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APPENDIX 545 In this account I have been assisted by the disquisition of J. Partsch, in the Prooemium to his edition of Coiippus, and by the narrative of M. Ch. Diehl, in L'Afrique byzantine. 19. THE EXARCHS— (P. 391, 423) The earliest mention of the name Exarch in connexion with the government of Italy is in a letter of Pope Pelagius II. to the deacon Gregory (Migne, Patr. Lat., vol. 82, p. 707 ; cp. Diehl, Etudes sur I'administration byzantine dans I'exarchat de Ravenne, p. 173), dated Oct. 4, 484. Seven years later we meet the earliest mention of an Exarch of Africa (Gregory the Great, Ep. i. 59), in July, 591. Under the Emperors Justin and Tiberius (a.d. .565-582) the supreme military governor is entitled magister militum. It is therefore plausible to ascribe to Maurice (Diehl, L'Afrique byzantine, p. 478) the investiture of the military governor with extraordinary powers and a new title designating his new position. Gennadius was the first exarch of Africa. From the first hour of the Imperial restoration in Africa military and civil governors existed side by side, and the double series of magistri militum (and exarchs) and Praetorian Praefects can be imperfectlj' traced till the middle of the seventh century. ^ On some exceptional occasions the two offices were united in a single individual. Thus Solomon was both magister militum and Praetorian Praefect in a.d. 535, and again in a.d. 539, &c. ; and Theodorus held the same powers in a.d. 569. Throughout, the tendency was to subordinate the civil to the military governor, and the creation of the exarchate, with its large powers, de- cisively reduced the importance of the Praetorian Praefect. 20. THE COMET OF A.D. 531— (P. 433-4) The identity of the comet of a.d. 1680 with the comets of a.d. 110<3, a.d. .531, B.C. 44, &c., is merel}' an ingenious speculation of Halley. See his Synopsis of the Astronomy of Comets, at end of Whiston's "Sir Isaac Newton's mathematick Philosophy more easily demonstrated " (1716), p. 440 sqq. The eccentricity of the comet of a.d. 1680 was calculated b_v Halley (Philosophical Transactions, 1705, p. 1882), and subsequent!}- by Encke, Euler, and others, — on the basis, of course, of the observations of Flamsteed and Cassini. Newton regarded its orbit as Ijarabolic (Principia, 3, Prop. 41) ; but it has been calculated that the eccentricity arrived at by Encke, combined with the perihelion distance, would give a period of 8813-9 years (J. C. Houzeau, Vademecum de 1' Astronome, 1887, p. 762-3). The observation.s were probably not sufficiently accurate or numerous to establish whether the orbit was a parabola, or an ellipse with great eccentricity ; but in any case there is nothing in the data to suggest 575 years, nor have we material for compari.son with the earlier comets which Halley proposed to identify. For the Chinese observations to which Gibbon refers, see John Williams, Observations of Comets from Chinese Annals, 1871 : for comet of b.c. 44, p. 9, for a doubtful comet (?) of a.d. 532, p. 33, for comet of a.d. 1106, p. 60. 21. ROMAN LAW IN THE EAST— (C. XLIV.) New light has been thrown on the development of Imperial legislation from Constantine to Justiiiian, and on the reception of Roman law in the eastern half of the empire (especially Syria and Egypt), by the investigations of L. Mitteis, in his work " Reichsrecht und Volksrecht in den bstlichen Provinzen dea romischen Kaiserreichs " (1891). The study is mainly based on Egyptian papyri and on the Syro-Roman Code of the fifth century, which was edited by Bruns and Sachau (1880). 1 See list of Diehl, L'Afrique byzantine, p. 596. 9 VOL. IV. He5