“They made considerable and remarkable changes in Deimos itself. One thing they did was to import a large stock of air. As I mentioned before the air was very light and thin, and visitors at first had to depend on their flasks of compressed air to a great extent. But after they got to making such frequent trips, it became a rule to always take up large flasks of compressed oxygen which was prepared and kept on hand to be carried up whenever a trip should be made. This was set free on the little moon. In that way in the course of time the air has been made quite passable. In order that visitors might not consume it and replace it with carbonic acid gas, they built several lines of tiny railroads reaching around Deimos on which they built movable gardens. These moved around the whole circuit of the little globe every 30 hours and 18 minutes, that being its period of revolution around Mars. These little gardens thus kept themselves directly under the sun all the time, and were thus always in a tropical climate. Their growth consumed the carbonic gas that accumulated there, and so kept the air pure. The power that moved the gardens was electricity generated by sun light. A large number of machines were placed at intervals all around the little moon so that the sun should always be shining on several of them.” CHAPTER XVI. Phobos. “Deimos is exceedingly valuable also as a sort of stepping stone from which to get onto Phobos. Once on Deimos and it is as easy getting onto Phobos as to step from one car to another in a running train. Phobos is 5,807 miles from the center of Mars. When the three are in line it is 8,740 miles from Deimos. Deimos travels 3,016½ miles an hour, Phobos 4,777. A body thrown off from Deimos towards Mars will retain the velocity of Deimos and will acquire in falling that 8,740 miles an additional velocity enough to bring its speed up to that of Phobos. So our Lunarians by close calculation and timely departure from Deimos have had little or no trouble in lighting on Phobos without a perceptible jar and have conveyed by that route all the machinery and apparatus they needed in making their improvements there.” “Was Phobos worth improving then?” “No, its motive power was simply harnessed so as to be utilized on Mars. They did it in this way. “A large number of powerful steel magnets were prepared on Mars together with the materials for a large basket or crate stout enough to hold them, also four cables made of wire, each two inches in diameter. These materials done up in proper shape were taken up the cable by repulsion to Deimos then again by repulsion cast off with a company of Lunarians in one of their cars to sail down to Phobos.” “I should think that being encumbered with such a lot of stuff would have added greatly to the risk of the trip,” said I.