52 



A MANUAL OF TOPOGRAPHIC METHODS. 



for the instrument is not unfrequently obtained by sawing off the top of a 

 high tree, and setting the instrument upon the stump, 50 or 75 feet above 

 the ground, the tree being guyed out by wire cables to prevent swaying in 

 the wind. The platform for the observer is supported by neighboring trees, 

 similarly sawed off and supported for the purpose. Similar devices are 

 resorted to also in the forests of West Virginia, Kentucky, and Tennessee. 

 In the secondary triangulation in these regions, the instrument support is, 

 in man}' cases, provided as above described, while the observer's platform, 

 instead of having an independent support, is attached to the same tree. This 

 is objectionable, but is often the best that can be done. 



Fig. 5 Coast Survey Heliotrope. 



In other cases it is more economical to support the instrument Upon the 

 ground, and to have openings made through the forest upon the station hill, 

 in the directions of the sight lines, or even to ha^e the whole summit cleared. 



It is not infrequently necessary to use more elaborate forms of signals, 

 especially when the point observed upon is below the horizon line, so that 

 the background, instead of being the sky, consists of forests or brown plains. 

 In such cases resort is had to heliotropes. These are simply instruments for 

 reflecting the sunlight to the observer at the instrument. The simplest form 

 is a circular mirror with a screw hinged at the back, giving a universal 

 motion. This is screwed into a stake or tripod over the center of the station 

 to be observed upon, and a ray of sunlight is thrown through a small hole 

 in a board nailed to a stake 10 or 15 feet away, and in the direction of the 

 observer at the distant station. This form has the advantage of simplicity, 



