SCIENCE. 



283 



the Pleiades was made, in part, so that the distances of 

 any pair of these stars might be used as a known celestial 

 arc 10 be determined in terms of the screw revolution. 

 Dr. Vogel, ©f Potsdam, determined the value of the screw 

 of the Leipzig retractor by measuring the difference of 

 declination between two stars with the micrometer, and 

 afterwards using the divided declination circle of the 

 equatorial to determine the whole arc. This method was 

 improved in the determination of the value of the screw 

 ot the Washington equatorial, by measuring with the 

 micrometer the difference of declination ot two standard 

 stars (1 and c Orionzs) a degree apart. In these last 

 methods the value of the known arc in the sky depends 

 upon cur knowledge of the positions of its two terminal 

 points. Dr. Winnecke, of Strassburg, has recently em- 

 ployed an ingenious way, which is even more simple. 

 The distance between some asteroid (whose orbit is well 

 known) and any star near it, is measured on several nights, 

 as the asteroid passes from north to south of the s'ar 

 (let us say). Then, although the absolute position of 

 the asteroid is not known, its daily motions are well de- 

 termined, and the arc moved over may be used as a 

 known distance from which the value ot the screw may 

 be determined. 



The following complete list of asteroids (21 in all) dis- 

 covered by the late Prof. James C. Watson, Director 

 of the Washburn Observatory, Madison, Wis., has been 

 compiled by the aid of the list of " Minor Planets," pub- 

 lished by Mr. A. N. Skinner in the American Journal 

 of Science and Arts, Vol. XVIII, Dec, 1879. All of these 

 asteroids, with one exception, were discovered at the 

 Ann Arbor Observatory, Michigan. Juewa was discov- 

 ered at Peking, China, where Prof. Watson was in charge 

 of one of the Transit of Venus parlies. 



Number. Name. Date of Discovery. 



79 Eurynome September 14, 1863. 



93 Minerva August 24, 1867. 



94 Aurora September 6, 1867. 



100 Hecate July 11, 1868. 



101 Helena August 15, 1868. 



103 Hera September 7, 1868. 



104 Clymene September 13, 1868. 



105 Artemis September 16, 1868. 



106 Dione October 10, 1868. 



115 Thyra August 6, 1871. 



119 Althaea April 3, 1872. 



121 Hermione May 12, 1872. 



128 Nemesis November 25, 1872. 



132 Aethra June 13, 1873. 



133 Gyrene August 26, 1873. 



139 Juewa October 10, 1874. 



150 Nuwa October 19, 1875. 



161 Athor April 19, 1876. 



168 Sibylla September 28, 1876. 



174 Phaedra September 3, 1877. 



175 Andromache October 1, 1877. 



The report of the Telegraphic Determination of 

 Longitudes on the East Coast of South America, by 

 Lieutenant Commanders F. M, Green, and C. H. 

 Davis, and Lieutenant J. A. Norris, U.S. N., has been 

 issued recently from the Hydrographic Office. This 

 work embraces the meridians of Lisbon, Madeira, St. 

 Vincent, Pernambuco, Bahia, Rio de Janeiro, Montevideo, 1 

 Buencs Ayres and Para, and is designed to supplement 

 the work done in 1877, under the direction of Lieutenant '. 

 Commander Green, in the West Indies and Central 

 America, by connecting important points in South Ame- 

 rica, whose longitudes have always been exceedingly 

 uncertain, with well-known places in Europe. 



Having made arrangements with the French Bureau 

 des Longitudes to furnish the party with the difference of 

 longitude between Lisbon and Paris, the work was begun 

 in December, 1877, by connecting Lisbon, Portugal, with 

 Funchal, Madeira, by means of an intervening station at 

 Carcavellos. This " transmitting " station was found 

 necessary in order to connect the submarine cables with j 



the land lines ; a direct connection endangering the safe- 

 ty of the cables. Partly by cables, and partly by the 

 overland wires, the stations from Lisbon to Buenos 

 Ayres were connected in the order named above, with 

 the exception of a break between Pernambuco and Rio 

 de Janeiro caused by a defect in the cable. These two 

 stations were connected with Bahia, and Pernambuco 

 with Para in 1879 ; and as the French Government had 

 failed to communicate to the Hydrographic Office the 

 longitude of Lisbon, it was determined to connect Lisbon 

 with Greenwich, in order to make the chain complete. 

 Tins last connection was effected by means of transmit- 

 ting stations at Porthcurnow, Lands End, and Carcavel- 

 los on the coast of Portugal. The reduction of com- 

 parisons of the Lisbon and Greenwich clocks "gives the 

 somewhat startling result that the longitude ot the ob- 

 servatory at Lisbon, has, up to the present time, been in 

 error more than two miles." The American de'ermina- 

 tion of the difference of longitude between these two 

 places being 9 11" 10.2", while that heretofore accepted 

 has been 9 9' 2.1". 



Of the instruments used, the Transit Instrument was ot 

 what is known as the " broken transit " pattern (the eye- 

 piece being at one end of the horizontal axis), especially 

 designed for this work by Mr. J. A. Rogers, and fitted to 

 be used as both transit and zenith-telescope. It was of 

 2.5 in. aperture and 30 in. focal length — made by Kahler, 



It seems to have combined considerable steadiness with 

 great portability, as it weighs in all but 125 lbs. In speak- 

 ing of the performance of this instrument, the report 

 says: "The results of the observations have demon- 

 strated that the reversal of the axis is almost inevitably 

 attended with a slight change of azimuth, and that a cor- 

 rection must always be introduced for flexure of the axis," 

 and adds further on, that these effects "are probably un- 

 avoidable in portable instruments of this pattern." 



In the reductions, no correction has been applied for 

 personal equation of the observers, either in noting tran- 

 sits of stars, or in receiving the deflections of the gal- 

 vanometer needle from the cables. After careful experi- 

 ment, it was found that the correction would be quite 

 small, and in view of the uncertainty involved in its de- 

 termination, it was decided to take no account of such 

 error, but to eliminate it, as far as possible, by placing one 

 observer alternately east and west of the other, com- 

 mencing at Lisbon. Advantage was taken of every op- 

 portunity to make latitude determinations with the 

 zenith-telescope, and the results in both latitude and 

 longitude show that nearly all of the stations occupied 

 have been up to this time considerably in error. 



The spectrum of Hartwig's comet has been observed 

 by Konkoly and Backhouse, and by Young in this coun- 

 try. It gives four bright lines, whose wave-lengths are 

 respectively 5609, 5492, 5169, and 4859 tenth- meters, and 

 a faint continuous spectrum. W. C. W. 



Washington, D. C, November 30, 1880. 



SWIFT'S COMET. 



Swift's comet is a faint object, and its distance from 

 the sun is so great, never less than 1.102, and therefore 

 always outside the earth's orbit, that no great changes of 

 form are to be expected, such as we see in comets that 

 pass near the sun. A. Hall. 



To the Editor of Science : 



Several interesting observations have been made by 

 me of Swift's latest comet. The last observation was 

 made on the evening of November 26th, at 7.20 P.M.T., 

 being then by estimation in about A. R. 2 hours 30 min- 

 utes, north declination 53 degrees 45 minutes. It was 

 quite a conspicuous object in the 5-inch Newtonian Re- 



