At four o'clock the two lads had their task completed, and at the same time Professor Henderson announced that the air and water pumps were now in good shape again. "Then let's get under way at once," suggested Mr. Roumann. "We have lost enough time as it is. Hurry inside, boys, and we'll start." The two chums were glad enough to do so, and in a few minutes they were again moving through the air toward the unknown portion of the moon. Below the travellers, as they could see by looking down through a plate-glass window in the floor of the projectile, were the same rugged peaks, the same large and small craters that had marked the surface of the moon from the time they had first had a glimpse of it. There was an uninteresting monotony about it, unrelieved by any save the very sparest vegetation. "I am beginning to think more and more that we will find people on the other side of this globe," remarked Mr. Roumann, as he made an observation through a telescope. "What strengthens your belief?" inquired Mr. Henderson. "The fact that the vegetation is growing thicker. There are many more plants below us now than there were before. This part of the moon is better able to support life than the portion we have just come from." This seemed to be so, but they were still some distance from the opposite side of the moon. "I don't see anything of those diamonds you talked so much about, Jack," said Mark, with a smile, a little later. "I guess all the Reonaris you get you can put in a hollow tooth." "You wait," was all Jack replied. The projectile was slowed up to permit the two professors to make some notes regarding a particularly large and deep crater, and a few minutes later when Mark, who was in the engine room, attempted to speed up the Cordite motor it would not respond. "Humph! I wonder what's wrong?" he asked of Jack. "Better call Mr. Roumann, and not try to fix it yourself," suggested his chum, when, in response to various movements of the lever, the machine seemed to go slower and slower. The German came in answer to the summons. "Ha!" he exclaimed, "that motor is broken again. We shall have to stop once more for repairs. I shall need to take it all apart, I fear. Get me the negative plate remover, will you, Mark?" The lad went to the tool chest for it. He opened the lid and fumbled about inside. "It doesn't seem to be here," he announced. "What! the negative plate remover not there?" cried the professor. "Why, it must be. It is one of the new tools we got, and it has not been used for anything; has it?". "Oh, by Jinks!" cried Jack suddenly. "What's the matter?" asked his chum. "That plate remover! Don't you remember you and I had it when we were fixing the pipes outside the projectile, when we had the other breakdown? We must have left it back there on the ground." Jack and his chum gazed blankly at each other. "I guess we did," admitted Mark dubiously.