The tubes were laid in the trench and supported by having the ground firmly tamped about them. Usually one tube was laid above the other, with an iron bracket between, but frequently this arrangement had to be departed from in order to avoid obstructions, such as gas- and water-pipes, sewers, man-holes, etc. The depth of the tubes below the pavement varied from two to six feet, and in one place, in order to pass under a sewer, the extreme depth of thirteen feet was reached. At the street crossings it was frequently difficult to find sufficient space to lay the tubes. At the intersection of Fifth and Chestnut Streets a six-inch water-main had to be cut and a bend put in. A seven-strand electric cable, used for telephoning and signalling, was laid upon the top of one of the tubes, protected by a strip of “vulcanized wood,” grooved to fit over the cable. The cable and protecting strip of wood were fastened to the tube by wrought-iron straps and bolts. The tubes enter the main post-office on the Chestnut Street side, through one of the windows, and are suspended from the ceiling along the corridor in the basement for a distance of nearly two hundred feet. Fig. 8 shows the tubes thus suspended. They terminate upon the ground floor about the centre of the building, and near the cancelling machines. [Illustration: FIG. 9. DUPLEX AIR-COMPRESSOR IN THE BASEMENT OF THE MAIN POST-OFFICE.] [Illustration: FIG. 10. TANKS AND TUBE IN THE BASEMENT OF THE MAIN POST-OFFICE.] =Air-Compressor—Method of Circulating the Air.=—The current of air that operates the tubes is supplied by a duplex air-compressor located in the basement of the main post-office. This machine is shown in Fig. 9, and requires no detailed description, as it does not differ materially from air-compressors used for other purposes. The stroke is twenty-four inches, the diameter of the steam-cylinders ten inches, and the air-cylinders eighteen inches. The air-cylinders are double acting, with poppet-valves, and have a closed suction. The speed of the machine varies slightly, being controlled by a pressure-regulator that maintains a practically constant pressure in the tank that feeds the tube. The engines develop a little over thirty horse-power under normal conditions. The pressure of the air as it leaves the compressor is usually six or seven pounds per square inch. Compressing the air heats it to about 156° F., but this is not sufficient to require water-jackets about the air-cylinders. From the compressor the air flows to a tank, shown on the right in Fig. 10, where any oil or dirt contained in the air is deposited. The principal purpose of the tank is, however, to form a cushion to reduce the pulsations in the air caused by the periodic discharge from the cylinders of the compressors, and make the current in the tube more steady. From this tank the air flows to the sending apparatus on the ground floor of the post-office and thence through the outgoing tube to the sub-post-office.