244 



IRRIGATION. 



the outside curve. Sometimes a canal needs to be carried 

 underground, beneath roads or buildings. A wooden 



bridge may be made as 

 shown in fig. 127, which, 

 when covered with earth, 

 appears as at fig. 128. It 

 consists of a piece of round 

 timber, to which short 

 planks are strongly nailed 

 by one of their ends. The 

 other ends are spread apart 

 as far as may be necessary 



Fig. 126. PBOTECTED CANAL. 



to give sufficient capacity to the canal. The bridge is put 

 together in the canal, and when it is finished is covered 

 with earth. Its triangular 

 form gives it great strength. 

 Where the soil is porous 

 and open in its character, con- 

 siderable loss of water often 

 occurs by percolation. This 

 is to be prevented by puddling 

 the bottom and sides with 

 clay or compact earth. The Flg * 127 -- BBIDGB 

 clay is deposited upon the banks, and as it is softened 

 and reduced to a plastic condition by the action of the 

 water, it is carried down and de- 

 posited in a layer upon the bot- 

 tom of the canal. Gravelly and 

 loamy soils may be made water- 

 tight by puddling the moist bot- 

 tom. This may be done by driv- 

 ing a flock of sheep up and down 

 V,& I the canal when the bottom is wet, 



Fig. 128.-COMPLETED 



CULVERT. horses. "When a canal must either 



cross a valley or be carried around it, it will often be 



