43 6 THE POPULAR SCIENCE MONTHLY. 



sible. Or, if any one is disposed, for the sake of argument, to assume 

 that a gas {at ordinary temperatures) may be as dense as water then 

 we need proceed but a few steps further, increasing the pressure about 

 18,000 times instead of 900 times, to have the density of platinum in- 

 stead of that of water, and no one is likely to maintain that our air 

 could exist in the gaseous form with a density equaling that of the 

 densest of the elements. We are still an enormous way behind the 

 number of twenty-one figures mentioned above ; and, in fact, if we sup- 

 posed the pressure and density to increase continually to the extent 

 implied by the number of twenty-one figures, we should have a den- 

 sity exceeding that of platinum more than ten thousand millions of 

 millions of times ! 



Of course this supposition is utterly monstrous, and I have merely 

 indicated it to show how difficulties crowd around us in any attempt 

 to show that a resemblance exists between the condition of Jupiter 

 and that of our earth. The assumptions I made were sufficiently 

 moderate, be it noticed, since I simply regarded (1) the air of Jupiter 

 as composed like our own ; (2) the pressure at the upper part of his 

 cloud-layer as not less than the pressure far above the highest of our 

 terrestrial cumulus clouds (with which alone the clouds of Jupiter are 

 comparable) ; and (3) the depth of his cloud-layer as about one hun- 

 dred miles. The first two assumptions cannot fairly be departed from 

 to any considerable extent, without adopting the conclusion that the 

 atmosphere of Jupiter is quite unlike that of our earth, which is pre- 

 cisely what I desire to maintain. The third is, of course, open to at- 

 tack, though I apprehend that no one who has observed Jupiter with 

 a good telescope will question its justice. But it is not at all essential 

 to the argument that the assumed depth of the Jovian atmosphere 

 should be even nearly so great. We do not need a third of our array 

 of twenty-one figures, or even a seventh part, since no one who has 

 studied the experimental researches made into the condition of gases 

 and vapors can for a moment suppose that an atmosphere like ours 

 could remain gaseous, except at an enormously high temperature, at a 

 pressure of two or three hundred -atmospheres. Such a pressure would 

 be attained, retaining our first two assumptions, at a depth of about 

 fourteen miles below the upper part of the cloud-layer. This is about 

 the six-thousandth part of the diameter of Jupiter; and, if any student 

 of astronomy can believe that that wonderfully complex and change- 

 ful cloud-envelope which surrounds Jupiter has a thickness of less 

 than the six-thousandth part of the planet's diameter, I would recom- 

 mend as a corrective the careful study of the planet for an hour or 

 tw^o with a powerful telescope, combined with the consideration that 

 the thickness of a spider's web across the telescopic field of view would 

 suffice to hide a breadth of twenty miles on Jupiter's disk. 



But we are not by any means limited to the reasoning here indi- 

 cated, convincing as that reasoning should be to all who have studied 



