He and the professor were soon deep in the discussion, while Jack and Mark, with the aid of the machinists, were busy constructing the main part of the projectile. The first thing to be done was to build the shell of the projectile. This consisted of plates of a new and peculiar metal, invented by Professor Henderson. The plates were riveted together, in the shape of a great cigar, two hundred feet long. This work took some time, but, as the professor had in his shop the proper machinery for it, a small force could accomplish a great deal of work. The rear of the projectile was to be occupied by the mysterious apparatus that was to drive it through space. In this compartment would be many strange machines, including the one which Mr. Roumann had invented to use the terrific and secret force of which he was the discoverer. There were apparatus for distilling water from the atmosphere, others for manufacturing oxygen, dynamos for furnishing light to the interior of the Annihilator, motors for working the various small machines, and a number of other appliances. Forward from the engine-room was a space to be used in storing away the food supplies, and the materials necessary for generating the force used, as well as for making a new supply of air when needed. Amidships was a living-room, with a plate-glass window on either side. There was not much space to move about in it, as, owing to the long and narrow shape of the projectile, economy of room was enforced. Still, the place was a lengthy one, with tables and chairs, which could be folded up out of the way when not in use. There was provision for a library of scientific and other books, and a piano played by electricity and brass disks, somewhat on the order of modern player-pianos. "What are those apertures in the sides of the living-room?" asked Jack of Mr. Roumann, as the lad glanced over a sheet of blue-print paper, on which was shown a plan of the projectile. "Those," said the German, "are for the guns." "Guns!" exclaimed Mark. "Why, they're too big for guns. They are large enough to put a cannon through." "And that is just what is going to be put through them, my boy," went on Mr. Roumann. "From those openings, and you will see that there are four of them, will protrude the muzzles of my electric cannons." "Do we need them?" asked Jack. "You can't tell what we'll need when we get to Mars," was the slow answer. "You must remember that we know nothing about the inhabitants of the planet. While I believe that the people there are of a very high grade of intelligence, we must be prepared for the worst. We may find them terrible savages, who will want to attack and destroy us. With the electric cannon we can defend ourselves." "That's so," admitted Jack. "We had to fight the Esquimaux up north," "And the putty-men in the center of the earth," added Mark.