Though John Rennie's reputation is based on his civil engineering works, he continued throughout his life to be interested in and to construct mill machinery, both for mills powered by water, wind, and horses, and for those powered by steam. He had been briefly employed by Boulton and Watt in 1784, and it is thought agreed then not to construct steam engines himself. As a quid pro quo, Boulton and Watt regularly gave Rennie the task of providing the machinery to go with their engines. Thus Rennie's papers provide information on almost all types of industrial activity undertaken at the beginning of the 19th century. For material relating specifically to steam engines, see MS.19825.