=Sending Time-Lock.=—In any system of large pneumatic tubes a short time should elapse between the despatching of carriers, in order that they may not collide in the tube, and to give the receiving apparatus at the stations time to act. To insure the impossibility of having carriers despatched too rapidly, we place on the sending apparatus a time-lock that will automatically lock it for a determined length of time after each carrier is despatched, the time-lock being adjustable for any desired time. The time-lock, W, is shown attached to the sending apparatus in Fig. 27. When the swing-frame is swung to despatch a carrier, it pulls up the rod X by means of a link and bell-crank, Y, thereby locking the controlling valve of the cylinder H and starting the time-lock W, which will unlock the controlling valve after the required time has elapsed. The details of this time-lock are shown in Fig. 30. It consists of a long vertical cylinder, A, containing a piston, B, and a spiral spring, C, that tends to force the piston to the bottom of the cylinder. The cylinder is filled with oil, and holes, D, in the piston allow the oil to pass freely through it when it is moved upward in the cylinder. When the piston moves downward an annular collar, E, forming a valve, closes the holes in the piston and prevents the oil from passing through. Extending from one side of the piston to the other is a by-pass, F, in the wall of the cylinder. When the piston moves downward the displaced oil is forced to flow through this by-pass. A small cock, G, is arranged in the by-pass to throttle the stream of oil flowing through it. The opening in this cock, or the amount of throttling, is indicated on the outside by an index and dial, Z (see Fig. 27). When the piston B is raised and allowed to descend by the force of the spring C, it forces the oil through the by-pass F and the cock G. If the latter is wide open the piston will descend quickly, but if it is nearly closed the piston will descend very slowly. In other words, the time of descent can be regulated by opening and closing the cock G. The reading on the dial Z can be made seconds of time that elapse while the piston is descending.