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into a great mesh of basket-work. Six- teen slender members form the longi- tudinals, running from bow to stern, and intersecting the spirals of wooden rope where they cross each other. The func- tion of the spirals and longitudinals acting together is to distribute the gas lift and strains evenly to all points of the hull.

There are, in reality, two hulls, the inner enclosing thirteen balloonets or gas bags and the outer supporting a waterproof and airtight envelope or skin. Twenty-nine ribs, or transverse girders, encircle the inner hull, and a spider web of wire cables stiffens the alternate ribs and forms the bulkheads between the balloonets.

Two eight-cylinder, sixty-horsepower motors have been installed, and by means of cable drives transmit the power to four propellers mounted high above the car, two being placed on each side of the slender torpedo-like hull.

In hot weather, or when the airship passes through a heated stratum of air, the gas expands, exerting more lifting power, and causing the airship to rise. To control this tendency, the gas has to be artificially cooled, or it will be neces- sary to release som.e of the valuable hydrogen to allow the ship to retake its proper altitude. On the contrary, if a sudden wave of cold air strikes the gas bag, the gas immediately contracts, and part of its lifting power is lost. If there is no means for heating the gas and ex- panding it, ballast will have to be dropped from the car, thus compensating the decreased lifting power of the gas by a lighter weight which it has to carry.

The control of the lifting power of the gas in the MacMechen dirigible is in the heating and cooling process. To keep the h>'drogen from cooling and losing its lifting power, hot vapor from the engine is blown into the foot-wide space between the balloonets and the outer skin of airtight cloth. To cool and condense the gas for descent, or to pre- vent its expansion to an extent that causes an undue inHation of the gas bags, cold air is introduced into the same space by means of a luminum disks with re- volving shutters at the bow and stern.

It is claimed that by this method of

��Popular Science Monthly

��construction a rigid airship has been built which is one-third lighter than it is possible to build a Zeppelin of the same relative size. The hull and car weigh 2,190 pounds, and the gas capacity is 108,000 cubic feet, or about one-tenth that of the latest Zeppelin monster. As hydrogen is usually rated by aeronauts, this quantity will lift about three and one-half tons, or seven thousand pounds. With engine equipment and crew, the airship weighs about 5,300 pounds, leav- ing a margin of 1,800 pounds for ballast, explosives and additional fuel. The length of the hull is 236 feet over all. The designers claim that their airship

���SPIDER WEBBING AT BULKHEADS

��will make about seventy miles an hour, or about ten miles an hour faster than the speed of a Zeppelin.

The Popular Science Monthly be- lieves that this airship will prove disap- pointing to its builders and to the British Government. Previous experi- ments with wooden frames in dirigibles ha\'e proved costly failures. The Zep- pelin's first rival, the Schiitte-Lanz dirigible, was built with wooden frame- work, and proved much heavier than a Zeppelin of the same dimensions. Lami- nated wood was used in the experiment and this was found faulty and discarded. The Zeppelin of to-day is the product of practical experience, as is the second, and successful, Schiitte-Lanz, which discarded the weblike wooden frame for the lighter metal ribs and strakes of the Zeppelin. Such a solid frame as that of the pigmy airship would not do for a

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