While she is going down the weighted pusher is moving slowly but surely over the threaded spindle; when it reaches the piston it pushes it against the pin in the air-valve and so opens it and keeps it open.
The instant the air-valve opens, the compressed air from the air tank (conning tower) rushes into the ballast tank, and because it is under a high pressure it forces the water out of the tank through the hole whence it came in.
When the water has been blown out of the ballast tank the boat is, of course, lighter, and naturally she rises to the surface again. This is your cue to be right there with a rowboat and get her and to pump more air into the compressed air tank before she makes another trip.
If you don’t do this and she ever goes down with her ballast tank full of water and there is no compressed air left to blow it out with, you can send a censored report to the daily papers that another U-boat has been sunk and that there was no time to save the crew.
But, anyway, you will have oceans of fun with your model, and your head will brim over with submarine lore.
CHAPTER III
HOW A REAL SUBMARINE IS MADE AND WORKS
The Construction and Operation of a Real Submarine Simply Explained