V 



ON THE SOLAR SYSTEM. 507 



cept the three small ones lately discovered; the eccentricity being one fifth 

 of the mean distance, and the inclination 7°. Of his density and his rota; 

 tion we know nothing but from conjecture. 



Venus is very nearly as. large as the earth; Dr. Herschel thinks her even 

 ahtcle 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 cir- 

 cular, inclined in an angle of 3° 24' to the ecliptic . Mr. Schroeter 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 ttie motions 

 of the other planets, and it has been supposed to be a little less thjin 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 tliat 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 comparison, 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 36"5days,6 hours,9minutes,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 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 



