398 



NATURE 



\Sept. 6, 1877 



communicated to the Smithsonian Institution, and by it, 

 to the European observatories. 



We subjoin the principal results of Prof. Newcomb's 

 calculations on the first ten days' measures, extracted 

 from a circular for which we are indebted to the 

 superintendent of the U.S. Naval Observatory. 



For the outer satellite Prof. Newcomb finds : — 



Major axis of apparent orbit at distance [9'S93o] 82"'5 



Minor axis ,, ,, '^^"'^ 



Major axis of orbit at distance unity 32"'3 



Position angles of line of apsides of apparent orbit. .70°. ..250° 

 Passagethrough west apsis (/ = 250°)... Aug. igat l6h'6 W.M.T. 



Inclination of true urbit to the ecliptic 25""4 



Longitude of ascending node 82°'8 



Peiiod of revolution 30I1. 14111. 



Hourly motion in aveocentric longitude Ii'-gi 



These elements give for the mass of Mars 



3,090,000 



For the inner satellite Prof. Newcomb finds- 



Major axis of apparent orbit at distance [9'593o] 33"'o 



Passage through eastern apsis (/ = 70").,, Aug. 20 at i3h'oW.M.T. 



Period of revolution 7h. 3Sm'5 



Hourly motion in areocentric longitude 47'"li 



If we refer the position of the orbit of the first satellite 

 to the earth's equator, the ascending node will be found 

 to be in 46°4, and the inclination 36° 2. The real dis- 

 tances of the satellites from the centre of Mars are about 

 14,500 and 5,800 miles respectively. 



Prof. Newcomb, writing to the New York Tribune on 

 August 22, gives some particulars relatmg to the new 

 satellites. He remarks that the first question which will 

 naturally arise is, Why have these objects not been seen 

 before.'' The answer is that Mars is now nearer to the 

 earth than he has been at any time since 1S45, when the 

 great telescopes of the present day had hardly begun to 

 be known. In 1862 when Mars was again pretty near 

 to the earth, we may suppose that they were not looked 

 for with the two or three telescopes which alone would 

 have shown them. In 1875 Mars^was too far south of 

 the equator to be advantageously observed in these 

 latitudes. The present opportunity of observing the 

 planet is about 'he best that could possibly occur. At 

 the next opposition in October, 1S79, there is hope that 

 the satellites may be again observed with the great 

 telescope at Washington, but Prof. Newcomb thinks that 

 during the following ten years, when owing to the great 

 eccentricity of the orbit of Mars, he will be much further 

 from the earth at opposiiion,the satellites may be invisible 

 with all the telescopes of the world. In the present year 

 it is hardly likely that they will be visible after October. 

 The satellites may be considered as by far the smallest 

 heavenly bodies yet known. " It is hardly possible to 

 make anything like a numerical estimate of their 

 diameters, because they are seen in the telescope only 

 as faint points of light. But one might safely agree to 

 ride round one of ihem in a railway car between two 

 successive meals, or to walk round in easy stages durin^:' a 

 very brief vacation. In fact, supposing the surface of the 

 outer one to have the same reflecting power with that of 

 Mars, its diameter cannot be much more than ten miles, 

 and may be less. Altogether these objects must be 

 regarded as among the most remarkable of the solar 

 system." Prof. Newcomb further points out that we may 

 regard M. Leverrier's mass of Mars as the product of a 

 century of observation, and several years of laborious 

 calculation by a corps of computers, while from the 

 measures on four nights of the new satellites, ten minutes' 

 computation gives a mass which is in striking agreement 

 with that of the dlustrious astronomer of Paris. 



The D'Angos Comet of 1784.— In continuation of our 

 remarks upon this object, so long considered apocryphal 



(Nature, vol. xvi. p. 124), we proceed to notice an investi- 

 gation by D'Arrest, undertaken in 1865. He refers to a 

 somewhat confused note in Cooper's '• Catalogue of 

 Cometic Orbits," where the orbit given by D'Angos is 

 introduced " with no slight misgivings," though the writer 

 thought his note was " considerably opposed to Encke's 

 statement," on what precise grounds, however, is not very 

 apparent. And D'Arrest considered the essay in Zach's 

 work may have received undue colouring from the editor, 

 and a further proof of the non-existence of the comet is 

 required. 



Deducing the sun's places from the Tables of Hansen 

 and Olufsen, D'Arrest compares the fourteen days' obser- 

 vations given by D'Angos in Bernouilli's and Hinden- 

 burg's Magazine, with the best of two orbits calculated by 

 Olbers from them, and finds what he terms " horrenden 

 Abweichungen." A similar comparison by means of 

 elements selected from numerous sets computed by 

 himself, exhibits also great differences, so that his first 

 conclusion is identical with that arrived at by Olbers and 

 Encke, the comet's places are irreconcilable with para- 

 bolic motion about the sun. On applying the general 

 method of the Tlieoria motus he was led to extra- 

 ordinarily small distances from the earth, in perfect 

 agreement with the results obtained by Encke on a 

 different method of procedure. 



D'Arrest then introduces the consideration of the 

 geocentiii hypotliesis. We are certain of cases where the 

 orbits of comets have undergone an entire change 

 through the action of the planet Jupiter, and there is now, 

 he urges, no difficulty in accepting the idea of an insig- 

 nificant and small vaporous mass, passing with a retro- 

 grade motion in such close proximity to the earth, that 

 the case treated of in the " M(?canique Celeste," book ix. 

 chap, ii., may enter. The question is a complicated one, 

 and D'Arrest contents himself with examining whether in 

 the series of observations of the comet, there is anything 

 to contradict the hypothesis that for a time it was moving 

 as a satellite of the earth. Necessarily omitting parallax, 

 he finds on this geocentric assumption the following 

 elements : — Time of passage by the perigee, April 1 5 ■ 1 3649 

 mean time at Paris ; ascending node 90° 52' 9" ; inclina- 

 tion 43" 8' 23" ; distance of perigee from ascending node 

 128° 7' 59" ; eccentricity 0^97320, and least distance from 

 the earth in units of the semi-axis major of the moon's 

 orbit 3 82290 ; motion, retrograde. When now the 

 comet's distances from the earth are found from the 

 observed angular motions, and again direct from the 

 elliptical elements, a comparison shows that the angular 

 motion with respect to the earth corresponds well with 

 the law of areas, and the latitudes from this approximate 

 calculation are sensibly better represented, than on the 

 supposition of heliocentric motion. 



The general conclusions arrived at by D'Arrest from 

 his investigation appear to be these : that theimpossibihty 

 of the observations published by D'Angos and the obvious 

 imposition previously attributed to him are not in accord- 

 ance with evidence. The comet may have been moving 

 for a time virtually as a satellite of the earth, or while 

 passing very near to it may yet have been without the 

 sphere of attraction, the definition of which is perhaps 

 somewhat arbitrary. The difficulty and uncertainty 

 attending further research, and the expense of time and 

 labour necessary, induced D'.\rrest to nst satisfied with 

 havmg thrown a more satisfactory light on the observa- 

 tions of the second comet of 1784, or at least of proxing that 

 a small body moving under certain conditions might have 

 occupied the positions in the heavens attributed to it. 

 We have yet to give some account of a paper by Gauss 

 on this subject. 



The First Comet of 1877 (Borrellv, February 8) 

 —The elements of this comet, so far as yet published, 

 appear to depend upon an interval of observation of eight 



