vicissitudes of the seasons. The planets are retained in their orbits by the com- 

 pound action of two mutually opposing forces : first, the centrijieial force, or ra- 

 vitation, by which a body is attracted towards the centre of gravity, which, in the 

 case of the planets, is the sun ; and, secondly, the centrifugal f<>rce, by which a body 

 in motion tends to proceed in a straight line. Thus, if a body be acted upon by 

 two forces impelling it in different directions, the body will obey neither, but take a 

 direction compounded of both, or between the two. It is the action then of this 

 universal law which retains the planetary bodies in their appointed orbits. 



Several of the planets are accompanied by satellites or moons, which supply light, 

 by reflection from the sun during his nightly absence, to their primaries. The 

 sa;ellites revolve round their primaries, and accompany them in their revolution round 

 the sun. Between the orbits of Mars and Jupiter have been discovered a number of 

 small planets, which are all classed under the term MINOR PLANETS. Their exis- 

 tence was unknown before the commencement of the present century ; but at this 

 time we are acquainted with thirty-five, and probably more may yet be discovered. 



MERCURY. This planet is, with the exception of the minor planets, the smallest 

 in the system, at the same time he is the most compact celestial body with which we 

 are acquainted. His density is about fourteen times that of water, or more than 

 equal to that of lead. On account of his small size and proximity to the sun, Mer- 

 cury is seldom distinctly seen, but with a good telescope he may sometimes be dis- 

 covered a little before sunrise and after sunset. The telescopic appearance of 

 Mercury is that of a planet having phases, or assuming that alternate increase and 

 decrease of form under which we see the moon, except that Mercury does not appear 

 quite full to us. The powerful telescopes of modern times have discovered to us 

 spots on the surface of this planet by means of which his axial motion has been 

 ascertained, and found to be 24 hours 5 minutes and 28 seconds; thus his day is 

 nearly the same length as our own. His year, however, consists only of 88 days, 

 being the period in which he completes his revolution round the sun, so that bis 

 seasons will each consist of only three or four weeks. 



VENUS. This is the brightest of the planets, and as she is usually visible at the 

 time of sunrise and sunset, she has received the name of the morning and evening 

 star. Venus would appear to be the sister globe to the earth ; her diameter differs 

 only by 200 miles from that of our own planet; her day only by a few minutes ; 

 she is surrounded, like the earth, with an atmosphere, through which clouds and 

 vapours float, indicating the existence of water beneath, from which they derive their 

 origin ; her surface is also diversified by mountains of vast height. The matter of 

 this planet is supposed to be somewhat denser than that of the earth. 



The phenomena of the seasons, which depend (as we shall hereafter explain) upon 

 the inclination of the axis of a planet to the plane of its orbit, are peculiar in the 

 case of Venus. Her axis is inclined about 75 degrees to her orbit, and as the decli- 

 nation of the sun on each side of the equator is equal to the inclination of her axia, 

 her tropics are only 15 degrees from her poles, and her polar circles at the same 

 distance from her equator. The variations of her seasons is so frequent that she 

 has two winters, springs, summers, and autumns in each of her annual revolutions. 



Venus exhibits the various lunar phases, except that she never appears quite full ; 

 for when the whole of her enlightened side is turned towards us, in her superior 

 conjunction, the solar rays interfere with her splendour. Her course is as follows: 

 Soon after her inferior conjunction, when she passes between the sun and the earth, 

 we behold Venus as a morning star, rising a little before the sun, exhibiting a fine 

 silver crescent. She gradually gains upon the luminary, rises more and more before 

 him, till her greatest angular distance westward is attained, when she appears a 

 semi- circle. Proceeding to her superior conjunction, she apparently returns to the 

 sun, rises later and later, appears gibbous, and then nearly full. On the east of the 

 sun Venus becomes an evening star, visible for a short time after his setting. 

 Passing to her eastern elongation, she sets later every night, and her appearance is 

 gradually reduced to a semi-circle j after which, returning to the sun, she becomes 

 a crescent, sets with him, and is invisible, the unenlightened half of the orb being 

 towards us. In a few days the phenomena of the morning star are repeated. 



Deferring for the present the description of the earth, we will pass on to ^the next 

 planet in order of distance from the sun. 



MARS. This planet, which has its orbit exterior to that of the earth, is about 

 one half the diameter of the latter. He sometimes presents a gibbous, and, at 

 other times, a circular appearance. He possesses an atmosphere which is very 

 dense, and of considerable extent. From the nature of this atmosphere arises, in 

 part, perhaps, the red colour which distinguishes this planet; tuough this may be 



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