SIGNALLING WJTE THE HELfOGRAPH 175 



seen to pass directly across the tops of the two range stakes. Move slowly back and 

 forth in the azimuth in which the station is known to lie until the call is acknowledged. 

 Then adjust on the flash from the distant station and proceed with the message. 



Section 110 Working at Short Ranges 



At ranges up to about 10 miles, it will be found that the flash from the heliograph 

 is extremely tiring to the eyes. For short ranges with 4-in. or larger mirrors, cover a 

 part of the station mirror with a pasteboard disk having a hole in the centre 2-in. in 

 diameter, or wear smoked glasses. Lookout men will generally be equipped with 

 coloured glasses 'and should wear them when reading heliograph signals. The heliograph 

 is more rapid and, at any but very short ranges, is more easily read than the flag, but it 

 has various disadvantages that will not ordinarily cause it to be preferred to the flag at 

 ranges under five miles unless it is necessary to signal through haze or smoke. For very 

 short ranges, the flag described in chapter XXII will be found preferable to the helio- 

 graph and more generally available. 



Section 111 Working through Haze and Smoke 



A remarkable and extremely valuable property of the heliograph flash in forest 

 protection is its ability to penetrate smoke. This increases with increase in size of the 

 mirror. With the standard American Army 4^-in. mirror the author has read 1 signals 

 with the naked eye at a distance of 15 miles, when even the outlines of the mountain 

 peak on which the sending heliograph was located could not be distinguished because 

 of smoke from forest fires. At the same time this station was in continuous communica- 

 tion with another, 30 miles distant, although neither was for days able to sjc the 

 mountain on which the other was located. This property, of course, has its limita- 

 tions and very dense smoke will make heliograph communication impossible but prob- 

 ably not till long after location of fires by lookout men has become fully as imprac- 

 ticable. 



Section 112 Heliograph Codes 



Four different codes have been used for heliograph signalling. These are: 

 (1) American Morse Code, (2) International Morse Code, (3) Myer Code, (4) Alpha- 

 betical Square Code. 



1 AMERICAN MORSE CODE 



This code is only used for telegraphic communication in the United States and 

 Canada. It is distinguished from the International Morse code by the "space" or 

 "pause" used between the elements of certain letters. It is not now used for visual 

 signalling although it may be so employed if desired, but has no advantages over other 

 codes. 



2 INTERNATIONAL MORSE CODE 



This is the telegraph code used generally throughout the world except in the 

 United States and Canadian telegraph services. It is also called the Continental code 

 and is the code most commonly employed for heliograph signals. All letters consist of 

 combinations of dots and dashes. With the heliograph these are made by short and 

 long flashes. 



The dash is represented by a flash of about two seconds' duration. 



The dot is represented by a flash one-third as long. 



The interval between flashes should be one-half second. 



The interval between letters, one second. 



The interval between words, two seconds. 



