posts". The purpose of the posts was to give notice of where the boundary ran so that no-one could claim ignorance of liability to pay the duties. However, in general, duties were not actually collected on the boundary. A known exception was the Grand Junction Canal: originally customs officers collected the duties at Grove Park, Hertfordshire. After the boundary was changed in 1861 a permanent house for the collector was built at Stockers Lock near Rickmansworth. The Queens Head Public House in High Street, Colney Heath, has a post standing close by and it has a "canted front bay said to have been used for the collection of coal tax". In other cases the railway and canal companies or local coal merchants calculated the sums due and paid the money to the Corporation. The railway companies were initially allowed some coal free of duty for their engines. Types of post There are five different forms of coal duty boundary markers in all. Granite obelisks, about high, erected beside canals and navigable rivers. Cast-iron posts about high. These form the majority of posts and are found beside roads – and also beside tracks and footpaths, sometimes in open countryside. They were cast by Henry Grissell at his Regents Canal iron works. Cast-iron boxes or plates, about square, built into parapets of road bridges. Stone or cast-iron obelisks, about high, found beside railways. Originally erected on previous boundaries and reused on the 1861 boundary. Cast-iron obelisks, about high, erected on railways after 1865. Almost all bear the City's shield or in some cases the full coat of arms. Most of the cast-iron posts are painted white, with the cross and sword of the shield picked out in red, but the stone ones are often of a sombre black, still bearing the stains accumulated on the smoky trackside. Most of the posts are Grade II listed buildings. How the duties were used The City of London had the right to collect dues for weighing and measuring coal entering the Port of London since medieval times. After the Great Fire of London in 1666, Acts of Parliament imposed further duties to help pay for the rebuilding. Although some of the proceeds were for general rebuilding purposes, most was to cover the costs of rebuilding St Paul's Cathedral and the City churches. After the completion of St Paul's, the duties were paid to the Commission