Dr. Young's Refractions, the Barometer being at 30 inches, and the internal Thermometer at 50, or the external at 47 degrees; with the Corrections for + one inch in the Barometer, and for one degree in the Thermometer of Fahrenheit. From page 19 of Vol. I. of Pearson's Practical Astronomy. The correction for an increase of altitude of one inch in the barometer, or for a depression of one degree in the thermometer, is to be added to the tabular refraction; but when the parometer is lower than 30 inches, or the thermometer higher than 47 degrees, the correction becomes subtractive. When great accuracy is required, 0.003 inch should be deducted from the observed height of the barometer for each degree that the thermometer near it is above 50 degrees, and the same quantity added for an equal depression. Logarithm for converting Sidereal into Mean Solar Time +9.9988126. Mean Solar into Sidereal Time + 0.0011874. 66 66 A second of time at the Equator contains 1521 feet. RECENT DISCOVERIES IN ASTRONOMY, RELATING TO THE COMETS. By Professor J. Lovering of Harvard University. COMETOGRAPHY has not fallen behind other branches of astronomy in the rapid growth it has made during the last ten years. This period has been marked by the appearance of one of the most remarkable comets on record, that of 1843; by the publication of diligent observations in the northern and southern hemispheres on the last appearance of Halley's comet in 1835; by the third return of Biela's comet to its perihelion (in 1846) since the discovery of its periodical character, and the singular display it made on that occasion; by four appearances of Encke's comet, namely, in 1842, 1845, 1848, and 1852, completing ten perihelion returns since its period was predicted; by the discovery of four new periodical comets, and many others not known to be so. Captain Smyth* observed in 1844, "Of the six or seven hundred comets which are on record, about one hundred and forty have had their orbits computed." Delambre in 1814 tabulated one hundred and seventeen.t Arago's list, which he referred to 1831,‡ contained one hundred and thirtyseven different orbits of comets. Mr. Hind has calculated the orbits of five or more comets from Chinese observations. § The catalogue of all the orbits of comets which had been computed, which was published by Professor Peirce in the American Almanac for 1847, contains the orbits of one hundred and seventy-seven independent comets. The reader will find, in some of the earlier volumes of this publication, ample information concerning comets in general, and the most distinguished comets of former times. It is only necessary here to complete the catalogue down to the present time, and give the results of recent observations on individual comets. In 1817, Damoiseau gained the Turin prize for calculating with precision the return of Halley's comet in 1835. In 1833, a prize was awarded to Pontecoulant for a similar service. The perihelion return of this comet was also calculated by Lubbock and Dr. Lehmann. || Rosenberger took account of all the perturbations from 1682 to 1835, and received for his labors the gold medal of the Astronomical Society of London. The perihelion return was assigned by these various astronomers respectively at November 4-5; November 7-8; October 30-31; November 26-27; and November 11-12. Lehmann's prediction was published on the 25th of ** July, 1835. The perihelion passage was really made, according to the computations of Stratford, on November 15-16. The comet was first seen at Rome by Dumouchel and De Vico on the 5th of August, 1835. On the 23d of August, Arago showed by his polariscope that the planet shone with [1853. reflected light. The comet was generally visible in the autumn of that year in the northern hemisphere, and passed the spring of the next year in the southern hemisphere. Never before had so conspicuous a comet been studied with so much care by gifted astronomers, and with excellent instruments, Struve at Pulkova,* Bessel at Konigsberg, Herschel at the Cape of Good Hope, Smyth at Bedford, § and Airy at Cambridge. || The following conclusions, drawn from the singular conduct of Halley's comet at this its last appearance, are presented in Herschel's Outlines: ¶ "Reflecting on these phenomena, and carefully considering the evidence afforded by the numerous and elaborately executed drawings which have been placed on record by observers, it seems impossible to avoid the following conclusions. "1st. That the matter of the nucleus of a comet is powerfully excited and dilated into a vaporous state by the action of the sun's rays, escaping in streams and jets at those points of its surface which oppose the least resistance, and in all probability throwing that surface or the nucleus itself into irregular motions by its reaction in the act of so escaping, and thus altering its direction. "2dly. That this process chiefly takes place in that portion of the nucleus which is turned towards the sun; the vapor escaping chiefly in that direction. "3dly. That, when so emitted, it is prevented from proceeding in the direction originally impressed upon it, by some force directed from the sun, drifting it back and carrying it out to vast distances behind the nucleus, forming the tail or so much of the tail as can be considered as consisting of material substance. “4thly. That this force, whatever its nature, acts unequally on the materials of the comet, the greater portion remaining unvaporized, and a considerable part of the vapor actually produced remaining in its neighborhood, forming the head and coma. 66 5thly. That the force thus acting on the materials of the tail cannot possibly be identical with the ordinary gravitation of matter, being centrifugal or repulsive, as respects the sun, and of an energy very far exceeding the gravitating force towards that luminary. This will be evident if we consider the enormous velocity with which the matter of the tail is carried backwards, in opposition both to the motion which it had as part of the nucleus, and to that which it acquired in the act of its emission, both which motions have to be destroyed in the first instance, before any movement in the contrary direction can be impressed. 66 6thly. That unless the matter of the tail thus repelled from the sun be retained by a peculiar and highly energetic attraction to the nucleus, differing from and exceptional to the ordinary power of gravitation, it must leave the nucleus altogether; being in effect carried far beyond the coercive * Beobachtungen des Halleyschen Cometen. Results, &c., pp. 393-413. Il Ibid., XVI. 337. † Astron. Nachr., XIII. § Mem. Astron. Soc., IX. 229. power of so feeble a gravitating force as would correspond to the minute mass of the nucleus; and it is therefore very conceivable that a comet may lose, at every approach to the sun, a portion of that peculiar matter, whatever it be, on which the production of its tail depends, the remainder being of course less excitable by the solar action, and more impassive to his rays, and therefore, pro tanto, more nearly approximating to the nature of the planetary bodies." * Laugier has recently presented two memoirs to the French Academy on ancient appearances of Halley's comet. He made use for this purpose of the description of comets which E. Biot obtained from the Chinese historians, and presented to the French Board of Longitude. Thus the comet can now be traced back through nine perihelion passages. Its disturbed period of revolution has varied from 77.58 to 74.91 years. Bessel calculated that if the mass of the comet had experienced a loss of one 23,000th of the whole quantity, the period of the comet would be diminished by 1,607 days. The history of the discovery of Encke's comet,† its orbit, its very short period of a little more than three years, and the speculations to which its premature return has led, are well known, and are recorded in other volumes of this Almanac. This comet came to its perihelion, for the seventh time since its period was computed, on the 12th of April, 1842. ‡ At that return, it was seen by Galle at Berlin on the 9th of February; on the 12th of March by Laugier and Mauvais at Paris; and on the 10th of March by Valz at Marseilles. § It was also seen at Philadelphia on the 28th of March by Professor Kendall, and was observed there by him and by Mr. S. C. Walker in March and April. || It was seen at Cambridge by Mr. B. A. Gould, Jr. on the 10th of April, this being the first fair evening after the comet was seen in the United States. T Encke's comet was observed at Hudson by Professor Loomis from March 28 to April 11. Encke did not expect that this comet, which he calls Pons's comet, but which all other astronomers call Encke's comet, would be visible in the northern hemisphere. As it had never been missed since 1819, he was anxious it should be observed in the southern hemisphere, and sent an ephemeris to England to be distributed among the Australian colonies of Great Britain. Encke's comet returned again to its perihelion on August 9, 1845.** But only five observations were made upon it; two at Rome, where it was first seen by De Vico,tt July 9, and again on the 14th; two at Washington, where it was seen July 10; and one at Philadelphia, ‡‡ by S. C. Walker, on July 4, who therefore had the first sight of it. Encke's comet returned again to its perihelion on the 26th of November, * Compt. Rend., XVI. p. 1006, XXII. and XXIII. 1846; and Astron. Nachr., XXIII. 377. † Astron. Nachr., XXI. 113-128. § Compt. Rend., XIV. 172. ↑ Ibid., XIX. 185. || Astron. Nachr., XXI. 231. Astron. Nachr., XXII. 203; Phil. Mag., XX. 137. ** Astron. Nachr., XXIII. 84-92. tt Ibid., XXIII. 255. # Ibid., XXIV. 131; Gould's Astron. Journ., I. 56, 134; Compt. Rend., XXVII. 343. |