84 FORESTRY WORK 



Explained briefly, the rules are: 



To find reduced level, deduct staff reading from height 

 of collimation. 



To find difference between two points, find difference of 

 the reduced levels. When changing stand of level, add 

 first sight after change to last reduced level for new line 

 of collimation. 



To find gradient, divide the total distance in feet between 

 two points by the difference in height. 



For instance, if the drain at C is to be 2 feet 6 inches = 

 2 '50, and the point C found by reference to the reduced 

 level column to be 3*70 feet higher than A, the difference 

 between the level of the bottom of the drain at C and A 

 is 870 -2-50=1 20 feet. The distance between these 

 two points is 100 yards=300 feet. Therefore, 300 feet 

 must be divided by 120, which gives 250 as answer. 

 That is, 1 foot fall in 250 feet=the gradient of the drain. 



After finding the gradient of the whole length, to find out 

 how much fall there is in each section of the length, multiply 

 total fall by length of section and divide by whole length. 



1-20 feet x 50 yards _ 1-20 x 150 feet_ 180-00 _. 6Q 

 100 yards 300 300 



60 fall in 50 yards. 



On the other hand, given a gradient, we must find the 

 depth at each point. 



Suppose a drain with a gradient of 1 in 250 is required, 

 the depth of the drain at each point is found by dividing 

 the section by the length of fall for one foot. 



Thus the fall in 50 yards=150 feet will be f J=.60. 



Add this to the previous reduced level, and subtract 

 the total from the ground level. 



