ON THE SOLAR SYSTEM. 403 



[four] small planets, Juno, Pallas, and Ceres [Vesta], twice 24 or 48, making 

 5$, for Jupiter ; twice 48 or 96 for Saturn, making 100 ; and twice 9G or 192, 

 making 1Q6, for the Georgian planet ; and these sums will represent the 

 d ; nces, without any material exception, in the nearest integer numbers. 



,/yTne planet Mercury is little more than one third as large as the earth in 

 Diameter. He performs his revolution in somewhat less than three months, 



v at about two fifths of the distance of the earth. His orbit is more eccentric, 

 t>^d more inclined to the ecliptic, than those of any of the planets except 

 the three [four] small ones lately discovered ; the eccentricity being one 

 fift^of the mean distance, and the inclination 7. Of his density and his 

 rocl&on we know nothing but from conjecture.* 



"Ve1'.7.s is very nearly as large as the earth; Dr. Herschel thinks her 

 even a little larger. Her revolution occupies about 7 months, her distance 

 from the sun being about seven tenths of that of the earth, and her orbit 

 nearly circular, inclined in an angle of 3 24' to the ecliptic. Mr. Schroeterf 

 attributes to her mountains much higher than those of the earth, he has 

 observed strong indications of an atmosphere surrounding her, and he 

 assigns for her rotation on her axis the period of 23 hours 21 minutes. 

 Her density has been estimated from the perturbations, occasioned by her 

 attraction, in the motions of the other planets, and it has been supposed 

 to be a little less than that of the earth. 



The distance of the earth from the sun is about 95 million English miles ; 

 and this determination is generally supposed to be so far accurate, that 

 there is no probability of an error of more than a million or two, at most, 

 although some authors are still disposed to believe that the distance may be 

 even greater than a hundred millions. The period of its revolution, with 

 respect to the equinoctial points, which are the usual standard of compari- 

 son, since their situation determines the annual return of the seasons, is 

 365 days, 5 hours, 48 minutes, and 48 seconds; and this is called its 

 tropical revolution ; that of its absolute or sidereal revolution is 365 days, 

 6 hours, 9 minutes, and 8 seconds ; the difference, which is 20 minutes and 

 20 seconds, being the time occupied in passing over the space, through 

 which the equinoctial points have retreated in the course of the tropical 

 year. By a day, we always understand the time which elapses during the 

 rotation of the earth with respect to the sun ; a sidereal day is about four 

 minutes shorter. 



At a distance from the sun exceeding that of the earth by one half, the 

 planet Mars revolves, in about a year and seven eighths. He is of half 

 the earth's linear dimensions : he has spots which change their form, and, 

 therefore, probably, an atmosphere. Dr. Herschel J found his rotation per- 

 formed in 39 minutes more than a day ; his equator inclined 28 42' to 

 the plane of his orbit, and his figure so much flattened at the poles, that 

 his axis is -^th shorter than his equatorial diameter. From this form, 



* Consult Lalande, Mem. de 1'Inst. v. 442. 



,f Beobachtungen, 4to, Erfurth, 1793. Aphroditographische Fragmenten, 4 to, 

 Helm. 1796. Journal de Physique, xlviii. 459. Beytrage, 8vo, Berlin, 1788. Ph. 

 Tr. 1792, Ixxxii. 309 ; 1795, Ixxxv. 117. 



J Herschel, On the Planet Mars, Ph. Tr. 1781, p. 115 ; 1784, p. 223. 



2 D 2 



