Above the cylinder is a cross-head, H, that moves up and down between vertical guides. This cross-head is moved by the rod X, also shown in Fig. 27, that receives its motion from the swinging frame of the sending apparatus. A piston rod, I, attached to the piston in the cylinder, extends up through the travelling cross-head but is not attached to it. On the piston-rod are two enlargements, J and K, one made a solid part of it, the other formed by two nuts. The travelling cross-head H carries a pawl, L, that engages under the shoulder formed by the nuts K. This pawl is kept against the piston-rod by the spring M. The enlargement, J, on the piston-rod forms a shoulder that bears against the bell-crank, N, that connects with the bolt, O, which locks the controlling valve. In the present down position of the piston and piston-rod, the enlargement J, by pressing against the bell-crank N, holds the bolt O in an unlocked position. When a carrier is despatched the cross-head H is lifted by the rod X, and carries with it the piston and piston-rod, compressing the spring C. This upward movement of the piston-rod allows the bolt O to be thrown by a spring, not shown in the figure, and so lock the controlling valve of the sending apparatus. As the cross-head continues its upward movement, the pawl L comes in contact with the end of the screw P and disengages the piston-rod. This allows the piston to descend as rapidly as the oil can pass through the by-pass and cock G. When the piston has reached nearly to the bottom of the cylinder, the shoulder J, on the piston-rod, engages the bell-crank N and withdraws the bolt O, thereby unlocking the controlling valve. The time that the sending apparatus is locked depends upon the time required for the piston to descend. While the sending apparatus is locked against the sending of another carrier, it is not so locked that the swing-frame cannot be returned to its normal position and another carrier inserted ready to be sent as soon as the necessary time expires. This time is usually not more than ten seconds. Not only may the second carrier be placed in the tube C, Fig. 29, ready to be sent, but the handle K may be pulled and fastened in the notch _a_, thereby compressing the spring S, which, as soon as the controlling valve is unlocked, will move the valve and automatically despatch the carrier. The controlling valve is locked by the passage of a bolt through the hole _b_, in a block carried on the end of the valve stem, when it returns to the normal position shown in the figure. Usually little or no time will be lost in thus locking the sending apparatus, for the small amount of time that the apparatus is locked will be needed in handling the carriers.