has been transmitted to the next adjacent ship.

486. On seeing the third repeater, ships astern (or ahead) hoist their third repeater

(1) At the yardarm as soon as ready to receive and record. The third repeater remains at the yardarm during the time the dispatch is being received and transmitted to the next in line. The dispatch should be transmitted part by part as received.

(2) Hauls down when the dispatch has been retransmitted to the next ship.

487. Complete transmission includes the reception of the received sign from the receiving unit. This procedure enables the originating ship under favorable circumstances to know how far down the line or column the dispatch has progressed at any instant.

488. In case a dispatch is passed down the line and is intended for a certain ship only, and therefore does not concern the ships through which it passes, the procedure sign "T" is to be used. In this case the letters PD L are not to be used.

FLASHING LIGHT METHODS OF TRANSMISSION.

489. In systems using the flashing light method of transmission the International Morse Alphabet and numerals and special Morse signs are used.

490. The characters are made by alternately exposing and obscuring the light, a short flash representing the "dot" and a long flash the "dash."

491. The characters shall be made deliberately and distinctly. 492. The dots and dashes, and the spaces between them, should bear the following ratio:

A dot is taken as a unit.

A dash is equivalent to three units.

An acknowledging flash is equivalent to six units.

A space between two elements of a letter or sign is equal to one unit. Thus, the letter "D" (—..) has three units for the dash, one unit space, one unit "dot," one unit space, one unit "dot." A space between two complete letters or signs is equal to a dash or three units.

A space between words, or groups, is equal to two dashes or six units.

493. This ratio should be carefully adhered to whatever be the rate of transmission. In especially slow transmission, as with heavy searchlight apparatus or with sluggish lights, it is difficult to appreciate the relative length of dots and dashes, and experience has shown it best to accentuate the dots making them shorter in proportion to the dashes.

494. Under favorable conditions transmission should be at the rate of 10 words per minute.

495. When transmitting at night care must be taken to select a position not in the immediate vicinity of, or in line with, other lights. All unnecessary lights in the vicinity of the signal bridge should be turned out.

496. Signals by flashing light are to be executed:

(1) On the termination of the 10-second flash of the "execute," or

(2) At the time designated in the signal, or

(3) On arrival at the position designated in the signal, or
(4) In the case of certain signals, as soon as made and
received.

497. Signaling by flashing light includes:

(1) Searchlight.

(2) Yardarm blinker.

(3) Blinker tube.

(4) Any other effective method of displaying or projecting light.

498. Searchlight.-At night the beam should be projected where it will be most clearly distinguished by the receiving unit(s), but not directly on the receiver.

499. During daylight, the searchlight must be accurately trained on the receiving unit. Signal searchlights should be fitted with a sighting device to insure this being accomplished. The searchlight is the most efficient method of transmitting visual signals long distances. Each ship shall have a portable signal shutter fitted to, and available for use with, one of the upper fighting searchlights for long-distance signals.

500. Yardarm blinker.-The yardarm blinker is an "all around" light located at the yardarms and operated by a key on the signal bridge.

501. Blinker tube.-When circumstances render it advisable that a signal light shall not be seen, except in the direction of the reiver, the blinker tube shall be used. This consists of a portable

tube about 24 inches long and about 3 inches in diameter, dull blackened on the inside. At the rear end a 5-candlepower quickacting electric lamp is fitted which is operated by a key. Blinker tubes may receive power from the ship's circuit, but, if so connected, additional blinker tubes receiving power from batteries only shall be fitted and retained for emergency. The tube must be pointed directly at the receiver. When less brilliancy is required, as on clear dark nights, the light may be dimmed by veiling it with one or more thicknesses of bunting; this veil should be inserted well down the tube in order that radiance from it shall not be visible laterally.

502. When transmitting signals or P. D. L. dispatches by blinker tube, two tubes shall be used, one toward the ship from which the signal is being received and one toward the next ship in the opposite direction. This permits the signal to be transmitted part by part as received and greatly reduces the time of transmission.

SIGNALS BY FLASHING LIGHT.

503. The following procedure is prescribed for signals transmitted by flashing light:

TRANSMITTING SHIP.

(1) Makes call of unit(s) for whom signal is intended several times or until repeated correctly by adjacent ships or unit leaders.

(2) Makes own call until repeated correctly by adjacent ships or unit leaders.

RECEIVING UNIT.

(1) Repeats call as made by transmitting ship.

(When the receiving unit is required to repeat she shall continue repeating as long as the transmitting ship continues to call.)

(2) Repeats call as made by transmitting ship (transmitting ship's call).

NOTE. In fleet searchlight signals under way, especially when supplementing flag or radio signals, the above may be omitted if no confusion will result and the procedure begun at (3).

Should there be a second signal to be executed simultaneously with the first, the second signal is separated from the first signal by the "full stop" (.—.—. The "full stop" shall be repeated by the receiving unit(s).

504. The following procedure is prescribed for dispatches by flashing light when transmitting to one ship:

505. The proper location of the prescribed procedure signs when their use is required is indicated in the example (art. 466).

506. When transmitting to more than one ship the procedure for the transmitting ship is the same as above. The procedure for the "receiving ship" is the same, except the answering flash required under (3), (4), (5), and (6) is omitted.

507. When transmitting to one ship the omission of the answering flash indicates the word or sign just made was not received, and that repetition is desired. The transmitting ship should repeat the last word or sign immediately. When the transmission is to more than one ship, the request for repetition shall not be made until the complete dispatch has been transmitted. As a rule, the commander in chief will repeat only when there are a number of ships requesting repetition. Individual ships failing to receive a dispatch intended for them should request a repetition from the nearest ship which received it, preferably from one of the same unit. If two ships are equally near, that which is nearer the transmitting ship shall be responsible for the repetition.

SIGNAL OF EXECUTION.

508. When signals are transmitted by more than one method, the signal should be executed on the first signal of execution received, whether it be by flag, radio, or flashing light. The possibility of a ship's signal halyards being carried away should be kept in mind.

SEMAPHORE.

509. Semaphore is the standard system of transmitting dispatches during daylight for short and medium distances. While under ordinary circumstances it would not be used for the transmission of signals, it may be used either as the primary method of transmission or as a secondary method to supplement flag signals then displayed.

510. It employs two hand flags from 15 to 18 inches square, and either blue and white similar to the international flag "P" or red and yellow similar to the international flag "O," the color to be used which affords the better contrast to the background. The flags should be attached to a light staff about 2 feet long.

511. The sender should select a background giving the greatest contrast. Except under special conditions of light and when the sun is in line with and back of the sender, the sky affords the best background.

512. As a general rule, conditions being favorable, semaphore dispatches should be sent at the rate of 20 words per minute. The arms must be placed at the exact position indicating the letters, a distinct pause being made at each position and the arms moved from position to position by the shortest route.