It has been repeatedly claimed for Mayer that he calculated the value of the dynamical equivalent of heat, indirectly, no doubt, but in a manner altogether free from error, and with a result according almost exactly with that obtained by J. P. Joule after years of patient labour in direct experimenting. This claim on Mayer's behalf was first shown to be baseless by W. Thomson (Lord Kelvin) and P. G. Tait in an article on "Energy," published in _Good Words_ in 1862, which gave rise to a long but lively discussion. A calm and judicial annihilation of the claim is to be found in a brief article by Sir G. G. Stokes, _Proc. Roy. Soc._, 1871, p. 54. See also Maxwell's _Theory of Heat_, chap. xiii. Mayer entirely ignored the grand fundamental principle laid down by Sadi Carnot--that nothing can be concluded as to the relation between heat and work from an experiment in which the working substance is left at the end of an operation in a different physical state from that in which it was at the commencement. Mayer has also been styled the discoverer of the fact that heat consists in (the energy of) motion, a matter settled at the very end of the 18th century by Count Rumford and Sir H. Davy; but in the teeth of this statement we have Mayer's own words, "We might much rather assume the contrary--that in order to become heat motion must cease to be motion." Mayer's real merit consists in the fact that, having for himself made out, on inadequate and even questionable grounds, the conservation of energy, and having obtained (though by inaccurate reasoning) a numerical result correct so far as his data permitted, he applied the principle with great power and insight to the explanation of numerous physical phenomena. His papers, which were republished in a single volume with the title _Die Mechanik der Wärme_ (3rd ed., 1893), are of unequal merit. But some, especially those on _Celestial Dynamics_ and _Organic Motion_, are admirable examples of what really valuable work may be effected by a man of high intellectual powers, in spite of imperfect information and defective logic. Different, and it would appear exaggerated, estimates of Mayer are given in John Tyndall's papers in the _Phil. Mag._, 1863-1864 (whose avowed object was "to raise a noble and a suffering man to the position which his labours entitled him to occupy"), and in E. Dühring's _Robert Mayer, der Galilei des neunzehnten Jahrhunderts_, Chemnitz, 1880. Some of the simpler facts of the case are summarized by Tait in the _Phil. Mag._, 1864, ii. 289.