THE IRRIGATION AGE. 



19 



the amount which the grade should rise in a distance equal to the 

 length of the base of the triangle to secure the fall which is best to 

 convey the water, according to character of soil, etc., a matter which 

 will be discussed later. Under one end of the base fasten a small 

 block with a thickness equal to the desired rise. Below is given a 

 table, showing the thickness of blocks which should be used on tri- 

 angles of different lengths to give various grades. 



Amount of fall secured and thickness of blocks required with triangles of several lengths. 1 



1 The numbers I to 10 ni the head of the columns are the number of feet of fall in the ditches per 

 mile of length The fractions below these numbers give in inches the fall which must be allowed in 

 the length of the triangle. These are correct to the nearest one-sixteenth of an inch 



When a block of required thickness to give the desired grade has 

 been fastened to the triangle, drive a peg at the starting point with 

 its top, say, 6 inches from the proposed bottom of the ditch. Place 

 the end of the leveling apparatus under which the block is fastened 

 upon this peg with the other end pointing in the general direction 

 from which the ditch is to come. The bottom of the block must rest 

 upon the top of the peg. Bring the apparatus to a level and set a peg 

 6' inches long so that its top just touches the bottom of the forward 

 end of the apparatus. The lower end of this last peg will then mark 

 the bottom of the proposed ditch. This operation will be simplified 

 by putting a leg just 6 inches long upon the forward end of the trian- 

 gle. It will then be only necessary to swing the triangle around un- 

 til the base is level, when this leg will rest upon the bottom of the 

 proposed ditch. Drive a peg here, which will, like the first, be 6 

 inches high from the ditch bottom, carry the triangle forward to this 

 peg, and proceed as before. 



Contour lines for checks or distributing ditches can be located wtth 

 the aid of the triangle. To locate a contour line (a line passing through 

 points of equal elevation), as required in the construction of a check 

 levee, drive a peg until its top has a convenient elevation from the 

 ground, say 1 foot. Put a leg of equal length on one end of the tri- 

 angle and rest the other end on the peg, then swing the triangle 

 around until the plumb line shows the base to be level. At this point 

 drive a second peg and proceed as before. If the pegs are driven so 



