USE OF THE HELIOGRAPH 



171 



The position of the sun must be the guide for determining whether one or two 

 mirrors should be used. When the sun is in front of the operator (that is, in front 

 of a plane through his position at right angles to the line joining the stations) the sun 

 mirror only is required; with the sun in the rear of this plane both mirrors should be 

 used. When one mirror is used the rays of the sun are reflected from the sun mirror 

 direct to the distant station; with two mirrors, the rays are reflected from the sun 

 mirror to the station mirror, thence to the distant observer. Care should be taken to 

 note in which direction the sun is moving so that it may not be necessary to change 

 from one mirror to two while signalling-. 



2 EMPLOYMENT WITH ONE MIRROR 



Run the winged nut down to the lower end of the threaded shank of the head- 

 bolt. Push the bolt up through the tripod head and slip over the lug the main mirror 

 bar (the one with hole for sighting-bar), taking care that the side of the bar bearing 

 the maker's name is up and that the bar points directly away from the distant station 

 and back toward the operator. Turn the lug on the head-bolt at right angles to the 

 long axis of the mirror bar and slip under it the angle foot of the screen, so that as 



Fig. 105 Forest ranger sending message by heliograph 



you stand facing the instrument and the distant station the spring of the shutter is on 

 the right. To accomplish this, the angle foot of the screen must, of course, be inserted 

 from the side toward the distant station. The screen should be at right angles to the 

 mirror bar. Clamp both bar and screen firmly in place by tightening up the winged 

 nut. 



