could form an explosive mixture in their gas-lit interiors. The first mills using fireproof construction were built in Shropshire and Derbyshire in the 1790s and Philips & Lee's mill built in Salford in 1801–2. Fireproofing took the form of cast iron columns and beams from which sprang jack arches that were infilled with ash or sand and covered with stone flags or floorboards. In some mills timber was also eliminated from the roof structure which was supported by cast or wrought iron trusses. Until the properties of cast iron were properly understood some mills constructed using the early technology collapsed. In Manchester extensive testing of cast iron as a structural material was carried out by Eaton Hodgkinson and William Fairbairn in the early 1820s. Fireproof construction was expensive and timber, sometimes clad in plaster or metal continued to be used throughout the 19th century. Rolled steel beams and reinforced concrete flooring was introduced in a limited way in the 1880s but not widely adopted in Lancashire mills until the 20th century. Other factors Cotton is sensitive to temperature and humidity. Heating systems used wrought iron pipes suspended at a height of to carry steam under pressure. In summer the system was barely used but in winter the boilers would be fired up two hours before the shift started to warm up the mill. As heat was applied the humidity dropped and system of humidifiers, either atomisers which played an air jet against a jet of water or ones that injected a stream/air mixture into the room. Early fire fighting systems used sprinklers supplied by water captured on flat roofs in shallow tanks. Later mills had a water tank at the top of the stair tower. Water for the sprinklers had to be protected from freezing and evaporation. Water pressure needed to be above 7 psi, and the header tank at least above the highest sprinkler. The provision of light, water tanks and heating system defined the structure and shape of the mill. Machinery Power The earliest cotton mills were driven by water, so needed to be situated on fast flowing streams. From about 1820, the stationary steam engine became the normal form of power for a cotton mill, water was still needed to produce the steam and to condense it, to maintain the humidity, for many of the finishing processes and for firefighting. Water was extracted from rivers and canals,