THE HELIOGRAPH 165 



tangent screws by which each mirror may be turned on its horizontal and vertical 

 axes. The details of this type are shown in figs. 100 and 101 as used both with 

 one mirror and with two. 



The screen shown ,'n Fig. 100 is 6& in. square and is operated by a downward 

 push on the key causing the thin vanes of which it is made to align themselves with 

 their edges across the path of the beam of light, thus permitting it to pass through 

 to the distant station. Once the flash is aligned with this type of instrument it 

 remains fixed! and all movements are confined to the screen by which the dots and 

 dashes of the code letters are formed. 



The advantages of this type of instrument are as follows : 



(1) It is very simple in design with few small parts and none that are subject 

 to much wear in operation. 



(2) The square mirror has a greater area, f<pr practically the same packing space, 

 than a round mirror of the same diameter. 



(3) The movements in operation being confined to the screen, and there being 

 110 motion to the mirrors, make it much less likely that the latter will ba thereby 

 thrown out of adjustment. 



(4) Vibration caused by high winds has less effect on the fixed flash as seen 

 from the distant station than on the moving flash. 



(5) There is no possibility of a double revelation of the flash caused' by being 

 focused too high, as there is with the moving flash. 



(6) The "shadow spot" by which the adjustment of the mirror is regulated 

 reT Yio - continuously on the guide disk and not intermittently as with the moving- 

 flash type. 



3 FOREST SERVICE TYPE 



There has recently been invented by D. P. Godwin of the United States Forest 

 Service, a type of fixed-flash heliograph which is to a certain degree a compromise 

 between the British and American types of instruments and which promises to super- 

 sede the army types for many purposes in forest protection. This type, while retain- 

 ing the screen and operating by the fixed-flash method, dispenses with the extra tripod 

 by placing the screen on the tripod carrying the mirror and directly over its head 

 where the operation of opening and closing the screen will be least likely to cause 

 serious vibration of the mirror. Further, the construction throughout is designed 

 to ensure the least practicable weight and bulk and the utmost simplicity of parts. 

 As it weighs only 4 pounds complete and has about the bulk of a common post-card 

 size pocket kodak with tripod, it is a thoroughly practical patrol device and much 

 superior for this purpose to the other forms. In all essential respects, however, this 

 instrument is similar to the American Army type. Its general appearance is shown in 

 Figs. 102 and 103. 



Section 102 Heliographs at Lookout Stations 



Extreme light weight is not a feature of much importance for lookout or other 

 permanent, or even semi-permanent, stations. The large types of army heliographs 

 weigh from 15 to 20 pounds complete, which is no bar to their successful use at 

 stations of this character. On the other hand, for lookout service the qualities 

 required are fairly long range, dependability, and especially rigidity when exposed 

 to high winds. These are secured to a greater degree with the two army types than 

 with the Forest 'Service instrument. 



Either the British Army form or the American Army form may be used for 

 station instruments at lookouts. The 4^-inch, square mirror of the latter has almost 

 the same reflecting area as the 5-in., round mirror and therefore the same effective 

 range. 



In considering the relative merits of the two types of instrument it will be 

 noted that the British type has the disadvantage that the manipulation of the key :s 

 liable to throw the mirror out of alignment. This is impossible with the American type 



