THREE TYPES OF DIRIGIBLES
There are three types of dirigible airships; the flexible, the semirigid, and the rigid. In the flexible type, as we have observed, the envelope must be kept tightly filled in order to hold its shape when driven against the air. The car is suspended from the gas bag. In the semirigid type a rigid frame or backbone serves as a keel for the gasbag and helps to prevent it from crumpling up. In the rigid type a casing incloses the gas or rather bags; for the gas Is contained in a number of separate bags fitted into separate compartments. The casing is composed of a framework of duralumin, which is an alloy of aluminum, with a percentage of copper and nickel. It weighs but little more than aluminum, but is five times as strong. Over the duralumin framework is stretched a sheathing of rubberized fabric. Because of the weight of this casing rigid dirigibles must be made in large size.