132 The Control of Mosquitoes 



control seepage water outcrops by tile drainage, 

 with a minimum grade limit of one foot in two 

 hundred. The plan adopted is to intercept the 

 seepage water by tile drains placed approximately 

 at right angles to the line of flow of seepage water. 



The line of seepage outcrop is determined 

 during the wet season, when the seepage is most 

 pronounced, and levels taken with an instrument. 

 The proposed ditch to take the tile drain is staked 

 in the field, and a profile map made. The profile, 

 compared to surface conditions, will show any 

 changes that can be made with advantage, and at 

 times it is essential to try out several lines in order 

 to obtain the best and most economical location 

 for the tile drain. When the uphill portion of the 

 tile line had a long steep grade, no harm was done 

 by using a flatter grade for a short length toward 

 the outlet, as the excessive rains in Panama 

 caused the pipes to run full section or nearly so. 



The tile ditches are made as narrow as conven- 

 ient, and their bottoms are kept at the established 

 grade. The openings left between the successive 

 tile lengths are from one eighth to one quarter of 

 an inch, and the joints are not wrapped with 

 muslin or covered in any way. If soft spots are 

 found in the bottom of the trench, stones are 

 rammed into place until a solid foundation is 



