Read it through once

On the wall, near the slat-frame, there was an outline drawing of the ship, and this betrayed the fact that this vessel has twenty-two considerable lakes of water in her. These lakes are in her bottom; they are imprisoned between her real bottom and a false bottom. They are separated from each other, thwartships, by water-tight bulkheads, and separated down the middle by a bulkhead running from the bow four-fifths of the way to the stern. It is a chain of lakes four hundred feet long and from five to seven feet deep. Fourteen of the lakes contain fresh water brought from shore, and the aggregate weight of it is four hundred tons. The rest of the lakes contain salt water—six hundred and eighteen tons. Upwards of a thousand tons of water altogether.