in the figure by a counterweight. When a carrier rests upon it, the weight of the carrier is sufficient to bear it down into a horizontal position; in this position the carrier rolls off on to a table or shelf. The platform, K, is connected by rods, bell-cranks, etc., to the piston slide-valve, so that when it swings downward by the weight of a carrier, the slide-valve is moved upward into its normal position, and this causes the receiving chamber to tilt back into a horizontal position ready to receive the next carrier. The time that elapses from the arrival of a carrier until the receiving chamber has returned to its horizontal position is not more than three or four seconds. Nothing could operate in a more satisfactory manner. =Main Post-Office Receiver.=—At the main post-office we have a receiver of a different type. It will be remembered that the pressure in the return tube at the main post-office is nearly down to zero or atmospheric, so that we can open the tube to allow the carriers to pass out without noise or an annoying blast of air. Figs. 15 and 16 show the main-office apparatus, and Fig. 17 is a drawing of the same. Here the receiver consists of a section of tube closed by a sluice-gate, located at B, Fig. 17. The air-current passes out through slots in the tube into a branch pipe leading to the return tank in the basement. These slots are located about four feet back of the sluice-gate, so that the portion of the tube between the slots and the sluice-gate forms an air-cushion to check the momentum of the carriers. The sluice-gate is raised and lowered by a piston moving in a cylinder located just above the gate. The movement of this piston is controlled by a piston slide-valve in a manner similar to the apparatus at the sub-post-office. Air for operating the piston is conveyed through the pipe D, Fig. 17, from the pipe leading from the air-compressor to the sending apparatus. This air is at about seven pounds pressure per square inch. [Illustration: FIG. 16. RECEIVING APPARATUS AT THE MAIN POST-OFFICE.] [Illustration: FIG. 17. _APPARATUS AT THE MAIN OFFICE—PHILA._]