instrument or from the SOUTH cabin instrument of the section, but only from one of them; and the act of taking out that staff automatically locks both instruments, and prevents the possibility of taking out any other staff from either instrument until the staff already removed is restored and inserted in one or other of the instruments. From the above description it will be seen that the electric train-staff instrument provides for the safe working of two or more trains proceeding, one at a time, in the same direction over a section of single line, each one being supplied with a train-staff, which must be handed over at the end of a section before another staff can be issued for a following train. Should the train-staffs accumulate in one instrument, in consequence of more trains running in one direction than another, a re-distribution of staffs is effected by the authorized persons according to fixed regulations. In the diagram sketch, Fig. 517, a piece of single line is shown divided into sections or blocks, with loops or passing-places at the stations. At the station E a train-staff taken out of the instrument F serves for the section up to the instrument L at the station H; and on the train-staff is a key which will open the detached locks on the points of the small intermediate station, G, as described in Fig. 507, in connection with the working of detached locks. At the station H the engine-driver receives another staff from the instrument M, which takes him to the instrument N at station K, and in like manner on this staff is a key which will open the detached lock on the colliery siding points at I. At stations H and K are shown loops, or short pieces of double line, with platform to enable an UP train to cross or pass a DOWN train. The distance apart of the electric train-staff stations will depend greatly upon the number of the trains, and for a frequent train-service it may be necessary to have the instruments at every station, whether large or small. The electric train-staff is of great advantage in the working of ballast or construction trains, as 347 a staff may be taken out of the instrument F at station E, which will give possession of the section as far as station H, and when the ballasting operations--which may be very near to E--are completed, the train can return to E, and deliver the staff again to the instrument F, instead of having to run the entire distance to station H. Although carrying a train-staff, the engine-driver must approach stations cautiously, and obey the fixed signals in the usual manner. CHAPTER VIII. 348 Railways of different ranks--Progressive improvements--Growing tendency for increased speeds, with corresponding increase in weight of permanent way and rolling-stock--Electricity as a motive-power.