174 METHODS OF COMMUNICATION FOR FOREST PROTECTION 



same time. This must be carefully guarded against in setting up the instrument, as 

 otherwise it becomes nearly impossible for one man to maintain proper adjustment 

 during operation. 



When receiving a message the operator must be careful to remain in position at 

 his instrument so that he may interrupt the sending station if necessary. He must 

 also keep his heliograph in adjustment constantly both when sending and receiving. 



Section 108 Maintenance of Adjustment 



The importance of careful attention to adjustment arises from the fact that the 

 lie-lit from a heliograph mirror is projected in the form of a cone, the lateral range 

 of which theoretically bears the same proportion to the distance between stations as 

 the diameter of the sun bears to its distance from the earth; that is approximately 

 1:107. Thus with the mirror aligned correctly on the distant station, the light will 

 be visible on either side of the station up to one-half of the distance between stations 

 divided by 107; for example, if the stations are six miles apart, the flash theoretically 

 will be visible about 50 yards on either side of the station. In practice, owing to 

 imperfections of mirrors, the lateral range is somewhat greater and the intensity of 

 the light is less at the outer edge of the cone than at its axis. This fact together with 

 the very small lateral distance in wnich the light is visible, makes it essential that 

 for the best results the alignment bu made carefully so as to have the axis of the cone 

 of light strike the distant station, and the adjustment maintained constantly. Perfect 

 adjustment is secured only by keeping the shadow spot uninterruptedly in the centre 

 of the paper disk, and, as this spot continually changes position with the apparent 

 movement of the sun, when two men are at a station, one should be in constant attend- 

 ance on the tangent adjusting screws. When working alone, operators must watch 

 their adjustment very closely to ensure that the light reaches the distant station. A 

 little practice will soon show how far the spot may move from the exact centre of the 

 paper disk before the flash becomes invisible at the other station. Extra care bestowed 

 upon preliminary adjustment is repaid by increased brilliancy of flash. Remember, 

 the distant observer is unquestionably the better judge as to the character of the flash 

 received; and if, therefore, adjustment is called for when the shadow spot is at the 

 centre of the disk, the alignment is probably at fault and should be looked after at 

 once. 



Section 109 Opening Communication 



In forest protection the locations of all permanent heliograph stations will be 

 known to all members of the force. To attract the .attention of a station once the helio- 

 graph is aligned on it, send a succession of flashes until answered. The heliograph flash 

 is strikingly noticeable, but attempts to attract attention, to be successful, must be 

 persistent. They should never be abandoned until every device has been exhausted. 

 When acknowledged, each station will then turn on a steady flash and 1 adjust. When 

 the adjustment is satisfactory to the station called, it will cut off its flash and the call- 

 ing station will proceed with its message. 



The exact location of semi-permanent stations, especially when placed at a low 

 level, is sometimes difficult. The compass bearings of all such stations from the helio- 

 graph stand should always be taken by each permanent station operator. A still better 

 method is to set two stakes about twenty yards apart with the tops directly in line with 

 the distant station, marking on the far stake the name of the station that it indicates. 

 Where stations are so located that smoke from forest fires is likely to obscure them, a 

 whole series of such guide-line posts should be permanently erected showing the line to 

 each station within the range of communication. With such, stakes accurately aligned 

 it is then possible to call any station within range even though the station may be 

 temporarily obscured by smoke. To call under these circumstances, loosen the catch on 

 the underside of the mirror bar and rotate one of the mirrors by hand until the flash is 



