Location of Mains and Laterals 



475 



lowest, and that if a straight main were laid through 

 these two points it would be given the course along 

 which surface water would naturally flow, which is 

 also the direction of steepest slope. 



The dotted lines in the figure are contours, or 



Fig. 156. Arranging drains to secure the maximum fall. 



lines of equal elevation, and as in this case these 

 are circumferences of circles with centers at station 

 1-1, it is clear that the shortest distance between any 

 two contours will be measured along their radii, and 

 hence, that there also will be the greatest fall. Since 

 the diagonal line from VI- 6 and the lines I and 1 



