404 LECTURE XLII. 



compared with the time of his rotation, it may be inferred that his 

 density must be very unequal in different pails : Laplace supposes it 

 from calculation to be on the whole about three fourths as great as that of 

 the earth. 



In the interval between Mars and Jupiter, and nearly at the 

 where, from a dependance on the regularity of the progression already men- 

 tioned, a number of astronomers had for some years been seeking for a 

 primary planet, the observations of Mr. Piazzi,* Dr. Olbers,t and M* . 

 Harding ; have placed three very small bodies, differing but little in their 

 mean distance and their periodical time. They have named them Cf-res, 

 Pallas, and Juno : none of them subtends an angle large enough ttV'je 

 measured by our best instruments ; and all the circumstances af-'their 

 motions are yet but imperfectly established. Juno, however, appears to be 

 somewhat less remote than the other two : all their orbits are considerably 

 inclined to the ecliptic, especially that of Pallas, which is also extremely 

 eccentric. Dr. Herschel does not admit, that they deserve the name of 

 planets, and chooses to call them asteroids. 



Jupiter is the largest of all the planets, his diameter being 11 times as 

 great as that of the earth, and the force of gravitation at his surface being 

 triple the terrestrial gravitation. He revolves in about 12 years, at a little 

 more than five times the earth's distance from the sun. His rotation is 

 performed in less than ten hours, his equator being inclined about three 

 degrees to his ecliptic, which makes an angle of 1 19' with ours. His belts 

 are supposed by many to be clouds in his asmosphere ; they seem to have 

 a rotation somewhat slower than that of the planet. 



The diameter of Saturn is ten times as great as that of the earth, but, on 

 account of the smaller density of his substance, the force of gravity at his 

 surface scarcely exceeds its force at the surface of the earth. He revolves 

 in 29 years and a half, in an orbit inclined 2^ to the ecliptic, at the dis- 

 tance of 9 1 semidiameters of the earth's orbit : his rotation occupies only 

 10^ hours, and his equator is inclined about 30 to our ecliptic. The most 

 remarkable circumstance attending him is the appearance of a double ring,|| 

 which is suspended over his equator, and revolves with a rapidity almost 

 as great as that of the planet. His figure appears also, according to Dr. 

 Herschel's observations, to be extremely singular ; deviating very consi- 

 derably from that of an elliptical spheroid, which is the form assumed by 

 all the other planets that appear flattened, and approaching in some degree 

 to a cylinder with its angles rounded off. Such a form can only be derived 

 from some very great irregularities in the density of the internal parts of 

 his substance. 



* Discovered Ceres, 1st Jan. 1801. Zach's Mon. Corresp. iv. 53. 



t Disc. Pallas, 28th March, 1802. 



I Disc. Juno, 1st Sept. 1804. 



A fourth, named Vesta, was discovered by Olbers, on 19th March, 1807. 



|| Pound. Ph. Tr. 1732, p. 240. Laplace, Memoire sur la Theorie de 1'An- 

 neau de Saturne. Herschel, Ph. Tr. 1790, pp. 4, 427 ; 1792, p. 1 ; 1794, p. 48 ; 

 1805-6-8. Bessel makes the inclination of the ring to our ecliptic to be 28 22', 

 Berlin, Ephem. 1814, 1822. He estimates the mass of the ring at T } g of that of 

 Saturn, Ast. Nach. Nos. 193-4-5. 



