If stations A, B, ... and H were arranged on a loop, as shown in circuit 6, Fig. 21, then station H, Fig. 26, would be at the central, or station A. If it were a single loop, like circuit 4, Fig. 21, then there would be only one sending apparatus and one receiving and transferring apparatus at the intermediate stations. A telephone circuit will include all stations, in order to give orders to the station attendants and to signal to the central station in case of an accident, when it might be necessary to stop the air-compressor. The telephone wires, in the form of a lead-covered cable, are laid in the same trench with the tubes and fastened to them. [Illustration: FIG. 27. SENDING APPARATUS.] [Illustration: FIG. 28. SENDING APPARATUS.—LONGITUDINAL SECTION.] =The Sending Apparatus.=—We have, in the preceding pages, frequently spoken of the sending apparatus, and have described it as mechanism by which carriers are inserted into the tube. In the Philadelphia postal line this apparatus consisted of a large valve, operated by hand. For an eight-inch tube such a valve would be too large and heavy to be manually operated. Furthermore, that type of apparatus is not suited to an intermediate station, where carriers have to pass through it. To meet all of these requirements we have designed an apparatus, of which Fig. 27 is a side elevation, Fig. 28 a longitudinal section, and Fig. 29 a cross-section. Referring to the longitudinal section, Fig. 28, the sending apparatus is shown inserted into the line of a pneumatic tube, A, A. We have a movable section of tube, B, that can be swung about the large bolt, G, at the top, into and out of line with the main tube, A, A. When the section of tube B is being swung to one side, the air-current has a by-pass through the slots E and F and the U-shaped pipe D. The joints at the ends of the movable section B are packed with specially-formed leathers. Referring to the cross-section, Fig. 29, when the movable section of tube B is swung out of line with the main tube, another and similar tube, C, takes its place. The two movable tubes, B and C, are made in one piece, so that they must always move together. They are connected together at each end by plates, M, that serve not only as connecting-plates, but covers for the ends of the main-line tube while the tubes B and C are being moved. The tubes B and C swing between four plates or wings, L, that extend out on each side of the apparatus. They serve as guards, and, at certain positions of the swinging tubes, prevent the air from escaping.