The way to do this is to cork up the hole in the pipe in the bottom that leads to the ballast tank and then set the boat in a tub full of water. Now lay the battery cells in the positions they are to occupy in the boat, as shown at _B_ and _C_ in Fig. 11, and see how far up the water-line comes on the hull—or, in other words, how deep the hull sinks into the water.
Next pour melted lead in between the sides of the ballast tank and the hull while the boat is still in the tub of water and distribute it so that the boat floats on a perfectly even keel. When you have poured enough lead into the hull to make her sink to within an inch or so of her _gunwales_ (the upper edge of the boat’s sides) and she is nicely balanced, let the lead cool, take the boat out of the tub and put her back on her stocks on your bench.
And now a couple of parting hints: (1) You can melt the lead in an iron ladle over a kitchen fire, and (2) put a little water in the ballast tank so that the hot lead will not open the soldered seams.
=Making the Superstructure.=—This consists of the top, or deck, and the conning tower, which in this model serves for the compressed air tank.
To make the deck, cut out a sheet of heavy tin the exact shape of and dimensions given in Fig. 18. Cut a ½-inch hole half way between the ends, and in the middle, for the air-valve pipe to pass through and which is screwed to the conning tower as shown in Fig. 15.
Cut out a 2½-inch hole in the aft end of the deck for a _hatch_, and make a cover, or hatch, for it 3½ inches in diameter; this hatch will allow you to get your hand through the deck and into the hull to reset the pusher device when your submarine is to make another trip.
[Illustration: FIG. 18. HOW THE DECK IS MADE.]
Cut out a rectangular hole ½ inch wide and 2 inches long in the for’ard end of the deck, for the screws and the wires of the switch to pass through. Screw the porcelain block of the switch to a board of the same size, with the tin deck in between them; this insulates the screws of the switch from the tin, which would otherwise _short circuit_ the battery and run it down. Run sealing-wax in and around the edges of both the porcelain and wood blocks to make a watertight joint, for water must not get into the boat.