Although Watt was a prolific writer when communicating with business partners and friends, he did not write any scientific papers to be published.

In a letter to Roebuck on 24 May 1768 Watt wrote "I speak within bounds when I say that every cubic foot of the contents of the cylinder will require only one cubic inch of water to be evaporated" [1]. As 12 inch make a foot Watt, with simple means had already evaluated that one unit of water transform into 12³ = 1728 units of steam. That was close to real figure of 1 liter of water at 4°C gives 1673 liter of steam of 100°C.


[1] source cited in H.W. Dickinson & R. Jenkins, "James Watt and the Steam Engine" first published 1929, as the "Doldowlod Papers"