The young inventor was idly handling some pieces of the very hard rock that had cropped out in the tunnel cut. Tom had tested it, he had pulverized it (as well as he was able), he had examined it under the microscope, and he had taken great slabs of it and set off under it, or on top of it, charges of explosive of various power to note the effect. But the results had not been at all what he had hoped for.  
"What's to be done, Tom?" repeated the contractor.
 
"Well, Mr. Titus," was the answer, "the only thing I see to do is to make a new explosive."
 
"Can you do it, Tom?"
 
The reply was characteristic.
 
"I can try."
 
And in the days that followed, Tom began work on a new line. He had brought from Shopton with him much of the needful apparatus, and he found he could obtain in Lima what he lacked.
 
A message to his father brought the reply that the new ingredients Tom needed would be shipped.
 
"The kind of explosive we need to rend that very hard rock," the young inventor explained to the Titus brothers, "is one that works slowly."
 
"I thought all explosions had to be as quick as a flash," said Walter.
 
"Well, in a sense, they do. Yet we have quick burning and slow-burning powders, the same as we have fuses. A quick-burning explosive is all right in soft rock, or in soil with rock and earth mingled. But in rock that is harder than flint if you use a quick explosive, only the outer surface of the rock will be scaled off.
 
"If you take a hammer and bring it down with all your force on a hard rock you may chip off a lot of little pieces, or you may crack the rock, but you won't, under ordinary circumstances, pulverize it as we want to do in the tunnel.
 
"On the other hand, if you take a smaller hammer, and keep tapping the rock with comparatively gentle blows, you will set up a series of vibrations, that, in time, will cause the hard rock to break up into any number of small pieces.
 
"Now that is the kind of explosive I want—one that will deal a succession of constant blows at the hard rock instead of one great big blast."
 
"Can you make it, Tom?"
 
"Well, I don't know. I'll do the best I can."
 
From then on Tom was busy with his experiments.
 
Work on the tunnel did not cease while he was searching for a new explosive. There was plenty of the old explosive left and charges of this were set off as fast as holes could be drilled to receive it. But comparatively little was accomplished. Sometimes more rock would be loosed than at others, and the native laborers, now seemingly perfectly contented, would be kept busy. Again, when a heavy blast would be set off hardly a dozen dump cars could be filled.
 
But the work must go on. Already the time limit was getting perilously close, and the contractors did not doubt that their rivals were only waiting for a chance to step in and take their places.
 
Nothing more had been seen or heard of the bearded man, Waddington, or Blakeson & Grinder. But that the rival firm had not given up was evidenced by the efforts made in New York to cripple, financially, the firm in which Tom was interested. In fact, at one time the Titus brothers were so tied up that they could not get money enough to pay their men. But Tom cabled his father, who was quite wealthy, and Mr. Swift loaned the contractors enough to proceed with until they could dispose of some securities.
 
It might be mentioned that Tom was to get a large sum if the tunnel were completed on time, so it was to his interest and his father's, to bring this about if he could.
 
Tom kept on with his powder experiments. Mr. Damon helped him, for that gentleman had succeeded in putting the affairs of the wholesale drug business on a firm foundation, and there was no more trouble about getting the supplies of cinchona bark to market. The natives seemed to have taken kindly to the eccentric man, or perhaps it was the reputation of Tom Swift and his electric rifle that induced them to work hard.
 
It must not be supposed that Professor Bumper was idle all this while.<............