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. ACROSS THE ATLANTIC IN A DIRIGIBLE Before the war large Zeppelins were built that were fitted with luxurious cabins and dining rooms and made regular scheduled voyages. The big British dirigibles are of the Zeppelin type. The R-34, was 672 feet long and 79 feet in diameter. It was fitted with nineteen gas bags and had a total capacity of over two million cubic feet of hydrogen. It was driven by five engines, each developing from 250 to 275 horsepower, and was capable of making from 50 to 75 miles per hour, depending upon whether or not the engines were pushed. The big dirigible left the Royal Naval Air Station, near Edinburgh, on the 2d of July, 1919, and landed at the Roosevelt Field near Mineola on the 6th, having made the trip in four days and two hours. The course covered about 3,100 sea miles, but the actual air mileage was about 6,300 miles because head winds were encountered. In aeronautic voyages it is the distance through the air that must be reckoned rather than the distance over the ground or sea. An airship may be traveling at the rate of 50 miles per hour through the air, but if there is a wind of 30 miles per hour blowing against the course of the dirigible, the latter will be making only 20 miles per hour over the ground, or if the wind is blowing with the airship it will be making 80 miles per hour over the ground. Because of the head winds the transatlantic flight of the R-34 was so much longer than had been anticipated that its stores of fuel were almost completely exhausted. And yet, when the airship started out from Edinburgh, it carried 81 tanks of gasoline, each containing nearly 70 gallons, or a total of 4,900 gallons. This fuel weighed nearly 16 tons. Almost a ton of oil and 3 tons of water added to the load and the baggage and crew amounted to 4 tons more. The total weight carried was over 24 tons and the dirigible fully loaded weighed altogether about 60 tons. When the dirigible started out it had to fly low, but as the fuel was consumed it grew lighter and rose higher. The