508 LECTURE xnr. 



is ^lyth shorter than his equatorial diameter. From this form, compared with 

 the time of his rotation, it may be inferred that his density must be very 

 unequal in different parts: 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 distance 

 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 pri- 

 mary planet, the observations of Mr. Piazzi, Dr. Olbers, and Mr. Harding 

 have placed three very small bodies, differing but little in their mean distance 

 and their periodical time. They have named them Ceres, Pallas, and Juno: 

 none of them subtends an angle large enough to be measured by our best 

 instruments; and all the circumstances of their motions are yet but imper- 

 fectly established. Juno, however, appears to be somewhat less remote tlian 

 the other tMo; all their orbits are considerably inclined to the ecliptic, espe- 

 cially 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 1 1 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° ly' with ours. His belts are supposed 

 by many to be clouds in his atmosphere ; they seem to have a rotation some- 

 what 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 incHned 24-° to the ecliptic, at the distance 

 4)f 94: 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 greSt as 



