in front of it, equal to the frictional resistance of the carrier in the tube. It is this difference of pressure in front and in rear of the carrier that moves it through the tube. When the carrier is in the position shown in the figure, the same difference of pressure will exist on opposite sides of the diaphragm, and it will be deflected into contact with the screw H, thereby closing the electric circuit. When the carrier has passed, equality of pressure on opposite sides of the diaphragm is established and the diaphragm takes its normal position, out of contact with the screw H. This apparatus is easily attached to the tube, and it contains no mechanism to get out of order. [Illustration: FIG. 33. OPEN RECEIVER.] =The Open Receiver.=—Wherever the pressure in the tube is down nearly to atmospheric, we can use an open receiver to discharge the carriers from the tube. This is a receiver that opens the tube to the atmosphere and allows the carrier to come out. Such a receiver is used at the main post-office in the Philadelphia postal-line, and was described in the last chapter. The present receiver is similar in operation, but contains some improvements in details. Fig. 33 is a side elevation of the apparatus, Fig. 34 is a longitudinal section, and Fig. 35 is a cross-section through the cylinder and valve, showing the sluice-gate. [Illustration: FIG. 34. OPEN RECEIVER.—LONGITUDINAL SECTION.] [Illustration: FIG. 35. OPEN RECEIVER.—SLUICE-GATE MECHANISM.] Referring to the longitudinal section, the apparatus is attached to the end of a pneumatic tube, A. The current of air from the tube A flows through the slots B into a pipe, C, that conducts it to a tank near the air-compressor. About the centre of the apparatus is a sluice-gate, E, that is raised and lowered by a piston in a vertical cylinder, F, located just above the sluice-gate. This piston is moved by air-pressure taken from some part of the system. When a carrier arrives from the tube A, it passes over the slots B and runs into the air-cushion D, where it comes gradually to rest. Checking the momentum of the carrier compresses the air in front of it considerably, and this excess of pressure is utilized to move a small slide-valve that controls the movement of the piston in the cylinder F, so that as soon as the carrier has come to rest the sluice-gate rises and allows the carrier to be pushed out with a low velocity on to a table. The small pipe G conducts a small portion of the air compressed in front of the retarded carrier to the controlling valve, H, seen in Figs. 33 and 35. Referring now to the section of the valve and cylinder, Fig. 35, the pipe G enters the top of a small valve-cylinder containing a hemispherical piston, I, that is held up by a spiral spring, J. This spring has just sufficient tension to hold the piston I up against the normal pressure of air in the tube. When a carrier arrives and compresses the air in the air-cushion, the excess of pressure