DAYS, NIGHTS, AND SEASONS. 



makes with" the terrestrial equator. The Saturnian seasons^ 

 zones, and_clim.ates are, therefore, absolutely similar to those of 

 the earth. The tropical and polar phenomena are the same. 



It is to be hoped that the recent improvements effected by 

 Lord Rosse, in the construction of reflecting telescopes, may 

 place it within the power of observers to determine the position 

 of the axes of Uranus and^Neptune, and the line of rotation of 

 the latter. 



5. So far as discovery has hitherto proceeded, it would appear 

 that a comparatively greater rapidity of rotation, and shorter 

 intervals of light and darkness, is a characteristic by which the 

 group of major planets are distinguished from the terrestrial 

 group. 



6. A second striking distinction between these two groups is 

 the comparative lightness of the matter composing the former. 

 It will be remembered that, in our notice of the terrestrial group, 

 we showed that the density of the matter composing the earth, 

 Venus, and Mars is nearly equal, and is five-and-a-half times that 

 of water, and about the same as that of iron-stone, while the 

 density of the planet Mercury is equal to that of gold. Now, it 

 appears that, on the contrary, the density of Jupiter very little 

 exceeds that of water, that of Uranus and Neptune is exactly 

 that of water, while Saturn is so light that it would float in 

 water like a globe of pine-wood. 



It must be admitted to be not the least striking among the 

 wondrous results of human sagacity, that these remote globes 

 have been submitted to such an analysis as enables us thus to pro- 

 nounce with certainty upon one, at least, of the physical characters 

 of their constituent parts. In some instances science has even gone 

 further, and has shown that the densities of Jupiter and Saturn 

 cannot be uniform, but must increase gradually as that of the 

 earth is known to do, from the surface to the centre, and from 

 this it follows that the mean density of the matter of their 

 surface must be much less than that of water. 



7. It follows, therefore, that the seas and oceans of these 

 planets must consist of a liquid far lighter than water. It is 

 computed that a liquid on Jupiter, which would be analogous to 

 the terrestrial oceans, would be three times lighter than sul- 

 phuric ether, the lightest known liquid, and would be such that 

 cork would scarcely float in it. 



8. The rapid rotation of these planets, combined with the 

 great length of their revolution round the sun, gives them years 

 consisting of a vast number of days. The year of Jupiter 

 is nearly twelve terrestrial years, or, more exactly, 4332^ ter- 

 restrial days. But as the Jovian days are shorter than the 



*2 35 



