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 instead 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 supposed the pressure and density to increase continually to the extent implied by the number of twenty-one figures, we should have a density 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 hundred 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 precisely what I desire to maintain. The third is, of course, open to attack, 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 changeful cloud-envelope which surrounds Jupiter has a thickness of less than the six-thousandth part of the planet's diameter, I would recommend as a corrective the careful study of the planet for an hour or two 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 indicated, convincing as that reasoning should be to all who have studied
