42 Forest Club Annual 



teen inches is a convenient size, in which case AB could be 

 taken as ten inches. This is laid off mid-way between the ends 

 perpendicular to the line which has been made parallel to 

 the top of the board. By proportion AB:BC = ab:bc. The 

 horizontal distance, ab, from the instrument to the point is now 

 decided upon. For areas with irregular boundaries one hundred 

 feet is most convenient for plotting. In the case of a section 

 of land and its subdivisions, five rods or eighty-two and one-half 

 feet is a convenient horizontal distance in plotting since it is a 

 common multiple of the dimensions of a section, a quarter sect- 

 ion, and a "forty." 



If a given height is taken, be, for instance two feet, three 

 quantities, AB, ab and be of the geometrical proportion are 

 known. The fourth quantity will be the size of the divisions 

 along the line, either way from B, the zero . point, and may be 

 calculated from the three known distances. Thi< gives each di- 

 vision on cf a reading of two feet elevation or depression for a 

 given horizontal distance. Let AB = 10 inches, ab = 100 feet and 

 be = 2 feet as already suggested. From the proportion AB:BC = 

 ab:bc. 10.0 inches : BC = 100 feet: 2 feet. Then 5C=0.20 

 inches. Points 0.20 inches apart are laid off along the line cf 

 either way from B as zero. Every fifth point is numbered 

 either way from the zero by the series 10, 20, 30, 40 etc. since 

 each division represents two feet. 



In taking a series of elevations in the field a rod is used 

 which has a mark on it at approximately the height of the top 

 of the board when the board is horizontal. The rodman holds 

 it on the desired point and the top of the board is sighted on 

 the mark. The reading is taken at the intersection of the plumb- 

 line and the line ef. If the horizontal distance from the instru- 

 ment to the rod is fifty feet instead of one hundred feet, the true 

 reading would be one-half of that shown by the board ; if eighty- 

 six feet the true reading would be eighty-six per cent and if two 

 hundred feet it would be twice that of the board and similarly 

 for other distances. 



It is well to equip the board with sights although experiment 

 has shown that if proper care is taken, readings can be made as 

 accurately by merely sighting along the smooth top of the board. 

 The instrument man, however, is likely to become careless is the 

 course of a day's work and in this case the sights will hold him 

 to a uniform manner of sighting. There is, moreover, less strain 

 on the eyes with sights than without, on account of the absence 

 of a haze which is always seen when sighting along the bare top 

 of the board. A peep sight at the end next to the eye and a 

 cross-hair ring at the opposite end make satisfactory sights. 



