82 Canal-locks How Used. 



of the canal with a gate on one side, at the bot- 

 tom, opening into the lower level, and another 

 on the opposite side, at the top, opening into 

 the upper level. These gates or doors can be 

 shut so as to be water-tight. 



Canal. A boat at low level about to enter a lock so as to reach 

 the upper level. Lower grate, A, opens to admit boat ; upper 

 grate, B, closed. 



10. When a boat is to go up hill, the door at 

 the top being closed, the one at the bottom is 

 opened, and the boat floats through into the 

 lock. 



Boat in canal-lock, lower gate, A, about to be closed. "Water 

 to be let into lock from upper canal until the lock is full. 



ii. That door is then closed and the upper 

 one, or a valve in it, is gradually opened, letting 



