The lights in the Ardois system--named after its inventor--sometimes called "shroud lights," are placed well up on the foremast. They are red and white electric bulbs. There are four of each placed in a line one above the other, in groups of two--- a red and white bulb together. Unlike the "wigwag" system, the whole letter is shown at once. The code is the same as the "wigwag." One is indicated by a red light, two by white, and three by the combination, white, white, red and white. Both systems may be mastered very easily by a little painstaking practice, and much amusement may be had through the mystification of those who do not understand it. A "wigwag" flag may be easily made by sewing a white square of muslin in the centre of a red bandana handkerchief. The best method of learning this system is to send simple messages, looking up the letters that there is any doubt about, and correcting mistakes as you go along. APPENDIX. NAVY CODE FLAGS. Messages sent by the navy code flags cannot be read except by the aid of the code book. There are ten numeral flags--1 to 9, and one for 0. All messages are made up by means of these ten flags headed by the code flag (whether it be geographical, telegraph, or navy list). For instance, a line of bunting is sent up on the flagship's signal halliards. It is read from the top down. The geographical flag flies first; then follow 7, 6, 3, 8. It means that the message can be found in the geographical list, number 7638. The repeaters are used to avoid confusion. Instead of putting two number 1 flags together, for instance, number 1 is flown with a repeater under it; second repeater repeats number 2, and so on. PREPARATORY.--Over hoist. Prepare to execute subjoined order. INTERROGATION.--Alone. What is that signal? or "I don't understand--repeat." Above hoist puts signal in interrogative sense. ANSWERING.--Flown by ship receiving message indicates that signal is understood. AFFIRMATIVE.--Alone. Yes. Above hoist puts message in affirmative or permissive sense. NEGATIVE.--Alone. No. Above hoist puts message in negative sense. MEAL or NUMERAL.--Alone. Crew at mess. Above or below hoist--the numeral flags are to be taken as numbers simply. CONVOY.--Alone at fore, means naval convoy. Above hoist means use navy list. POSITION.--In manoeuvres, hoisted by each ship as it gets into position ordered; lowered when next ship gets into place. GUARD or GUIDE.--As its name implies--flown by guard or guide ship. TELEGRAPH.--Use telegraph list. DESPATCH or GEOGRAPHICAL.--Alone at fore, indicates that the ship flying it is carrying despatches. Above hoist. Use geographical list. CORNET.--Alone. Ship about to sail. Over number. Official number of ship. GENERAL RECALL.--Recalls all small boats. POWDER.--Hoisted alone in port. Taking powder on board. Alone at sea. Distress. RATING MARKS IN THE UNITED STATES NAVY. THE INSIGNIA OF RANK OF COMMISSIONED, WARRANT, AND PETTY OFFICERS.