sentences:  The stop lock at Hall Green, which has a fall from the Macclesfield to the T&M, was originally built as two chambers end to end, to allow for either canal to be higher.The second chamber fell into disuse when the weir level on the top pound of the Trent and Mersey Canal was permanently lowered after nationalisation to improve the clearance in Harecastle Tunnel.Whilst the fall of the lock is small, it cannot be taken out of use (as many stop locks have), because lowering the bottom pound of the Macclesfield Canal would lead to draft problems on it (as it is a fairly shallow canal), and raising the top pound of the Trent and Mersey would lead to air draft problems through Harecastle tunnel.