164 FOREST PLANTIITG. 



In large, extended plains a fall of 1.6000 may be suf- 

 ficient for a regular flow of the water ; usually the pro- 

 portion of 1.3000 is considered desirable for the proper 

 discharge of the water, and in smaller ditches even a 

 greater fall may be given without danger of injury to 

 the base or sides of the ditches. 



The transverse ditches which discharge their water 

 into the main ditch should have the least possible fall, 

 because, if thus constructed, they can easily be shut up 

 whenever required and the water therein retained during 

 a dry season. Usually they are so planned that during 

 the wet seasons they discharge the water at a proper 

 depth, while during the dry summer seasons, the water 

 in the main ditch is stopped and forced to flow back into 

 the lateral ditches, in order to furnish the soil with the 

 necessary moisture. This is especially desirable in sandy 

 soils. 



The above suggestions may be sufficient for guidance 

 in draining smaller areas, and where no great local dif- 

 ficulties have to be overcome. However, where the proj- 

 ect assumes larger dimensions, and the obstacles to be 

 surmounted are of more than usual magnitude, an ex- 

 pert should be called in for surveying, mapping and 

 laying out the field, while the employer will be greatly 

 assisted by the rules hereinbefore given to ascertain that 

 the planning and execution of the work will prove 

 satisfactory. 



