In the summer of 1895 the sub-post-office was removed from Chestnut Street to the basement of the Bourse (see Fig. 7). This required the abandonment of a few feet of tube on Chestnut Street and the laying of a slightly greater amount on Fourth Street, thus increasing the total length of the tubes a little. Wrought-iron tube, coated with some alloy, probably composed largely of tin or zinc, was used for this extension. The wrought-iron tube is not as good as the bored cast iron. [Illustration: FIG. 7. BOURSE BUILDING, PHILADELPHIA.] [Illustration: FIG. 8. PNEUMATIC TUBES SUSPENDED IN THE BASEMENT OF THE MAIN POST-OFFICE.] =Description of the Tubes, Method of Laying, etc.=—This Chestnut Street line consists of two tubes, one for despatching carriers from and the other to the main post-office. The distance between the two stations is two thousand nine hundred and seventy-four feet, requiring five thousand nine hundred and forty-eight feet of tube. The inside diameter of the tube is six and one-eighth inches, and it was made in sections each about eleven feet long, with “bells” cast upon one end, in order to join the sections with lead and oakum, calked in the usual manner of making joints in water- and gas-pipes, with this exception, that at the bottom of the bell a counter-bore was turned to receive the finished end of the next section. By thus machining the ends of each section of tube and having them fit accurately together, male and female, a practically continuous tube was formed with no shoulders upon the interior to obstruct the smooth passage of the carriers. Joints made in this way possess another great advantage over flanged and bolted joints, in that they are slightly yielding without leaking, and so allow for expansion and contraction due to changes of temperature. Each joint takes care of the expansion and contraction of its section, which is very slight, but if all were added together would amount to a very large movement. Another advantage of the “bell” joint is that it permits slight bends to be made in the line of tube without requiring special bent sections. Where short bends had to be made, at street corners, in entering buildings, and other similar places, brass tubes were used, bent to a radius of not less than six feet, or about twelve times the diameter of the tube. (One of the brass bends may be seen in Fig. 10.) The bends were made of seamless tubing, bent to the desired form and radius in a hydraulic machine. To prevent them from being flattened in the process of bending, they were filled with resin, which was afterwards melted out. Flanges were screwed and soldered to the ends of the bent brass tubes, and they were bolted to special flanged sections of the iron tube.