THE MOOR PLANETS. 



THERE is no subject of inquiry to which the improved powers of the tel- 

 escope have been directed with greater effect than the investigation of the 

 physical condition of the several planets composing the solar system. We 

 shall on the present occasion take a review of some of these bodies, and shall 

 state the chief circumstances which have been discovered respecting them. 



In a general survey of the system, the planets composing it will naturally 

 be classed in three distinct groups, the first of which we shall call the minor 

 planets, the second the new planets, and the third the major planets. 



Proceeding from the sun outward in the system, the four planets which are 

 nearest to that luminary are Mercury, Venus, the Earth, and Mars. Between 

 these bodies there prevails a striking analogy. We find that they are not 

 very different in magnitude ; that they correspond closely, so far as we can 

 discover, in their geographical character ; that they receive in not very differ- 

 ent proportions the influence of the sun. The close alliance between them 

 has also occurred to other astronomical writers, inasmuch as they are some- 

 times called the terrestrial planets, from their analogy to the earth. 



OF THE PLANET MERCURY. 



The planet Mercury revolves at a distance from the sun of about thirty-six 

 millions of miles, completing his periodical revolution in about eighty-eight days, 

 or something less than three of our months. The diameter of this pl.-met is 

 about three thousand two hundred miles, or four tenths of that of the earth, and 

 consequently its volume or bulk is about a sixteenth of that of our jjlobe. As 

 Mercury revolves round the sun in an orbit enclosed within that of the earth, it 

 follows that his illuminated hemisphere, which is always presented to the sun 

 in the course of each revolution, must assume every possible variety of position 

 in regard to the earth. Thus when Mercury is between the sun and earth as 

 at A, in what is called inferior conjunction, his dark hemisphere is turned tow- 



