INTRODUCTION. J 



In the following table the inclinations of the axis of the planets to 

 their orbits are given ; and the meaning of this term it may be neces- 

 sary to explain. We have already said that the annual motion of the 

 earth occasions the diversity of seasons : but this would not happen 

 were the axis of the earth exactly parallel to, or in a line with, the 

 axis of its orbit ; because then the same parts of the earth would 

 be turned towards the sun in every diurnal revolution ; which would 

 deprive mankind of the grateful vicissitudes of the seasons arising 

 from the difference in length of the days and nights produced by 

 this inclination of the axis. The axis of several others of the pla- 

 nets are in like manner inclined to the planes of their orbits, and 

 the angle of their inclination is given in the following table. 



A TABLE OF THE DIAMETERS, PERIODS, Sec OF THE 

 SEVERAL PLANETS IN THE SOLAR SYSTEM. 







Mean distances 





i 









Names of the 

 planets. 



Diame- 

 ters in 

 English 

 miles. 



from the sun, as 

 determined from 

 observations of 

 the transit of 



Annualpe- Diurnal ro 

 riod round tation on 

 the sun. [its axis. 



Hourly mo- 

 tion in its 

 orbit. 



Hourlymo 

 tion of its 

 equator. 



Inclination 

 of axis to 

 orbit. 







Venus in 1761. 



















V. 



D. H. J). H. M. 









Sun 



890,000 







'25 14 





3.318 



8° 0' 0" 



MtTCury 



3,2-45 



36,841,468 







87 23J unknown 



109,699 



unknown 



unknown 



Wnus 



7,743 



68,891.486 







224 17j 23 22 



80.295 



1,043 



15 



Earth 



7.942 



95,173.000 



1 



23 56 



68,243 



1,042 



23 29 O 



. D 



2.162 



ditto 



1 







29 12 44 



22.290 



9 1-2 



2 10 



Man 



4.220 



145,014,148 



1 



321 i3 



1 40 



55,287 



556 







■Jupiter 



89,»>00 



494 990.976 



U 



115 14 



9 56 



29,083 



25,920 







Sai am 



79,600 



907,956,110 



29 



174 2 



10 16 



22,101 



22,400 



28 



Gi or^ram 



















Sid us 



39,900 



1,815,912,260 



83 



150 18 



unknown 



15.000 



unknown 



unknown : 



Comets. — Besides the primary and secondary planets already 

 enumerated, there are other bodies which revolve round the sun, 

 and, consequently, make a part of the solar system. These are 

 called Comets, and appear occasionally in every part of the hea- 

 vens. Descending from the far distant parts of the system with 

 great rapidity, they surprise us with the singular appearance of a 

 train, or tail, which accompanies them ; become visible to us in the 

 lower parts of their orbits ; and, after a short stay, go off again to 

 vast distances, and disappear. Though some of the ancients had 

 "more just notions of diem, yet the opinion having prevailed, that 

 they were only meteors generated in the air, like to those we see 

 in it every night, and in a few moments vanishing, no care was ta- 

 ken to observe or record their phenomena accurately till of late. 

 Hence this part of astronomy is very imperfect. The general doc- 

 trine is, that they are solid compact bodies, like other planets, and 

 regulated by the same laws of gravity, so as to describe equal areas 

 in proportional times by radii drawn to the common centre. They 

 move about the sun in very eccentric ellipsis, and are of much 

 greater density than the earth ; for some of them are heated in 

 every period to such a degree as would vitrify or dissipate any sub- 

 stance known to us. Sir Isaac Newton computed the heat of the 

 comet that appeared in the year 1680, when nearest the sun, to be 

 2000 times hotter than red-hot iron, and that, being thus heated, it 

 must retain its heat till it comes round again, although its period 

 should be more than 20,000 years ; and it is computed to be only 

 575. The number of comets is very much greater than that of the 



