Gasholders or gasometers are large vertical cylindrical storage vessels for holding town gas at constant pressure prior to distribution to consumers. The oldest design of gasholder, made from wrought-iron plates dates from an 1824 patent. It telescoped vertically by means of grooved wheels on vertical guide rails. Later design by W. Gadd in 1890. Small gasholders were used in factories and mills which had their own gas manufacturing plant for providing lighting. By the 1870s a typical gas manufacturing works comprised of: a retort house containing rows of retorts where the gas was made, a hydraulic main to provide a water seal to prevent explosive gas blowbacks when a retort was opened, a condensor to cool the gas and allow liquid tar to drain out, an exhauster to draw out the gas from the retorts, a scrubber in which water removed ammonia from the gas, purifiers to remove contaminants, a meter to measure the volume of gas, one or more gasholders, a governor to control the pressure at which gas was distributed to customers, covered storage for coal and coke, a boiler and steam engine to drive the exhauster and a water pump for the scrubber (William Jones, 1996, Dictionary of Industrial Archaeology, EG Stewart, 1958, History of the gas, light and coke co. 1812-1949, T. Williams, 1981, History of the British Gas Industry).