METHODS OP FLUSHING. 



151 



above the obstacle are charged with water. Water also 

 accumulates in the subsoil and soil, and in fact the 

 whole portion of the field under the influence of the 

 drains, becomes filled with water as completely as may be 

 desired. At any time when the drain may be opened, 

 there is a rush of water through the drains, by which 

 any sediment is effectively carried 

 away, and the drains left free and 

 clear. The operation may be re- 

 peated upon each division of the 

 field consecutively from the foot up- 

 wards. Instead of the plug above de- 

 scribed, an iron rod, having a curved 

 sheet of zinc, or tinned or galva- 

 nized iron, attached to the end, see 

 fig. 77, may be used. The curved 

 sheet reaches quite round the well, 

 and when drawn up opens the drain, 

 but when pushed to the bottom, 

 closes it. If the well is square, a slide 

 made to move in grooves may be used to close the drain. 

 The simpler the method, the less risk there is in its use ; 

 but the need of permanence of structure is obvious, for 

 if it gets out of flrder, nothing remains but to take up 

 the well and replace it. 



Whatever system of drainage is adopted, is immaterial, 

 if the main points here touched upon are provided for. 

 It must not be forgotten, however, that drainage is in- 

 dispensable, and that except under rare circumstances, 

 thorough subsoil drainage only will be sufficient to meet 

 all the requirements of the case ; and that surface drain- 

 age may be an unsatisfactory makeshift for the more per- 

 fect method. The size of tile used, is one inch for the 

 small drain, two inches for the laterals, and three or four 

 inches, or even larger than that, for the main drains. 

 The size of the main drains should bear a proper propor- 



Fig. 77. A CURVED 



DRAIN-STOPPEB. 



