as Tyndall has shown, and thus the nights are warmer than where the air is dry. Now, in Jupiter's atmosphere there is much water, for observers, armed with that wonderful instrument, the spectroscope, have recognized the very same dark bands upon the spectrum of the planet which appear in the solar spectrum when the sun is low down, and therefore shining through the lower and denser atmospheric strata. The spectroscopist knows that these bands are due to the aqueous vapor in the air, because Janssen saw the very same bands when he examined the spectrum of a powerful light shining through tubes filled with steam. So that there is the vapor of water—and that, too, in enormous quantities in the atmosphere of Jupiter. But though we thus recognize the very quality necessary for an atmosphere which is to retain the solar heat, our difficulty is not a whit lessened; for it is as difficult to understand how the invisible aqueous vapor finds its way thus into the planet's atmosphere, as to understand how the great cloud-masses are formed.
Aqueous vapor in the atmosphere, whether its presence is rendered sensible to the sight or not, implies the action of heat. Other things being equal, the greater the heat the greater the quantity of watery vapor in the air. In the summer, for instance—though many imagine the contrary—there is much more of such vapor in the air than there is in winter, the greater heat of the air enabling it to keep a greater quantity of the vapor in the invisible form. In winter, clouds are more common, and the air seems moister; yet, in reality, the quantity of aqueous, vapor is reduced. Now, it cannot but be regarded as a remarkable circumstance that, though the sun supplies Jupiter with only one twenty seventh part of the heat which we receive, there should yet be raised from the oceans of Jupiter such masses of clouds as to form veritable zones; and that, moreover, above these clouds there should be so large a quantity of invisible aqueous vapor that the spectroscopist can recognize the bands of this vapor in the planet's spectrum.
Even more perplexing is the circumstance that the cloud-masses should form themselves into zones. We cannot get rid of this difficulty by a mere reference to the planet's rapid rotation, unless we are prepared to show how this rotation is to act in forcing the cloud-masses to become true belts. The whole substance of Jupiter and his whole atmosphere must take part in his rotation, and to suppose that aqueous vapor raised from his oceans would be left behind in the upper air, like the steam from a railway-engine, is to make a mistake resembling that which caused Tycho Brahe to deny the rotation of the earth, because bodies projected into the air are not left behind by the rotating earth. Nor is it conceivable that belts which vary remarkably, from time to time, in position and extent, should be formed by sun-raised clouds in the Jovian atmosphere, if the planet's surface is divided into permanent lands and seas.
