It would necessarily follow that Mars, having such small force of gravity, could not long retain a heavy atmosphere, even if it had set out with such a one. No molecule of gas moving at a greater speed than three and thirteen-hundredths miles a second could be held by Mars in its atmosphere, and so much as it may have had of the rarer gases which move with great rapidity must have escaped long ago. But it did not begin life with an atmosphere heavy in proportion to that which the larger planets have. We have seen, in the case of Mercury, that being one of the small planets entails many restrictions in development. Such planets not only lose their atmosphere more quickly than the larger ones, but it is less dense to begin with. The atmosphere of Mars is probably no denser than we have at the tops of our highest mountains, more than likely not even so dense as that. There is some water vapor, and there are a few clouds most of the time; but in the main the atmosphere is so clear and thin that we can without any doubt see the actual surface of the planet. It is not certain that the clouds we see are formed from water vapor, as clouds of the ordinary kind are. It has been suggested that they may be simply dust-clouds. But this is as yet not much more than a suggestion, and nothing convincing has been offered to substantiate the idea. Even dust-clouds would need currents of air to create and carry them; so, whether dust or vapor, the presence of clouds implies an atmosphere. The famous white polar caps, which furnish so many news items to the journals, are also of uncertain origin, and their true nature can be determined only by a fuller knowledge of the atmosphere of Mars. They appear in the winter season on the planet and disappear in its summer, so there seems to be no doubt that they are dependent in some way on the temperature in the polar regions of Mars. If they are hoar-frost or snow, they are condensations of water vapor; and, in that case, when they disappear there must be sufficient heat to melt them. It has been contended that the sun’s rays fall too obliquely on the poles of Mars to melt more than a few inches of snow, but that the caps may be light snow or frost, and thus capable of being dissolved by even such oblique rays of sunlight as they receive. Also it has been suggested that the deposit resembling snow may be carbon dioxide, which condenses into a white substance at a temperature more than a hundred degrees (-109° Fahr.) lower than is necessary to produce snow and melts at a correspondingly low temperature. What the nature of the phenomenon seen at the poles of Mars is depends largely upon what the temperature is; and the temperature in turn is dependent in some measure on the density and constitution of the atmosphere, as well as the planet’s distance from the sun.