278 SANITARY ENTOMOLOGY 



system. The tiles are laid at right angles to the flow of tlie seepage 

 at the highest seepage water level, with a space of one-eighth to a quarter 

 inch between joints. The grade of the trench bottom must be true. 

 Tiles must not be located on soft mud where they may sink. The outlet 

 should be well above the ground surface (see Le Prince and Orenstein, 

 pp. 130-136). 



Dr. C. W. Metz (1919) has set down certain very valuable principles 

 in drainage, and describes the methods of surface and vertical drainage 

 used by the Public Health Service. The treatment depends upon the 

 sources of the water. The methods described above will suffice for rain 

 water. For seepage water where tile drainage is not to be used tJie 

 ditches must be dug at right angles to the flow of the seepage water, that 

 is, across the exposed end of the water table. These ditches may be con- 

 nected to main ditches which will carry the flow down the hillside parallel 

 to the seepage flow. If the water table is too deep to be intercepted by 

 one ditch, it may be necessary to dig additional intercepting laterals at 

 intervals lower down. A swiftly running ditch is better than a sluggish 

 one. Water, confined in a narroAV channel will run more swiftly, give less 

 surface, and be easier to oil, hence V-shaped ditches are usually pref- 

 erable to wide-bottomed ones. The shape of the ditch will largely depend 

 upon the nature of the soil. Where wide ditches are apt to form puddles 

 in dry season, a small V-shaped ditch the width of a shovel may be made 

 down the middle of the large ditch. 



Vertical drainage consists of sinking wells to conduct the water 

 through relatively impervious soil into water-bearing sand or gravel. 

 Such drainage is advisable only where surface drainage is difficult or 

 expensive. In case the underh'ing stratum is deep down, holes should 

 be bored and drain heads installed. The drain head will consist of a 

 culvert-like box at the level of the bottom of the lake or pond which will 

 conduct the water to the well. The receiving end will be screened to keep 

 out debris with a coarse screen and a fine screen. The other end of the 

 culvert is closed. Over the well will be a hole about one-fourth or one- 

 third the diameter of the well, and this likewise will be covered with a 

 screen. A pipe or funnel from the hole in the culvert into the well will 

 reduce washing and crumbling of the sides of the hole. Soft soils will 

 require that the well be cased with tile or iron pipe. 



Any one engaged in marsh drainage should familiarize himself with 

 the methods in vogue in the great salt marsh drainage work of the State 

 of New Jersey (Headlee, 1915). 



When ditches become matted with algae and other matter and contain 

 mosquito larva, in some localities it is possible to construct water gates 

 to permit temporary impounding of water, whicli will enable the ditch 



