History of the British Thermal Unit

THE date of the phrase ``British thermal unit'' can be pushed back by fifteen years; it occurs in W. J. M. Rankine, ``Manual of the Steam Engine'' (1859), p. 300; but here, as in the 1875 quotation given by Dr. Powell¹, it is a descriptive phrase and not a technical term; Rankine is in fact contrasting the British thermal unit and the French thermal unit.