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THE POPULAR SCIENCE MONTHLY. 



of course, excepted) in the solar system. The reader is doubtless 

 aware that the sun's mean density is almost exactly one-fourth of the 

 earth's ; Jupiter's is almost exactly the same as the sun's ; but Saturn's 

 is little more than half the sun's, being represented by the number 

 13 only, where 100 represents the earth's. Thus, instead of exceeding 

 the earth nearly 700 times in mass, as he would if he were of the same 

 density, he exceeds her but about 90 times. But this disproportion 

 must still be regarded as enormous, especially when it is added that 

 the combined mass of the four terrestrial planets amounts to little more 

 than the forty-fourth part of Saturn's mass. The combined mass of 

 Uranus and Neptune, though these are members of the family of 

 major planets, falls short of one-third of Saturn's mass ; yet, by com- 

 parison with Jupiter, whose mass exceeds his more than threefold, 

 Saturn appears almost dwarfed. And it may be noted as a striking 

 circumstance — one that is not sufficiently recognized in our astronomical 

 treatises — that, while Jupiter's mass exceeds the combined mass of all 

 the other planets (including Saturn) about two and a half times, Saturn 

 exceeds all the remaining planets in mass about two and three-quarter 

 times. So unequally is the material of the planetary system dis- 

 tributed. 



Fig. 2. 



Saturn and his Moons. 



The mighty globe of Saturn rotates on its axis in about nine hours 

 and a half, the most rapid rotation in the solar system so far as is yet 

 known. 



But what shall we say to indicate adequately the dimensions of 

 that enormous ring-system which circles around Saturn? Here we 

 have no unit of comparison, and scarcely any mode of presenting the 

 facts except the mere statement of numerical relations. Thus the full 

 span of the rings, measured across the centre of the planet, amounts 

 to 167,000 miles ; the full breadth of the ring-system amounts to 

 35,600 miles. But these numbers convey only imperfect ideas. Per- 

 haps the best way of indicating the enormous extent of the ring-sys- 

 tem is to mention that circumnavigation of the world by a ship sail- 

 ing from England to New Zealand by the Cape of Good Hope, and 

 from New Zealand to England by Cape Horn, would have to be re- 

 peated 21 times to give a distance equaling the outer circumference of 



