DRAINAGE. 161 



point to the more inclined part of the place, and the 

 outlet will be the spot which is situated lowest. 



On most of extended tracts, we find empty beds of 

 old rivulets or other water courses. These are the best 

 guides for ascertaining the natural level of the place. 

 However, the site and run of the river, stream or other 

 channel, into which the principal ditch is supposed to 

 pass off its contents, is the decisive moment in settling 

 the question of the direction of this ditch. If feasible, 

 the direction should be the straightest possible, not only 

 for the sake of saving territory and labor, but because 

 the flow of water is then less damaging to the slopes of 

 the ditch than when checked by curvatures and 

 windings. 



In order to determine upon the width and depth of 

 the main ditch, the quantity of water which runs over 

 the place not only during summer time, but also during 

 floods and in wet seasons must be ascertained. Finally, 

 an examination of the quality of the soil, especially in 

 regard to its different layers, ingredients, water-table, 

 porosity, etc., must be made, because the correct laying 

 out of the trenches depends for the most part upon 

 these considerations. 



In planning out the particulars of a drainage system 

 for moors there should not only be taken into considera- 

 tion the best way to let off the water, and to prevent 

 forever the degeneration of moors into swamps, but 

 also the means to stop the outflow during the dry sea- 

 sons at will. If the subsoil be sandy, a deepening of 

 the level of the ground water sometimes becomes highly 

 objectionable because the vegetation in such case, dur- 

 ing a dry season, will not be so sufficiently developed as 

 it would be if the necessary humidity of the soil were 

 retained. 



Usually forest-trees thrive where the land extends 



