Freventioii and EradicafioN 77 



possible between two tertiary canals. A glance at fig. 37, a shows 

 at once that where a village is on the bank of a main or secondary 

 canal the bilharzia eggs and embryos carried on in the main stream 

 passing through the village are liable to infect all the tertiary 

 canals supplied from the canal in the section down stream of the 

 village. This arrangement one sees frequently on the Suez section 

 of the sweet water canal. Where the village is on a tertiar}' 

 canal (fig. 37, b) as happens at El Marg, the water passing through 

 the village is dissipated on the surrounding fields so that the area 

 of infectivity of the village is limited practically by its own 

 communal boundaries. Where a village lies between two tertiary 

 canals, as in fig. 37, c, the liability to contamination of the water 

 channels is practically restricted to those paths leading from the 

 village and such branches as are used for washing and other 

 domestic purposes. 



(3) The village w^ater supply should be derived from " sakias " 

 or deeper wells. Each irrigation unit should possess paired supply 

 canals and drains, so that these may be alternately dried without 

 interfering with the irrigation. At Marg, when the canals became 

 practically dry during the rotations, a certain amount of water 

 was drawn from a sakia in the middle of the village. On one 

 occasion when the shortage was becoming serious, a neighbouring 

 land proprietor diverted a generous supply of clear artesian water 

 into the Marg Canal. This, however, revived enormous numbers of 

 molluscs which otherwise would, undoubtedly, have been killed by 

 drying before the completion of the rotation cycle ! 



(4) Surface-water drains should be reduced, as far as possible, 

 by the utilization of infiltration drains. 



The Mosseri system of field drainage, which is said to be simple, 

 economical, and extraordinarily effective, seems, of the various 

 systems of land reclamation at present in use, to be the one most 

 likely to produce those conditions that are unfavourable to the 

 spread of bilharziosis. 



Its dual system of " collecting " drains affords more complete 

 control. From the section shown in fig. 39, it will be apparent that 

 the bulk of the surface water, after, irrigating the land, rapidly drains 

 by a separate surface drain into the main drain, while the water 

 which has soaked into the soil is drawn off by a deep infiltration 

 drain to be pumped later into the main drain. The surface water 

 drain can therefore be readily sterilized during the summer by 

 drying, while if need be, the deep infiltration drain can be treated 

 with chemical agents, or periodically cleared. 



