CROSSING CANALS OR ROADS. 349 



Sometimes it is necessary to carry one channel across 

 another at the same level, and yet keep them separate. 

 This is done by constructing a pipe of plank, see fig. 

 133, in the shape of an inverted siphon, and carrying it 

 beneath the channel to be crossed. This pipe is sunk 

 in the channel of which it is a continuation. At the en- 

 trance to the pipe a well or basin, c, is sunk in the chan- 



133. MANNER OP CROSSING A CANAL. 



nel, in which matter suspended in the water may fall and 

 be caught so that it may not obstruct the pipe. In the 

 same manner a canal may be carried beneath a road. 



In forming the smaller permanent channels, some labor 

 may be saved by taking advantage of natural depressions 

 in the ground, and forming the channels there, using the 

 excavated earth for filling up the depression. Thus in 

 the case illustrated by fig. 134, the earth is removed from 

 the center of the hollow, and placed on each side, 

 raising the sides, as shown by the dotted lines, and 



Fig. 134. CANAL IN A HOLLOW. 



ing the canal of the shape indicated. A case in which 

 a canal is needed upon sloping ground is shown at fig. 135. 

 The earth excavated is placed as before on each side, and 

 the level raised as shown by the dotted lines. 



The distributing furrows upon cultivated grounds can- 

 not be permanent. They are destroyed at every plowing, 

 and must be re-made for every crop. For the majority of 

 crops that are planted in hills or drills, the furrows be- 



