Returning now to Fig. 31, we have on the top of the chamber D, in addition to the electro-magnet A and its armature B, a differential cylinder and piston, K, L, whose function is to close the valve C when the chamber D is filled with air. The piston K is smaller than the piston L, and sustains a constant air-pressure, supplied through the small pipe M M, from the pipe E, which leads to the main tube. When the chamber D becomes filled from the pipe E through the valve C, the pressure in the chamber moves the piston L upward against the pressure on the piston K, because of the greater area of the piston L. This movement of the differential piston raises the lever I, which passes through a slot in the stem of the differential piston, and thus closes the valve C. The air in the chamber now gradually escapes to the atmosphere through a small orifice Q; in fact it has been escaping here all the time while the chamber was being filled, but the opening through the valve C is so many times larger than the orifice Q that the escape of air was not sufficient to prevent the chamber from filling. Now, however, that all supply to the chamber is shut off, the air in the chamber is gradually being discharged through the orifice. When nearly all the air has escaped, the piston F will return to its normal position, shown in the figure, and unlock the controlling valve. The time required for the air to escape from the chamber, D, is the time that the sending apparatus will be locked, and this time can be regulated by varying the size of the orifice Q. The opening of the orifice, or the time that the sending apparatus is locked, is indicated by an index and dial, P. This locking mechanism is secured to a bracket on the side of the large cylinder H, Fig. 27, in a position where it can be easily inspected. The moving parts of the electro-magnetic valve—for such is the valve C, with the magnet A, Fig. 31—are made very light, in order that they may respond easily and quickly to the closing of the electric circuit.