"The engines are built at the expense of the proprietors ; Messrs Boulton and Watt furnish such drawings, directions and attendance, as may be necessary to enable a resident engineer to complete the machine ; and, in lieu of all profits, they take one-third of the annual savings in fuel, which their engine makes, when compared with a common engine of the same dimensions, in the neighbourhood. Mr. Watt's new engine will raise 20 to 24 thousand cubic feet of water to 24 feet high, by 1 cwt. of good pit coal." (= 480 to 576 thousand cubic feet 1 foot high)." [2]


[1] Proposal made by Mr. Boulton to the Carron Company, to rebuild their returning engine, on the new system cited in John Farey, A treatise on the steam engine : historical, practical, and descriptive, London 1827, page 327; available INTERNET ARCHIVE

[2] John Farey, page 328