One of only four such boat weighing machines to have been built in Britain, the weighing dock on the Glamorganshire Canal was constructed in 1834 by Brown, Lenox & Co of Pontypridd at a cost of £643. Capable of weighing up to forty tons, the machine consists of six round pillars with plain capitals supporting a structure from which hangs a cast-iron cradle suspended on four radial rods. A boat would be floated into the lock where the machine was placed and the gate shut. The water would then be emptied, depositing the boat on the cradle so that the weight could be taken by means of a weightbeam which was positioned on a fulcrum in order to exert a force of 112 to 1. Inside the weight house were counterpoises which were placed on a suspended pan to calculate the measurement.

The weighing dock was originally situated on the eastern site of Ton Lock (NPRN 91465) at the southern end of Tongwynlais, north of Cardiff. In 1850, the machine was moved to Crockherbtown Lock (NPRN 85382), north of present-day Queen Street, Cardiff. It was moved again in 1894 to a site on North Road, Cardiff, north of North Road lock, no.49. During this move, the machine was restored and reconditioned, after which the mechanism was found to be sensitive to fourteen pounds. The boat weighting machine was in regular use until c.1914. In 1944, the Glamorgan canal was purchased by the Cardiff Corporation and the canal company's last working boat, No. 451, was deposited in the weigh dock, not having arrived at the West Warf in time for the takeover ceremony.

Following the takeover, the National Museum of Wales requested that the weigh dock be preserved in situ, but no action was taken until 1955 when the Cardiff Corporation presented the boat weighing machine to the British Transport Commission by whom it was dismantled and place into storage in a dockland warehouse. However, from 1963 the machine was on permanent exhibition at the Waterways Museum in Stoke Bruerne, Northamptonshire, where it was exhibited with the Grand Union narrow boat Northwich, and it has been on display in the garden of the National Waterfront Museum, Swansea, since 2013.

(Sources: Rowson and Wright, The Glamorganshire and Aberdare Canals, vol. II, ch. 9 `The Boat Weighing Machine? (2004); NMR Site File, Glam/IND/ST17NE; NMR Additional Information, Glam/IND/ST17NE; National Waterfront Museum, Swansea)
A.N. Coward, RCAHMW, 02.07.2018