CONDITIONS INFLUENCING PEEVALENCE 171 



It is obvious therefore that hydrauhc sanitation can do 

 much to diminish malaria, tiiough in some cases, as in the 

 Roman Campagna, the works required may, as pointed 

 out by Celli (CM., p. 126), be of such magnitude as to be 

 beyond our present financial and engineering resources ; 

 but there are many cases where much good can be effected. 

 The level of the true subsoil water can as a rule be 

 modified only by works of a public character, as the neces- 

 sary works must usually be undertaken at a considerable 

 distance from the properties they are intended to benefit. 

 AVhat is usually spoken of by agriculturalists as " subsoil 

 drainage " only indirectly affects the ground water level, 

 as the pipes or rubble drains, &c., employed are placed at 

 but a little depth from the surface and nearly always much 

 above the true subsoil water level. The only works that can 

 directly affect the ground water level are the straightening 

 and regrading of the channels which form its natural 

 outlets. 



Harbour works for example, which involve the removal 

 of tidal bars, may have an important influence of this kind, 

 and there are other cases in which locks shutting off the 

 flood tide from estuaries may have an equally good effect. 



It must be remembered in tliis connection that as a 

 rule the surface of a river is the lowest point of a section 

 of the ground water level across its basin at any given 

 point of its course. Now, whatever may be the fall of a 

 river bed, the main obstacle to its efficiency as a ground 

 water effluent is friction, and the more tortuous and there- 

 fore longer its course, the flatter will be its gradient and 

 the greater the total friction, which may also be greatly 

 increased by the presence of " snags," boulders and similar 

 obstructions, or by water plants, as in the instance of the 

 "sudd" of the upper Nile. Fortunately, in many cases, 

 works of this sort are not only of sanitary, but also of 

 obviously commercially economic importance, and it is very 

 possible that the desire to make "trade follow the flag" 

 which actuates our attempts to cut a way through the 

 " sudd" may result in a really far more economically im- 

 portant improvement in the health of the Soudan, whereby 



