PRINCIPLES OF MOSQUITO CONTROL 281 



constructed promptly show a rapid and vigorous growth of foxtail 

 which serve to hold the soil of the dike together and give it much greater 

 resistance to damage by the high tides. 



In the construction of these dikes, at various points it is necessary 

 to cross soft spots or deep soft ditches and thorofares. In such cases it 

 has been customary to run a single row of heavy sheet piling across the 

 mouth of the ditch and to bank the soil deeply on either side. In other 

 cases two rows of sheet piling from six to twelve feet apart have been 

 driven and the filling of mud has been tamped in between. More recently 

 the foxtail weeds have been cut and laid in the mud crosswise, forming 

 a mat over the soft spot or stream to be crossed. The soil is then 

 thrown upon these reeds and tamped down, giving a crosswall without 

 any other reinforcement. Thus far, this method seems to be fairly satis- 

 factory in preserving the dike at such points. 



At points where streams or larger ditches cross the dikes, sluice 

 boxes and tide gates were introduced. The largest sluice box used meas- 

 ured inside four feet high, six feet wide, and twenty-four feet long. It 

 was made of two-inch lumber nailed to outside ribs eighteen inches 

 apart. The box was set on two rows of two-inch sheet piling and then 

 covered with soil. A large heavy wooden door was suspended over the 

 downstream end of the box to serve as a tide gate. 



More recently some larger sluice boxes and tide gates have been con- 

 structed. Still more recently a proposition of using the same cross- 

 section in the form of numerous small sluices and tide gates at the same 

 spot has been tried. It is held that this last method is materially cheaper 

 and at least equally as satisfactory as the larger sluices. Still another 

 modification in tide gates has come into use. This is the use of gal- 

 vanized corrugated iron pipe carrying a cast iron flap or gate. This 

 sluice and gate is bedded on sheet piling and is rather extensively used. 

 In general, the results seem to be about the same as the results of the 

 wooden sluices. 



In earlier practice along the Delaware Bay coast sluice boxes were 

 made small and so deeply buried that at all times they were completely 

 under water. In much of the mosquito work the sluice boxes have been 

 set at a point where they are not continuously covered by water. There 

 is no doubt in the writer's mind that the practice of setting the sluice 

 box and gate so low that it is continually covered by water results in 

 longer and better service, but the cost of the operation of setting sluices 

 and tide gates this low is much greater. 



Experience has clearly demonstrated that for all types of tide gates 

 the piling must be driven well into the soil substratum for any move- 



