I.'i2 PRELIMINARY WORKS. 



can 1)0 disposed of ]>\ evaporation, lilterinj:^ inio the snl)soil, 

 or surface drainage. An excess of water may be due to ex- 

 cessive rainfall, inundation, underground currents, or springs; 

 in the first two cases the swampiness may be only temporary. 

 The natural draining away of the excess water may be im- 

 peded by an insufBcient local gradient, by an impermeable 

 soil, or by both combined, the last being the usual case. 



]3efore removing the surplus water from a swampy piece of 

 ground, the expense and the effect of drainage on the sur- 

 rounding lands should be carefully considered. The cost is, 

 as a rule, considerable and sometimes prohibitive ; draining 

 a swamp may, especially in a hilly country, seriously reduce 

 the necessary degree of moisture of adjoining areas, the level 

 of underground water, the continuous flow of water in the 

 ordinary water channels, or even the amount of rain and 

 dew in the neighbourhood. Existing woods, which have 

 become accustomed to a certain degree of moisture, may 

 thus be seriously injured.* 



If, after full consideration, it has been decided to remove 

 the excess water, this can be done either by diverting the 

 water l)efore it reaches the swampy ground, or l)y draining 

 the latter. 



a. Dirrrfiion of Jurrss Wr/fcr. 



This is done by embankments in the case of inundation 

 water coming from rivers in low lands, or by ditches in the 

 case of spring water or surface drainage coming from higher 

 ground. 



Inundation water moving over the surface of tlie land may 

 be kept out of a certain locality by a surface embankment ; 

 if the water moves underground, it can only be stopped by a 

 substantial underground embankment, such as a stone or 

 concrete wall. 



Spring water and surface drainage in hilly ground is caught 



* It is belic-vfd that sonic of ihv finis in tlic Lonjr AValk. Windsor Vixrk. 

 died in cojisciiuiiici' of (ir!iinat,'t' can ird out sonic tinieajro; after tliese drains 

 liad. ill consciincncc. bt'eu blocked again, the remaining trees recovered. 



