"The thing that weakened the propeller. Look at this, and smell!" He held out a piece of wood toward Ned. The bank employee saw where a half-round hole had been bored in what remained of the blade, and from that hole came a peculiar odor. "It's some kind of acid," ventured Ned. "That's it!" cried Tom. "Someone bored a hole in the propeller, and put in some sort of receptacle, or capsule, containing a corrosive acid. In due time, which happened to be when we took our first flight, the acid ate through whatever it was contained in, and then attacked the wood of the propeller blade. It weakened the wood so that the force used in whirling it around broke it." "Are you sure of that?" asked Ned. "As sure as I am that I'm here! Now I know what caused the accident!" "But who would play such a trick?" asked Ned. "We might all have been killed." "Yes, I know we might," said Tom. "It must be the work of some of those foreign spies whose first plot we nipped in the bud. I must tell Marbury of this, but don't mention it to dad." "I won't," promised Ned. Lieutenant Marbury agreed with Tom that someone had surreptitiously bored a small hole in the propeller blade, and had inserted a corrosive acid that would take many hours to operate. The hole had been varnished over, probably, so it would not show. "And that means I've got to examine the other two blades," Tom said. "They may be doctored too." But they did not prove to be. A careful examination showed nothing wrong. An effort was made to find out who had tried to destroy the Mars in midair, but it came to nothing. The two men in custody declared they knew nothing of it, and there was no way of proving that they did. Meanwhile, the torn gas bag was repaired, and Tom began working on the problem of doing away with the gun recoil. He tried several schemes, and almost was on the point of giving up when suddenly he received a hint by reading an account of how the recoil was taken care of on some of the German Zeppelins. The guns there were made double, with the extra barrel filled with water or sand, that could be shot out as was the regular charge. As both barrels were fired at the same time, and in opposite directions, with the same amount of powder, one neutralized the other, and the recoil was canceled, the ship remaining steady after fire. "By Jove! I believe that will do the trick!" cried Tom. "I'm going to try it." "Good luck to you!" cried Ned. It was no easy matter to change all the guns of the Mars, and fit them with double barrels. But by working day and night shifts Tom managed it. Meanwhile, a careful watch was kept over the shops. Several new men applied for work, and some of them were suspicious enough in looks, but Tom took on no new hands. Finally the new guns were made, and tried with the Mars held on the ground. They behaved perfectly, the shooting of sand or water from the dummy barrel neutralizing the shot from the service barrel.