LAYING OUT A DRAIN OR SYSTEM 113 



To determine the distance between laterals when they 

 enter the main at a distance other than 100 feet, multiply 

 distance by the relation factor of the angle at which they 

 approach the main. Example : If laterals enter at 

 distance 70 feet and approach at an angle of 50 (the 

 factor for 50 is .766), 70 X. 766 = 53.620 feet. The dis- 

 tance between laterals is 53.62 feet or 53 feet 7| inches 

 nearly. 



153. Laterals. The laying out of a lateral is in no way 

 different from that of a simple drain, as described in 

 paragraph 146, excepting that the laterals discharge at 

 their lower terminal into the main or sub-main, and not 

 at an outlet. It is most convenient to drive the grade 

 stakes at intervals of 50 feet, for reasons given in para- 

 graph 147, and for the further reason that a greater dis- 

 tance than 50 feet increases the difficulty in using the 

 boning line. 



154. The angle of approach for laterals. It is com- 

 mon, in systems like that illustrated in Fig. 25, to locate 

 the laterals so that their upper angle to the main shall 

 be less than 90 degrees. If, however, it should be deemed 

 advisable to run the lateral at right angles to the main, 

 as shown in Fig. 26, then they should be turned slightly 

 as they approach the main so as to enter at an angle of 

 less than 90 degrees, the reason being that if the water 

 from the lateral is discharged into the main at an angle 

 of 90 degrees, it is likely to interfere with the movements 

 of the water and also with the ready movement of the silt 

 which may be carried by the waters of the main. An- 

 other factor, however, that must enter into the angle of 

 approach is the position and shape of the area requiring 

 drainage. (See Figs. 28 and 29.) The angle of approach 

 must be determined by the needs of the land and economy 



