to develop extensively in the late 18th century, with rapid expansion throughout the 19th century and early 20th century when the industry peaked. The location of the coalfields helped to make the prosperity of Lancashire, of Yorkshire, and of South Wales. The Yorkshire pits which supplied Sheffield were only about 300 feet deep. Northumberland and Durham were the leading coal producers and they were the sites of the first deep pits. In much of Britain coal was worked from drift mines, or scraped off when it outcropped on the surface. Small groups of part-time miners used shovels and primitive equipment. Scottish miners had been bonded to their "maisters" by a 1606 Act "Anent Coalyers and Salters". A Colliers and Salters (Scotland) Act 1775, recognised this to be "a state of slavery and bondage" and formally abolished it; this was made effective by a further law in 1799. Before 1800 a great deal of coal was left in place as extraction was still primitive. As a result, in the deep Tyneside pits (300 to 1,000 ft deep) only about 40 percent of the coal could be extracted. The use of wooden pit props to support the roof was an innovation first introduced about 1800. The critical factor was circulation of air and control of dangerous explosive gases. At first fires were burned at the bottom of the "upcast" shaft to create air currents and circulate air, but replaced by fans driven by steam engines. Protection for miners came with the invention of the Davy lamp and Geordie lamp, where any firedamp (or methane) burnt harmlessly within the lamp. It was achieved by preventing the combustion spreading from the light chamber to the outside air with either metal gauze or fine tubes, but the illumination from such lamps was very poor. Great efforts were made to develop better safe lamps, such as the Mueseler produced in the Belgian pits near Liège. Coal was so abundant in Britain that the supply could be stepped up to meet the rapidly rising demand. In 1700 the annual output of coal was just under 3 million tons. Between 1770 and 1780 the annual output of coal was some 6¼ million long tons (or about the output of a week and a half in the 20th century). After 1790 output soared, reaching 16 million long tons by 1815 at the height of the Napoleonic War. By 1830