This illustration is of a Roman hypocaust system discovered close to Lincoln's Exchequergate in 1739.
In a letter dated 18 April 1740, William Stukeley described it:
"This work was found 13 foot under ground, at the Exchecquer as it is called, the gate before the front of the minster; above it they dug up some stone coffins, which, I suppose, belonged to some church there before the minster was built.
"The plan of the hypocaust is a long square, thick sett with pillars, in rows, about 5 foot high; they were covered with large tyles, 2 inches thick, reaching from pillar to pillar.
Those were covered with a strong terrace cement, on which was layd a tessellated pavement, all white …
There was a fire-place by this subterranean room, whence the heat, steam, and smoke, passed through it, and was conveyed away by two funnels at the opposite end; above was the hot room."
Courtesy of Lincolnshire County Council, The Collection