In the case of a ship constructed with a double bottom for water ballast, if the space between the inner and outer plating thereof is certified by a surveyor of ships to be not available for the carriage of cargo, stores, or fuel, then the depth required by the provisions of Rule I. relating to the measurement of transverse areas shall be taken to be the upper side of the inner plating of the double bottom, and that upper side shall, for the purposes of measurement, be deemed to represent the floor timber referred to in that Rule.