Prevention and Eradication 



69 



object might be attained by the provision of an alternative route 

 for the " rotation " w^ater from the secondary canal to the fields. 



The small collections of residual water might he 



Agents treated chemically so as to destroy the surviving 



molluscs. As the v^ater so treated would be carried 



on to the land at the commencement of the following "rotation" 



it would be essential that the chemical used should not be injurious 





V ^ 



Fig. 33.— The bed of the Marg Canal ia the village during a summer rotation. 



to the crops. Certain chemicals are used nowadays on a large scale 

 as manures. It was found experimentally that some of these, 

 especially ammonium sulphate, in weak solution killed the molluscs 

 within a few hours. This chemical manure can therefore be used 

 with safety, and ivitliout idtlmate loss, to kill off those molluscs 

 which had escaped destruction by drying. 



The small drains, such as that figured on page 39, 

 are less cared for, as a rule, than the small supply 

 canals. They consequently become over-grown 



Closed 

 Drains. 



