"Wouldn't that do, Tom? Check the recoil of the gun with whatever stuff is in that arrangement!" A sudden change came over Tom's face. It was lighted up with a gleam of understanding. "By Jove, Ned, old man!" he cried. "I believe you've struck it! And to think that has been under my nose, or, rather, over my head, all this while, and I never thought of it. Hurray! That will solve the problem!" "Do you think it will?" asked Ned, glad that he had contributed something, if only an idea, to Tom's aerial warship. "I'm almost sure it will. I'll give it a trial right away." "What's in that door-check?" Ned asked. "I never stopped before to think what useful things they are, though at the bank, with the big, heavy doors, they are mighty useful." "They are a combination of springs and hydrostatic valves," began Tom. "Good-night!" laughed Ned. "Excuse the slang, Tom, but what in the world is a hydrostatic valve?" "A valve through which liquids pass. In this door-check there may be a mixture of water, alcohol and glycerine, the alcohol to prevent freezing in cold weather, and the glycerine to give body to the mixture so it will not flow through the valves too freely." "And do you think you can put something like that on your guns, so the recoil will be taken up?" Ned wanted to know. "I think so," spoke Tom. "I'm going to work on it right away, and we'll soon see how it will turn out. It's mighty lucky you thought of that, for I sure was up against it, as the boys say." "It just seemed to come to me," spoke Ned, "seeing how easily the door closed." "If the thing works I'll give you due credit for it," promised Tom. "Now, I've got to figure out how much force a modified hydrostatic valve check like that will take up, and how much recoil my biggest gun will have." "Then you're going to put several guns on the Mars?" asked Ned. "Yes, four quick-firers, at least, two on each side, and heavier guns at the bow and stern, to throw explosive shells in a horizontal or upward direction. For a downward direction we won't need any guns, we can simply drop the bombs, or shells, from a release clutch." "Drop them on other air craft?" Ned wanted to know. "Well, if it's necessary, yes. Though I guess there won't be much chance of doing that to a rival aeroplane or dirigible. But in flying over cities or forts, explosive bombs can be dropped very nicely. For use in attacking other air craft I am going to depend on my lateral fire, from the guns mounted on either beam, and in the bow and stern." "You speak as though you, yourself, were going into a battle of the air," said Ned. "No, I don't believe I'll go that far," Tom replied. "Though, if the government wants my craft, I may have to go aloft and fire shots at targets for them to show them how things work.