OF THE PLANET NEPTUNE. 427 



examination we shall see that whilst certain general characteristics 

 belong to the two great divisions of the primary planets, yet when 

 these are examined in detail there is by no means an uniform agree- 

 ment amongst them individually. 



The eight primary planets composing the Solar System are divided 

 into two groups of four each, separated by the space which comprises 

 the orbits of the Asteroid planets. The four interior planets, viz : 

 Mercury, Venus, the Earth, and Mars have greater densities than the 

 exterior Planets, are of less size, and with one exception are unpro- 

 vided with moons. They rotate on their axes in rather more than 

 double the time of the exterior planets — they move round the sun 

 with far greater velocity. Their year varies from about three months, 

 the year of Mercury, to a year and eleven months, the year of Mars, 

 (in round numbers), and their day is about twenty-four hours long. 



The four exterior planets, viz., Jupiter, Saturn, Uranus, and Nep- 

 tune have less densities than the interior ones ; but their size is vastly 

 greater. They move rouud the sun much slower — their year varies 

 from nearly twelve years, which is the length of the year in Jupiter, 

 to one hundred and sixty-four years, which is the length of the year 

 in Neptune ; their day, as far as has been ascertained, is about 

 ten hours long. Thus we see that increased velocity of axial rotation, 

 and, consequently, increased centrifugal force, with its corresponding 

 diminished force of gravity at the surface, is a characteristic of the 

 four superior planets. 



But these greater bodies have also increased means for compensa- 

 ting the reduced amount of light they receive from the sun, for they 

 are all provided with moons in greater or lesser numbers ; Jupiter 

 has four moons ; Saturn has eight, and several rings ; Uranus four, 

 and Neptune but one. There are many other differences existing 

 between these two groups which I have not time to dwell upon. 



But with regard to the differences in density, size, &c, amongst 

 the particular bodies of these two great divisions of the Solar Sys- 

 tem — although Mercury, the nearest to the sun, is the densest body 

 of the Solar System, yet Mars, which is outside the Earth, is denser 

 than Venus which stands next to Mercury in proximity to the Sun, 

 and Neptune which is the " outsider" of the system is denser than 

 either Uranus or Saturn — Saturn being the lightest body in the 

 System. 



lin, the Earth has a moon, but Mars, which is outside the Earth, 

 has none. Venus was long supposed by Cassini, Short, and other 

 astr to have a moon, but the fine telescopes of our time have 



failed to discover it, and, therefore, Venus must be deemed moonless. 



