LETTERS

摘要

The two letters about water used in locks by boats (CB, Apr) really didn't help with the explanation. Look at it this way (and let's simplify by assuming the lock has vertical sides): Going down for example, you've got your boat in the lock with the gates closed - the volume of water in the lock is the cross sectional area of the lock times the lock-full depth of water, minus the volume displaced by your boat. After opening the bottom paddles, the volume of water left is the area of the lock times the lock-empty depth of water, minus the volume displaced by your boat. So when you subtract these two volumes, the volume displaced by the boat cancels out and the volume used is the lock area times the drop in level.