A Boat Driven by Air
If you are one of those boys who like to experiment with things, here is something that will assume you. Get a lightly built model rowing boat and put a small length of metal tubing through the stern end, so that it projects under water. Then blow up an air ballon and close the neck by means of a wire clip. Fit the end of the neck to the inner end of the tube and tie it on securely. The ballon now lies in the boat and must be fastened down to prevent it blowing up. Now put the boat in water, release the wire clip and the escaping air will drive it along.