110 LAND DRAINAGE 



or simple ditch, excepting that the grade stakes may be 

 driven at intervals other than 50 feet. In laying out the 

 main, the grade stake usually establishes also the point 

 at which the laterals connect with the main and, there- 

 fore, the starting point of the laterals. If the laterals 

 are to be located at intervals of 100 feet, and the laterals 

 on opposite sides of the main are to alternate and to lie 

 at right angles to the main, as shown in Fig. 26, then 50 

 feet is the proper interval to be adopted between grade 

 stakes. If the laterals on one side of the main are to be 

 located at intervals of 60 feet, and are to lie at right 

 angles to the main, then the grade stakes should be set 

 at intervals of 30 feet. In other words, the interval be- 

 tween any two grade stakes is one-half the interval be- 

 tween any two laterals on one side of the main. This 

 is, of course, under the assumption that the intervals 

 between laterals are uniform. 



151. Fifty-foot intervals. The 50-foot interval be- 

 tween grade stakes is chiefly desirable because in thinking 

 of, and discussing, the fall of drains, the fall in inches 

 is almost invariably compared with 100 feet of length of 

 drain. When a drain is said to have a fall of three inches, 

 a fall of three inches in 100 feet of drain is meant. Fifty 

 feet is just one-half of 100 feet, and if the rate of fall is 

 3 inches for 100 feet, then the fall in 50 feet is 1| inches. 

 If the interval between stakes is any other than 50 feet, 

 some other factor than one-half must be used in deter- 

 mining the fall between stakes. The next easiest dis- 

 tance to use for intervals between stakes is 25 feet, which 

 is one-quarter of 100 feet, and the next is 33 feet 4 inches, 

 which is one- third of 100 feet. 



152. The relation of angle of approach to the main 

 to the actual distance between laterals. In some 



