438 THE POPULAR SCIENCE MONTHLY. 



an appreciable extent. The substance of a mass of iron forty or fifty 

 feet high, would be the same in structure at the bottom as at the top 

 of the mass ; for the strength of the metal would resist any change 

 which the weight of the mass would (otherwise) tend to produce. But 

 if there were a cubical mountain of iron twenty miles high, the lower 

 part would be absolutely plastic under the pressure to which it would 

 be subjected. It would behave in all respects as a fluid, insomuch 

 that if (for convenience of illustration) we suppose it inclosed within 

 walls made of some imaginary (and impossible) substance which would 

 yield to no pressure, then, if a portion of the wall were removed near 

 the base of the iron mountain, the iron would flow out like water * 

 from a hole near the bottom of a cask. The iron would continue to 

 run out in this way, until the mass was reduced several miles in 

 height. In Jupiter's case a mountain of iron of much less height 

 would be similarly plastic in its lower parts, simply because of the 

 much greater attractive power of Jupiter's mass. Thus we see that 

 the conception of a hollow interior, or of any hollow spaces through- 

 out the planet's globe, is altogether inconsistent with what is known 

 of the constitution of even the strongest materials. 



How, then, are we to explain the relatively small mean density of 

 Jupiter's globe ? On the supposition that his atmosphere is less than 

 fourteen miles deep, we cannot do so ; for there is nothing hypothet- 

 ical in the above considerations respecting a solid globe as large as 

 Jupiter's, excepting always the assumption that the globe is not 

 formed of substances unlike any with which we are familiar. Even 

 this assumption, though it is one which few would care to maintain in 

 the present position of our knowledge, amounts after all to an admis- 

 sion of the chief point which I am endeavoring to maintain : itis one 

 way — but a very fanciful way — of inferring that Jupiter is utterly dis- 

 similar to the earth. Rejecting it, as we safely may, we find the small 

 density of Jupiter not merely unexplained, but manifestly inexplicable. 



All our reasoning has been based on the assumption that the at- 

 mosphere of Jupiter exists at a temperature not greatly differing from 

 that of our own atmosphere. If we assume instead an exceedingly 

 high temperature, abandoning of course the supposition that Jupiter 

 is an inhabited world, we no longer find any circumstances which are 

 self-contradictory or incredible. 



To begin with, we may on such an assumption find at once a par- 

 allel to Jupiter's case in that of the sun. For the sun is an orb at- 

 tracting his atmospheric envelope and the material of his own solid 

 or liquid surface (if he has any) far more mightily than Jupiter has 

 been known to do. All the difficulties considered in the case of Jupi- 

 ter would be enormously enhanced in the case of the sun, if we forgot 

 the fact that the sun's globe is at an intense heat from surface to cen- 



1 The effect of pressure in rendering iron and other metals plastic has been experi- 

 mentally determined. Cast-steel has been made to flow almost like water, under pressure. 



