[2] The Observatory, No. 43, p. 613. [3] Nature, vol. xxv. p. 537. [4] Silvered glass is considerably more reflective than speculum-metal, and Mr. Common’s 36-inch mirror can be but slightly inferior in luminous capacity to the Lick objective. It is, however, devoted almost exclusively to celestial photography, in which it has done splendid service. The Paris 4-foot mirror bent under its own weight when placed in the tube in 1875, and has not since been remounted. [5] E. Holden, “The Lick Observatory,” Nature, vol. xxv. p. 298. [6] Monthly Notices, R. Astr. Soc. vol. xiv. p. 133 (1854). [7] Phil. Trans. vol. cxlviii. p. 455. [8] Captain Jacob unfortunately died August 16, 1862, when about to assume the direction of a hill observatory at Poonah. [9] The height of the mercury at Guajara is 21·7 to 22 inches. [10] Phil. Trans. vol. cxlviii. p. 477. [11] We are told that three American observers in the Rocky Mountains, belonging to the Eclipse Expedition of 1878, easily saw Jupiter’s satellites night after night with the naked eye. That their discernment is possible, even under comparatively disadvantageous circumstances is rendered certain by the well-authenticated instance (related by Humboldt, “Cosmos,” vol. iii. p. 66, Otte’s trans.) of a tailor named Schön, who died at Breslau in 1837. This man habitually perceived the first and third, but never could see the second or fourth Jovian moons. [12] Sir W. Herschel’s great undertakings, Bessel remarks (“Populäre Vorlesungen,” p. 15), “were directed rather towards a physical description of the heavens, than to astronomy proper.” [13] Am. Jour. of Science, vol. xiii. p. 89. [14] The characteristic orange line (D_{3}) of this unknown substance, has recently been identified by Professor Palmieri in the spectrum of lava from Vesuvius—a highly interesting discovery, if verified. [15] The Sun, p. 193. [16] R. D. Cutts, “Bulletin of the Philosophical Society of Washington,” vol. i. p. 70. [17] This instrument may be described as an electric balance of the utmost conceivable delicacy. The principle of its construction is that the conducting power of metals is diminished by raising their temperature. Thus, if heat be applied to one only of the wires forming a circuit in which a galvanometer is included, the movement of the needle instantly betrays the disturbance of the electrical equilibrium. The conducting wires or “balance arms” of the bolometer are platinum strips 1/120th of an inch wide and 1/25000 of an inch thick, constituting metallic _antennæ_ sensitive to the chill even of the fine dark lines in the solar spectrum, or to changes of temperature estimated at 1/100000 of a degree Centigrade. [18] Defined by the tint of the second hydrogen-line, the bright reversal of Fraunhofer’s F. The sun would also seem—adopting a medium estimate—three or four times as brilliant as he now does. [19] Annales de Chimie et de Physique, t. x. p. 360. [20] S. P. Langley, “Nature,” vol. xxvi. p. 316.