A weigh lock is a specialized canal lock designed to determine the weight of barges in order to asses toll payments based upon the weight and value of the cargo carried. This requires that the unladen weight of the barge be known.
A barge to be weighed was brought into a supporting cradle connected by levers to a weighing mechanism. The water was then drained downstream and the scale balance adjusted to determine the barge gross weight. Subtracting the tare weight (the weight of the barge when empty) would give the cargo weight.
An example of a weigh lock (now dry), once part of the Erie Canal, may be found in downtown Syracuse, New York, along with a canal barge and interpretive displays at the Erie Canal Museum.