The superintendent did so; then the men started upward. At Rush's suggestion the party divided. The mine captain had been picked up in the meantime, and the four men divided themselves into two parties, each party taking a level through which it moved, visiting every place where men were at work, questioning each sharply as to whether any of their shift had left their work during the last hour. The search was fruitless. There were not many men working on the night shift, and beyond considerable ladder climbing, the two parties had finished their search within a few hours. The four men met on the surface shortly after midnight. The Iron Boys had nothing to report; neither had Mr. Penton nor the mine captain met with any better results. The mystery was still unexplained. "Rush, you usually have ideas on most subjects. What do you think about this affair?" "I hardly know what to think. I have an idea, however, as to where the charge was dropped from." "From where was it dropped?" demanded the superintendent sharply. "From the first sub-level below the surface. You see, it would be very easy for anyone to go down that ladder there at night, without attracting attention. He could have dropped the charge down through the shaft and been out and away long before you telephoned up here." "That is very true. It may explain that part of the affair." "We found a fuse on the first sub-level near the shaft, but of course that doesn't prove anything. It may have been dropped there by any one of fifty different men." The superintendent's face was stern as he pondered over the matter that was disturbing them all. "Rush, I want you boys to devote your time, during the next few days, to working on this case. I know of no one else better qualified to do it. If you can't get to the bottom of the mystery, I know of no one who can. In the meantime I shall be pursuing some investigations of my own." "Very well, sir; do you wish us to drop our work?" "As a matter of fact, yes; but you need not appear to have done so. Pretend to make your rounds, but devote your time to running down this mystery. The officials will be back here within the next few days. I want this affair cleared up before they get here, so work fast. You have my authority to go to any length necessary to solve the mystery and to discover the guilty ones. Now, show me what you can do." "It is a pretty big contract you have given us, sir, but we will do the best we can. We have personal reasons for wanting to succeed, as you can understand." "We have," affirmed Bob Jarvis grimly. The boys bade the superintendent good night and went to their boarding place. The following morning found Rush suffering from a severe cold. He could barely speak, but he went to the mine, nevertheless. The cage was not yet ready for use, but the superintendent had had the ore skips rigged to carry men down, which was done at greatly reduced speed, but at the usual time the mine was in full operation.