"Please don't think that I am in favor of war, Ned," went on Tom earnestly. "I hate it, and I wish the time would come when all nations would disarm. But if the other countries are laying themselves out to have aerial battleships, it is time the United States did also. We must not be left behind, especially in view of what is taking place in Europe." "I suppose that's right," agreed Ned. "Have you any of your guns ready?" "Yes, all but the mounting of them on the supports aboard the Mars. I haven't dared do that yet, and fire them, until I provided some means of taking up the recoil. Now I'm going to get right to work on that problem." There was considerable detailed figuring and computation work ahead of Tom Swift, and I will not weary you by going into the details of higher mathematics. Even Ned lost interest after the start of the problem, though he was interested when Tom took down the door-check and began measuring the amount of force it would take up, computing it on scales and spring balances. Once this had been done, and Tom had figured just how much force could be expected to be taken up by a larger check, with stronger hydrostatic valves, the young inventor explained: "And now to see how much recoil force my guns develop!" "Are you really going to fire the guns?" asked Ned. "Surely," answered Tom. "That's the only way to get at real results. I'll have the guns taken out and mounted in a big field. Then we'll fire them, and measure the recoil." "Well, that may be some fun," spoke Ned, with a grin. "More fun than all these figures," and he looked at the mass of details on Tom's desk. This was the second or third day after the fire in the red shed, and in the interim Tom had been busy making computations. These were about finished. Meanwhile further investigation had been made of clues leading to the origin of the blaze in the shed, but nothing had been learned. A photo-telephone had been installed near Eradicate's quarters, in the hope that the mysterious stranger might keep his promise, and come to see about the mule. In that case something would have been learned about him. But, as Tom feared, the man did not appear. Ned was much interested in the guns, and, a little later, he helped Tom and Koku mount them in a vacant lot. The giant's strength came in handy in handling the big parts. Mr. Swift strolled past, as the guns were being mounted for the preliminary test, and inquired what his son was doing. "It will never work, Tom, never!" declared the aged inventor, when informed. "You can't take up those guns in your air craft, and fire them with any degree of safety." "You wait, Dad," laughed Tom. "You haven't yet seen how the Newton hydrostatic recoil operates." Ned smiled with pleasure at this.