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.
NATURE, May 9, p. 525.
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WOLEDGE, G. History of the British Thermal Unit. Nature 149, 613 (1942). https://doi.org/10.1038/149613c0