108 



MOSQUITO ERADICATION 



In cases where no instrument is available for checking the 

 accuracy of the bottom-finishing, water may be admitted to 



Fig. 70. 



(Photos by E. B. Johnson, C. E.) 

 Above, extensive swamp; below, ditch that drained it. 



the ditch at its upper end and allowed to descend to the lower 

 end. Presence of pools will indicate a ridge or a depression, as 

 the case may be, and enable the finishers to correct their errors. 



