His earliest work, “Clavis Astronomiæ,” first appeared during 1725, but it is only in the second and enlarged new edition of it in Horrebow’s “Operum Mathematico-Physicorum,” Havn. 1740, Vol. I. p. 317, that will be found the passage (s. 226) in which the luminous process of the sun is characterized as a perpetual northern light. Humboldt, who mentions the fact (“Cosmos,” 1859, Vol. V. p. 81) suggests that a comparison be made of Horrebow’s statement with the precisely similar views held by Sir William Herschel (1738–1822) and Sir John Frederick William Herschel (1792–1871). He says that Horrebow, who did not confound gravitation with magnetism, was the first who thus designated the process of light produced in the solar atmosphere by the agency of powerful magnetic forces (“Mémoires de Mathématiques et de Physique, présentés à l’Académie Royale des Sciences,” Vol. IX. 1780, p. 262; Hanow, in Joh. Dan. Titius’s “Gemeinützige Abhand. über natür. Dinge,” 1768, p. 102), and, with reference to the Herschels he thus expresses himself: “If electricity, moving in currents, develops magnetic forces, and if, in accordance with an early hypothesis of Sir Wm. Herschel (_Phil. Trans._ for 1795, Vol. LXXXV. p. 318; John Herschel, “Outlines of Astronomy,” p. 238; also, Humboldt, “Cosmos,” Vol. I. p. 189), the sun itself is in the condition of a perpetual northern light (I should rather say of an electro-magnetic storm) we should seem warranted in concluding that solar light transmitted in the regions of space by vibrations of ether, may be accompanied by electro-magnetic currents” (“Dict. of Nat. Biog.,” for John and William Herschel, Vol. XXVI. pp. 263–274). REFERENCES.--Larousse, “Dict. Univ.,” Vol. IX. p. 397; Wolf, “Hist. Ordbog.,” Vol. VII. pp. 194–199; Nyerup, “Univ. Annalen”; Houzeau et Lancaster, “Bibliographie,” 1882, Vol. II. p. 166. Three of the children of Peter Horrebow, almost equally distinguished for their learning, are: Nicolas Horrebow (1712–1760), who made physical and astronomical observations in Iceland and published an able report thereon during 1752; Christian Horrebow (1718–1776), who succeeded his father in 1753 as astronomer in the Copenhagen University and who wrote several important scientific treatises; and Peter Horrebow (1728–1812), who was professor of mathematics and philosophy, and published works on geometry, meteorology and astronomy. Much of interest concerning the above will also be found in the “Abstracts of Papers ... Roy Soc.,” Vol. II. pp. 208, 249, 251, and in the “Catalogue of Sc. Papers ... Roy. Soc.,” Vol. III. pp. 322–328; Vol. VI. p. 687; Vol. VII. p. 965.