170 METHODS OF COMMUNICATION FOR FOREST PROTECTION 



mirror thus equipped is then known as the "station mirror" and reflects the light to 

 the distant station. The other reflects the light from the sun to the station mirror 

 and is known as the "sun mirror." All metal parts are brass or aluminium bronze and 

 are finished in dull black. 



(c) Screen. The screen consists of a metal frame 4f in. square. Within this 

 frame, swung on pivots, the lines between which are horizontal, are four thin metal 

 leaves or vanes. At one side of the screen is a movable bar connected by levers to the 

 vanes in such a way that when it is pressed downward the vanes rotate on their hori- 

 zontal axis through an angle of 90 and thus open the screen. On releasing the bar, 

 a spring causes it to fly back to its original position and with it the vanes, thus closing 

 the screen. On the bottom of the screen frame is an angle foot by which it is attached 

 to the head of the tripod. Near the bottom of the sliding-bar is a small projecting 

 lug with a hole in it through which is passed a string by which the screen or shutter is 

 opened and closed while being used for signalling. An adjusting screw near the top of 

 the retractile spring enables it to be tightened if it becomes weakened through use. 



(d) Sighting -rod. This is a round metal bar S| in. long, flattened and pointed 

 at one end. To this flattened portion a small metal vane is pivoted which carries a 

 white target at the top for use in adjusting. This sighting-rod fits in a round hole in 

 one of the mirror bars in which it is held by a set-screw. 



(e) Mirror Bars. Two are required. They are similar in construction except 

 that the one used with the sun mirror has a hole near the tripod attachment for the 

 insertion of the sighting-bar, and a set-screw for holding it in place. This ! bar is also 

 equipped with a tangent adjusting screw at the end, by means of which the mirror 

 mounted on it may be rotated on its vertical axis. The bare are 9 in. long, rectangular 

 in cross-section and enlarged at the end that fits on the tripod to a circle 2 in. in 

 diameter. At the other end, both have a round, tapered hole in which the studs on 

 the frames of the mirrors fit. A pivoted catch on the underside of each bar holds 

 the mirrors when in place. The slow-motion adjusting screw provided on one of 

 the bars together with the similar screw on the mirror enable the operator to rotate 

 the sun mirror simultaneously on both its vertical and its horizontal axes. These axes 

 intersect at the centre of the mirror at the point where the unsilvered spot is located, 

 and by means of the two adjusting screws the mirrors may be made to follow the sun 

 while the instrument is in use. 



(/) Tripod. A light but strong tripod with a flat, circular, brass head-plate is 

 provided. The cross-section of the legs is one-third of a circle, so that when folded 

 the tripod forms a neat cylindrical package. In the centre of the brass head is a 

 round hole in which is fitted a bolt having a three-wing nut at the lower end. The 

 upper end is provided with a lug. This lug fits in holes in the mirror bars and over 

 the angle foot of the screen, and by tightening the winged nut the various parts may 

 be attached firmly to the head of the tripod. A cylindrical sole-leather cup with strap 

 and buckle fits over the points of the tripod legs when folded for transportation. 



Section 105 Using the Godwin Heliograph 



1 SETTING UP 



Set the tripod firmly on the ground with one leg toward the distant station and 

 the metal head as level as possible. A station heliograph on hard ground, rock, or a 

 wooden platform may have the legs set in pails, boxes, etc. filled with earth. To pre- 

 vent vibration in heavy wind, suspend a bag filled with sand or earth beneath the 

 tripod so that it touches the ground just enough to keep it from swaying (Fig. 105). 

 In the Godwin type this cannot be done until setting up is completed as the weight 

 must be attached to the tripod head-bolt. Other types are provided with special 

 anchoring hooks for this purpose. 



