Thus, from B carriers can be sent and received in either direction. In order to prevent the possibility of a collision of carriers by attempting to despatch one at station B at the instant another is passing through the sending apparatus, an automatic lock is attached to each sending apparatus. Just outside the station B, say three hundred feet on each side, are located manholes, and in these manholes boxes are attached to the tube containing an electric circuit-closing apparatus, so arranged that when a carrier passes it will close an electric circuit leading to the sending apparatus in the station. These manholes and circuit-closers are shown and located on the diagram at _v_ and _w_. Wires _x_ and _y_ lead from them to the sending apparatus _r_ and _q_. When a carrier from station A passes the box _v_, it closes the electric circuit _x_, which sets a time-lock on the sending apparatus _r_, holding this apparatus locked, so that it is impossible to despatch a carrier for, say, twelve seconds, a sufficient time for the carrier coming from the station A to pass station B and get three hundred feet beyond it. After the twelve seconds have elapsed the sending apparatus is unlocked and a carrier can then be despatched. In a similar manner a carrier coming from station H, in passing the box _w_, closes the electric circuit _y_ and locks the sending apparatus _q_ for a sufficient length of time to let the carrier pass the station. This resembles, in some respects, the “block system” as used on railroads. A “block” about six hundred feet in length, depending upon the speed of the carriers, is made at each intermediate station with the station in the centre of the block. Whenever a carrier enters this “block” the sending apparatus at the station is locked, and a carrier cannot be inserted into the tube to collide with the one which is passing. It will be noted that a carrier in passing out of the “block” does not unlock the sending apparatus; this is done automatically at a definite time after the carrier entered the block. The unlocking is entirely independent of the carrier after it has entered the block, and the reason it is so arranged is this: suppose that a second carrier enters the “block” before the first one leaves it; if the first carrier unlocked the apparatus when it left the “block,” then it would be unlocked with the second carrier in the block and a collision might occur, but by arranging it as we have done, if a second carrier enters the “block” before the first has passed out, the sending apparatus remains locked for a period of time beginning with the arrival of the first carrier in the “block” and ending, say, twelve seconds after the arrival of the last carrier, which is sufficient time for the last carrier to pass out of the block. Of course, if a carrier becomes wedged in the tube a collision may occur, but this very seldom if ever happens. The details of the locking apparatus will be described in another place.