Complex sentence: His most notable contribution to the industrial revolution happened at Armley Mills which he had leased in 1804, it was badly was damaged by fire he bought the ruins and ordered the rebuilding include cast iron internal frames and other fireproofing measures.
Split into simpler sentences: His most notable contribution to the industrial revolution happened at Armley Mills, which he had leased in 1804. When it was badly damaged by fire he bought the ruins and ordered that the rebuilding include cast-iron internal frames and other fireproofing measures.