ENTOMOLOGY

278 SANITARY ENTOMOLOGYsystem. The tiles are laid at right angles to the flow of the seepageat the highest seepage water level, with a space of one-eighth to a quarterinch bet'Veen joints. The grade of the trench bottom must be true.Tiles must not be located on soft mud where they may sink. The outletshould be well above the ground surface (see Le Prince and Orenstein,pp. 180-186).Dr. C. W. Metz (1919) has set down certain very valuable principlesin drainage, and describes the methods of surface and vertical drainageused by the Public Health Service. The treatment depends upon thesources of the water. The methods described above will suffice for rainwater. For seepage water where tile drainage is not to be used theditches must be dug at right angles to the flow of the seepage water, thatis, across the exposed end of the water table. These ditches may be connectedto main ditches which 'will carry the flow down the hillside pa.rallelto the seepage flow. If the ,vater table is too deep to be intercepted byone ditch, it may be necessary to dig additional intercepting laterals atintervals lower down. A swiftly running ditch is better than a sluggishone. Water. confined in a narrow chdnnel will run more swiftly, give lesssurface, and be easier to oil, hence V-shaped ditches are usually preferableto wide-bottomed ones. The shape of the ditch will largely dependupon the nature of the soil. Where wide ditches are apt to form puddlesin 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 waterthrough relatively impervious soil into water-bearing sand or gravel.Such drainage is advisable only where surface drainage is difficult orexpensive. In case the underlying stratum is deep down, holes shouldbe bored and drain heads installed. The drain head will consist of aculvert-like box at the level of the bottom of the lake or pond which willconduct the water to the well. The receiving end will be screened to keepout debris with a coarse screen and a fine screen. The other end of theculvert is closed. Over the well will be a hole about one-fourth or onethirdthe diameter of the well, and this likewise will be covered with ascreen. 'A pipe or funnel from the hole in the culvert into the well willreduce washing and crumbling of the sides of the hole. Soft soils willrequire that the well be cased with tile or iron pipe.Anyone engaged in marsh drainage should familiarize himself withthe methods in vogue in the great salt marsh drainage work of the Stateof New Jersey (Headlee, 1915).When ditches become matted with algre and other matter and containmosquito Iarvre, in some localities it is possible to construct water gatesto permit temporary impounding of water, which will enable the ditch