METHODS OF CONVEYING INFORMATION 15 



an important element. In one of these, the chronosemic method, the basis is the time 

 interval that elapses between two displays of the same object. To form the letters of 

 the alphabet by the International Horse Code it is only necessary to be able to make 

 two symbols, a dot and a dash. This may be done by allowing a short time interval 

 between two displays of a flag, for instance, to stand for a dot and a longer interval 

 to stand for a dash. It is a method that is valuable, however, only when using for 

 display something that cannot be easily controlled as to the length of time it remains 

 visible. Such, for instance, are rockets or bombs of only one colour. The interval 

 between displays of these devices may be easily controlled and varied but the duration 

 of the display itself cannot be controlled. Smoke puffs or flashes from a hand mirror 

 are somewhat similar but any visible object may be used if desired. 



5 FLASH SIGNALS 



Instead of forming the dot and dash of the code letters by the time interval 

 between displays of an object, it is readily possible to form them by the time interval 

 during which the object is visible. This, of course, pre-supposes that the duration of 

 visibility is entirely under control. Although a great many different objects may be 

 used for this purpose, the actual application of the method is mostly made through the 

 medium of sun flashes from mirrors, or flashes from different forms of signal lamps. 

 The apparatus used being provided with suitable devices for controlling the duration 

 of the flash, it becomes readily possible to form the code letters by making a short 

 flash for a dot and a long flash for a dash. Other objects less frequently used are flags 

 and disks. 



6 ELECTRO-TELEGRAPHIC SIGNALS 



The ordinary telegraph may be easily equipped so as to furnish a written or 

 printed message. This might, perhaps, be included under visual signals though this 

 term is generally confined to non-electrical means of communication cnly. Similarly, 

 the mirror galvanometer used in submarine electric telegraphy is also a form of visual 

 signal. 



7 SHAPE AND COLOUR SIGNALS 



In addition to the foregoing there is a numerous and important class of signals in 

 which the operator depends upon using two or three objects of easily distinguishable 

 shape, or two or three lights of different colours, to form by their arrangement all the 

 letters of the telegraphic code. For the two-symbol codes, such as the International 

 Morse Code, only two shapes or colours are required. Each represents an element 

 of the code letters and by displaying them simultaneously in the proper relation to 

 each other, or separately in the proper sequence, the entire alphabet may be readily 

 formed. Thus, for day signals a ball may represent a dot, a cone may stand for a 

 dash, and with combinations of these two shapes displayed together all the letters of 

 the code may be formed. At night several methods are employed using coloured 

 lights, rockets, coloured balls, or bombs, and coloured lanterns. One method, known 

 as the Ardois lantern system, employs an ingenious arrangement of electric lanterns 

 which can be operated by a special switchboard from a distance and at considerable 

 speed. The range over which such signals may be seen, however, is comparatively 

 short. 



Section 16 Types of Audible Signals 



Audible signals are not of such a varied character as are the visual signals. They 

 are of two main classes : non-electrical and electrical, and may be tabulated as 

 follows : 



Non-electrical, (1) Detonations revolver or rifle shots, dynamite explosions, (2) 

 whistle or trumpet blasts, (3) bells. 



Electrical, (1) Electric telegraph or telephone with wires, (2) electric telegraph 

 or telephone without wires. 



