164 



METHODS OF COMMUNICATION FOR FOREST PROTECTION 



cation, operates on the fixed-flash system. In this system the revelation and obscura- 

 tion of the flash is accomplished not by the motion of the mirror itself but by 

 means of a separate screen placed between the mirror and the distant station in such 

 a way as to entirely cut off the light when closed but permit it to pass freely when 

 opened. Two tripods are required, one to carry the screen and the other the helio- 

 graph proper. The latter consists of two square plane mirrors, each 4| in. to a side, 

 and a mirror bar on which these are mounted when in use. As the mirrors when 



Fig. 100 American Army type of heliograph, one-mirror 



Fig. 101 American Army type of heliograph, two-mirror 



A, square mirror ; B, mirror yoke ; C, mirror adjusting screw ; D, unsilvered spot ; E, 

 tripod ; F, hook for weight ; G, sighting-rod ; H, sighting-rod vane ; I, mirror bar ; 

 J, screen ; K, screen key ; L, screen spring 



once set and aligned do not need to be moved except to adjust them to follow the 

 motion of the sun, the auxiliary parts are very simple, consisting merely of two 



