Mayer left behind him a considerable quantity of manuscript, part of which was collected by G. C. Lichtenberg and published in one volume (_Opera inedita_, Göttingen, 1775). It contains an easy and accurate method for calculating eclipses; an essay on colour, in which three primary colours are recognized; a catalogue of 998 zodiacal stars; and a memoir, the earliest of any real value, on the proper motion of eighty stars, originally communicated to the Göttingen Royal Society in 1760. The manuscript residue includes papers on atmospheric refraction (dated 1755), on the motion of Mars as affected by the perturbations of Jupiter and the Earth (1756), and on terrestrial magnetism (1760 and 1762). In these last Mayer sought to explain the magnetic action of the earth by a modification of Euler's hypothesis, and made the first really definite attempt to establish a mathematical theory of magnetic action (C. Hansteen, _Magnetismus der Erde_, i. 283). E. Klinkerfuss published in 1881 photo-lithographic reproductions of Mayer's local charts and general map of the moon; and his star-catalogue was re-edited by F. Baily in 1830 (_Memoirs Roy. Astr. Soc._ iv. 391) and by G. F. J. A. Auvers in 1894. AUTHORITIES.--A. G. Kästner, _Elogium Tobiae Mayeri_ (Göttingen, 1762); _Connaissance des temps, 1767_, p. 187 (J. Lalande); _Monatliche Correspondenz_ viii. 257, ix. 45, 415, 487, xi. 462; _Allg. Geographische Ephemeriden_ iii. 116, 1799 (portrait); _Berliner Astr. Jahrbuch_, Suppl. Bd. iii. 209, 1797 (A. G. Kästner); J. B. J. Delambre, _Hist. de l'Astr. au XVIII^e siècle_, p. 429; R. Grant, _Hist. of Phys. Astr._ pp. 46, 488, 555; A. Berry, _Short Hist. of Astr._ p. 282; J. S. Pütter, _Geschichte von der Universität zu Göttingen_, i. 68; J. Gehler, _Physik. Wörterbuch neu bearbeitet_, vi. 746, 1039; Allg. _Deutsche Biographie_ (S. Günther). (A. M. C.) MAYER, JULIUS ROBERT (1814-1878), German physicist, was born at Heilbronn on the 25th of November 1814, studied medicine at Tübingen, Munich and Paris, and after a journey to Java in 1840 as surgeon of a Dutch vessel obtained a medical post in his native town. He claims recognition as an independent a priori propounder of the "First Law of Thermodynamics," but more especially as having early and ably applied that law to the explanation of many remarkable phenomena, both cosmical and terrestrial. His first little paper on the subject, "_Bemerkungen über die Kräfte der unbelebten Natur_," appeared in 1842 in Liebig's _Annalen_, five years after the republication, in the same journal, of an extract from K. F. Mohr's paper on the nature of heat, and three years later he published _Die organische Bewegung in ihren Zusammenhange mit dem Stoffwechsel_.