"Wonder why there's always been so much mystery about Mercury?" pondered the architect invitingly. "Looks as though the big five-foot telescope on Mt. Wilson would have shown everything." "Ask doc," suggested Smith, diplomatically. Jackson turned and hailed the little man on the other side of the car. He looked up absently from the scientific apparatus with which he had been making a test of the room's chemically purified air, then he stepped to the oxygen tanks and closed the flow a trifle, referring to his figures in the severely exact manner of his craft. He crossed to the group. "Mercury is so close to the sun," he answered the architect's question, "he's always been hard to observe. For a long time the astronomers couldn't even agree that he always keeps the same face toward the sun, like the moon toward the earth." "Then his day is as long as his year?" "Eighty-eight of our days; yes." "Continual sunlight! He can't be inhabited, then?" The architect knew very little about the planets. He had been included in the party because, along with his professional knowledge, he possessed remarkable ability as an amateur antiquarian. He knew as much about the doings of the ancients as the average man knows of baseball. Dr. Kinney shook his head. "Not at present, certainly." Instantly Jackson was alert. "Then perhaps there were people there at one time!" "Why not?" the doctor put it lightly. "There's little or no atmosphere there now, of course, but that's not saying there never has been. Even if he is such a little planet--less than three thousand, smaller than the moon--he must have had plenty of air and water at one time, the same as the Earth." "What's become of the air?" Van Emmon wanted to know. Kinney eyed him in reproach. He said: "You ought to know. Mercury has only two-fifths as much gravitation as the earth; a man weighing a hundred and fifty back home would be only a sixty-pounder there. And you can't expect stuff as light as air to stay forever on a planet with no more pull than that, when the sun is on the job only thirty-six millions miles away." "About a third as far as from the Earth to the sun," commented the engineer. "By George, it must be hot!" "On the sunlit side, yes," said Kinney. "On the dark side it is as cold as space itself--four hundred and sixty below, Fahrenheit." They considered this in silence for some minutes. The builder went to another window and looked at Venus, at that time about sixty million miles distant, on the far side of the sun. They were intending to visit "Earth's twin sister" on their return. After a while he came back to the group, ready with another question: "If Mercury ever was inhabited, then his day wasn't as long as it is now, was it?"