=Carriers.=—The carriers are similar in all respects to those used in the Philadelphia postal-line, that have been described in the preceding chapter and illustrated in Figs. 18, 19, and 20. When there are intermediate stations upon the lines, means are provided for attaching disks to the front end of the carriers. The disks have a central stem that secures them to the bolt in the centre of the head, and are so arranged that they can be quickly attached or removed. Many experiments have been made to find the best material for bearing-rings, but thus far nothing better than a specially-prepared woven fabric has been found. These rings will run about a thousand miles, when they become so reduced in diameter that they have to be replaced by new ones. The most essential elements of a carrier are strength, lightness, and security of the contents. Aluminum has frequently been proposed as a suitable material for the bodies of carriers, but for the same weight steel is much stronger, especially in thin rolled sheets, and for this reason it has been used. One of the most perplexing problems that presented itself in working out the details of the system was to design a secure and reliable lock for the lids of the carriers. We believe that the one which has been adopted fulfils all requirements in a satisfactory manner. Some experiments have been made with carriers that open on the side, but structurally they are weak and unsuited to stand the blows that carriers frequently receive. They are not so easily and quickly filled and emptied as those that open on the end. These remarks apply to carriers for large tubes. In small tubes for the transportation of cash in retail stores, carriers with side openings are found convenient. When United States mail is sent through tubes not used exclusively for postal service, carriers with special locks can be used, so that they can be opened only by post-office employees. =Air Supply.=—This completes the description of the special apparatus used in this system, but we have yet to say something regarding the machines that supply the air. In Paris the water from the city mains has been used to compress or exhaust the air used in small tubes, but to operate large tubes in most of our cities steam is the only available power. Except in isolated cases, an independent steam plant will be erected to supply the air for a system of tubes. This plant should be designed with a view to obtaining the maximum economy in coal consumption, labor, water, cartage, and incidental expenses. We might say that the same general rules of economy which govern the design and construction of electric-lighting plants should be applied to the plans and construction of air-compressing plants. Three types of blowing machines are used,—viz., centrifugal fans, positive blowers, and air-compressors.