"Easy!" grinned Jarvis. "Martian gravity and Martian air — that's the answer. Figure it out: First, the dirt they dug only weighed a third its earth-weight. Second, a steam engine here expands against ten pounds per square inch less air pressure than on earth. Third, they could build the engine three times as large here with no greater internal weight. And fourth, the whole planet's nearly level. Right, Putz?"

The engineer nodded. "Ja! Der shteam — engine — it iss sieben-und zwanzig — twenty-seven times so effective here."

"Well, there does go the last mystery then," mused Harrison.

"Yeah?" queried Jarvis sardonically. "You answer these, then. What was the nature of that vast empty city? Why do the Martians need canals, since we never saw them eat or drink? Did they really visit the earth before the dawn of history, and, if not atomic energy, what powered their ship? Since Tweel's race seems to need little or no water, are they merely operating the canals for some higher creature that does? Are there other intelligences on Mars? If not, what was the demon-faced imp we saw with the book? There are a few mysteries for you!"

"I know one or two more!" growled Harrison, glaring suddenly at little Leroy. "You and your visions! 'Yvonne!' eh? Your wife's name is Marie, isn't it?"

The little biologist turned crimson. "Oui," he admitted unhappily. He turned pleading eyes on the captain. "Please," he said. "In Paris tout le monde — everybody he think differently of those things — no?" He twisted uncomfortably. "Please, you will not tell Marie, n'est-ce pas?"

Harrison chuckled. "None of my business," he said. "One more question, Jarvis. What was the one other thing you did before returning here?"