U NTIL QUITE RECENTLY historians of physics have invariably maintained that the famous vis viva controversy was summarily and unequivocally ended in 1743 by Jean D'Alembert. 1 In the preface to his Traite de dynamique D'Alembert pointed out that the disputants had been seeking a unitary definition of moving force when in fact the effects of a force could be measured in two different ways, thereby making it possible to use both conservation of momentum (my) and conservation of vis viva (my2) in the same system of mechanics. By calling the debate" une dispute de mots," 2 D'Alembert was thought to have persuaded his contemporaries that what they had viewed as an important debate about the empirical measure of moving force was simply a semantic dispute. Recently certain doubts have been raised about D'Alembert's claim to priority in this matter. Specifically it has been suggested that several years before 1743 some scientists had dismissed the vis viva controversy as being no more than a dispute of words, incapable of definitive experimental resolution. 3 Such doubts, although based on clear-cut evi-* University College, London. 1 The historiographical tradition which credits D'Alembert with resolving the vis viva debate dates back at least as far as 1803, when Dugald Stewart asserted:" the honour of placing this very subtle and abstruse question [the question about the correct measure of force] in a point of view calculated to reconcile completely the contending parties, was reserved for M. D'Alembert."(See Stewart's introduction to The Works of Thomas Reid, ed. W. Hamilton, Edinburgh, 1858, p. 6.) Since then, many distinguished historians of physics have repeated and embellished this claim. Ernst Mach, for instance, wrote in 1893 that" the dispute raised by Leibniz... lasted fifty-seven years, till the appearance of D'Alembert's Traite de Dynamique in 1743"(The Science of Mechanics, trans. McCormack, LaSalle, Ill.: Open Court, 1960, p. 365). In 1899, F. Cajori adopted the same view in his History of Physics (New York: Macmillan, 1929), pp. 58-59. Following Mach and Cajori, J. Merz asserted in 1912 that the controversy was" settled by D'Alembert in his Traite de Dynamique (1743) by stricter definition"(A