THE HELIOGRAPH 



163 



or on the near side of the distant station and is not visible therefrom. This mirror 

 is mounted directly over the head of the tripod on which the instrument is sup- 

 ported when in use, and various parts required for its proper alignment, adjustment, 

 and operation are provided. For use when the sun is behind the operator, a second 

 mirror is employed. The details of this instrument may be seen in Figs. 98 and 99, 

 wherein it is shown as employed with one and with two mirrors. 



HELIOGRAPH WITH DUPLEX MIRROR 



Fig. 99 British Army type of heliograph, two 



List of Parts, Figs. 98 and 99: A, signalling mirror in metal frame; B, U arms for ditto; C, 

 tangent box with lid below ; D, tangent screw head ; E, key ; F, key spring ; G, capstan screw, 

 H, collar ; I, vertical rod in vertical socket ; J, clamping screw for ditto ; K, jointed arm ; 

 L, clamping screw for ditto ; M, gun clip ; N .clamping screw at end of arm ; O, sighting rod ; 

 P, sighting vane and mark ; Q, unsilvered spot ; R, German silver ball attached to claw R' ; 

 S, base plate ; T, key bridge ; U, key bearing ; V, anchoring hook ; W, duplex mirror ; X, 

 U arms for ditto; Y, butterfly or pivoting screw; Z, tripod. 



This type of instrument has the following advantages : 



(1) The sending key is also one of the adjusting screws and the operator can 

 therefore have both hands on the adjusting screws even while sending. 



(2) Only one tripod 1 is required and no screen is necessary. 



(3) The instrument is manufactured in Canada and more readily procurable 

 than other types. 



(4) Operators skilled in the use of this type may be secured in Canada. Other 

 types are practically unknown. 



There are some disadvantages, however, as compared to other ^ types, several of 

 which are particularly objectionable for the uses to which the heliograph is mostly 

 put in forest protection. These will be referred to in considering the advantages 

 of the several other types .available. 



2 AMERICAN ARMY TYPE 



The heliograph adopted by the United States Army Signal Corps,^ which is the 

 type that has been most extensively employed to date in forest-protection communi- 



79211 Hi 



