When a carrier arrives, after passing the slots that allow the air-current to flow into the branch pipe, it compresses the air in front of it against the gate. This compression checks its momentum, and it comes gradually to rest. The air compressed between the carrier and the sluice-gate operates to move the piston slide-valve, thereby admitting air to the gate cylinder under the piston, which rises, carrying with it the sluice-gate. The tube is now open to the atmosphere, and there is just sufficient pressure in the tube to push the carrier out on to a table arranged to receive it. As the carrier passes out of the tube it lifts a finger out of its path. This finger is located at E, Fig. 17, and when it is lifted by the passing carrier it moves the piston slide-valve, and the sluice-gate is closed. A valve is located in the branch-pipe that conducts the air to the return tank in the basement. If the pressure in the tube is not sufficient to push the carrier out on to the table, this valve is partially closed, thereby increasing the pressure to a desired amount. [Illustration: FIG. 18. CARRIER.] [Illustration: FIG. 19. CARRIER.] =The Carrier.=—We have frequently spoken of the carrier, which contains the mail and other parcels that are transported from one office to the other. In Fig. 13, showing the sub-post-office apparatus, we see one of these carriers being despatched by the attendant and another being delivered from the tube. In Fig. 15 several carriers may be seen standing on the floor. Fig. 18 shows a carrier with the lid open, ready to receive a charge of mail, and Fig. 19 shows the same closed, ready for despatching. The construction of the carrier is shown by the drawing, Fig. 20. The body of the carrier is steel, about one-thirty-second of an inch in thickness. It is made from a flat sheet, bent into a cylinder, riveted, and soldered. The length outside is eighteen inches, and the inside diameter is five and one-quarter inches. The front end is made of a convex disk of steel, stamped in the desired form, and secured to the body of the carrier by rivets, with the convex side inward. It is necessary to have a buffer upon the front end of the carrier to protect it from blows that it might receive, and this buffer is made by filling the concave side of the front head with felt, held in place by a disk of leather and a central bolt. The leather disk is made of two pieces, riveted together, with a steel washer between. The steel washer is attached to the head of the bolt. The carrier is supported in the tube on two bearing-rings, located on the body of the carrier a short distance from each end. The location of these rings is so chosen that it permits a carrier of maximum length to pass through a bend in the tube of minimum radius without becoming wedged. This is a very important feature in the construction of carriers, but does not appear to have been utilized in other systems. [Illustration: FIG. 20. _MAIL CARRIER.—PHILA._]